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Depends how much you integrate the heat pump with a PV system. Most heat pumps now have a "SG Ready" contactor, which lets you turn the thermostat up whenever it is contacted across - most obviously when there is excess PV available. There is going to be a minimum power level at which this makes sense, so for small PV systems it won't work very well and the Sunamp solution would be better - resistance heating is much more flexible in the rate at which it can absorb heat - but in those cases the majority of your energy isn't going to be coming from PV anyway. A number of inverters (SMA certainly do) now have the ability to trip a relay whenever export goes above a certain level, so implementing this shouldn't be hard. I've been playing around with a spreadsheet that takes daily average air temperatures (used to predict the COP) plus half hourly PV data to work out self-consumption rates. A bit crude, but it suggests that this approach should provide >50% of annual hot water and a big chunk of the space heating too - can't remember the exact numbers and don't have it to hand right now, but using real-life data from 2015-18 gave "COP"s based only on the consumption of imported electricity of greater than 10:1 compared to heat demand. Given that heat demand was modelled for a 180m2 Passivhaus using a fair bit of hot water, to me it suggests that this approach should work pretty well. I would also take issue with the idea that heating is mostly required at night in a well-insulated house - you're normally in bed and would want a slightly lower temperature for comfort. With a well insulated & high decrement delay house the time constant for it to cool down is very long, so with no heating on overnight the temperature drop is likely only to be a degree or so, making the value of shifting space heat from day to night pretty minimal. Where you do need to shift heat is from summer to winter, which unless you go for the container sized storage units isn't happening with a Sunamp and there is significant value in increasing the COP since there won't be much surplus PV available after plug loads are taken care of. Oh, and if I thought building a Passivhaus would lead to me spending more time on the beach I wouldn't even consider it ?

@jack does exactly this with a 5kW heat pump though, and both Panasonic and LG do ~3kW units. There is typically a minimum water volume requirement in the UFH circuit - for Panasonic that's 30 litres. 16mm pipe has a 12mm bore - that means any layout with more than 265 metres of pipe in it is stated by the manual to not require a buffer vessel. That's a pretty small house. Note also that some units (Panasonic and Samsung I know about, not sure on others) can modulate down to 25°C for a big improvement in COP over 40°C. If the heating capacity is 3.2kW, 5°C dT gives 4.186 kJ/kg.K - 20.93 kW@1 l/sec, and thus a hot water flow rate of 9.2 l/min (I have a Panasonic manual for a 7kW unit which gives 20 l/min at max flow rate which is consistent with this). An 80m2 ground floor slab with 16mm OD pipes at 150mm centres would have 5 x ~100m loops of pipe in it, with each pipe containing 11 litres of water - so even with no heat transfer to the slab at all and no modulation on the heat pump that's ~6 minutes of heat pump running (0.3 kWh heat input before there is any possibility of the outlet temperature increasing), giving a heat pump which runs once per hour to meet the typical heating demand. If there is an anti-short-cycle limit then it will only be hit if the heat demand is >~1kW (12W/m2) and there is no heat transfer between the slab and water. The simplest remedy for this would be to increase the flow temperature slightly - increasing the flow temperature to 30°C would double the heat dumped to the system per cycle - thus doubling the heat input per cycle with no heat transfer to the slab but also greatly increasing the heat transfer rate between water and slab. So if you have a heat pump which can modulate down to a low enough temperature that you don't need a mixing valve, a buffer tank is superfluous in the vast majority of circumstances. I'm not quite sure how it works if you go down the mixing valve route however - I suspect that matching the power output of the heat pump to the rate at which the slab can accept the heat will need some playing around with circulation pump flow rates, mixing valve settings, etc. and that if they aren't right then short cycling will be the result. If I've understood correctly, most circulating pumps will be in the 20 l/min zone, at which point you'd see temperature rises in the return line after 2-3 minutes and thus a reduction in the dT seen by the heat pump.

I've got that going around in my head at the moment and as much as I like the simplicity of the concept I really struggle to make the logic work for a well insulated/low energy house: They've usually got a long time constant (decrement delay) with a significant heat capacity - so little or no benefit from adding volume from a buffer tank unless the heat pump can't take it. Provided the heat pump is a small one and the volume in the floor pipes is large enough, there shouldn't be any requirement for this - just connect them up directly as @jack did. That takes you down to one cylinder, and hopefully the mixing valve on the UFH goes as well improving the COP a bit. Vacuum insulation always makes me nervous with regards to long-term lifetime - this is a vastly better application of it than using it structurally within a building, but I spent 10 years of my life as a vacuum engineer looking for leaks so am suspicious of how long the insulation in an unpumped system will last, given that no matter what we did we always found more leaks. The manufacturers claim that when manufactured correctly the expected leakage will cause the U-value to double over 30 years - as I understand the process this isn't checkable however and relies on the build process being well controlled. Most (emphatically not all) people building a low energy house do so to reduce energy consumption rather than increase comfort. Delivering heat at a COP of 3 is a huge advantage here - to achieve the same performance you need to reduce energy loss by the same factor of 3, which is likely to be expensive. The PCM34 is in theory well suited to use as a hot water preheat tank. Problem is, you can do nearly the same thing with a shower heat exchanger at lower cost, which would lead to the heat delivered from the PCM34 being pretty minimal (when providing hot water to a shower the uplift to 34°C would be from ~25°C rather than 5°C - a 2/3rds reduction in power). That means the majority of hot water comes from another system - either a hot water tank (in which case why bother preheating) or a PCM58 unit run on resistance heat (minimum charge temperature 65°C - too much for a heat pump). Even if you only achieve a COP of 2 on the heat pump, that's a huge difference to the energy consumption for hot water and goes back to the question of what type of house you're trying to build. When they eventually commercialise the PCM43 material, I suspect all this will change. 43°C is hot enough for just about all DHW applications and eliminates the need for a TMV to deal with scalding, while the minimum charging temperature will be ~50°C which is comfortable for a heat pump. I think split heat pumps are probably underrated on here - finding an air conditioning company to make the connections can't be that hard (there are about a dozen within a 5 mile radius of me, so costs **should** be reasonable), you eliminate the need for glycol or equivalent and reduce the length of the primary hot water circuit to nearly zero with an associated reduction in losses. Heat pumps can do cooling - where the additional cost over an alternative solution is modest, that starts looking very attractive in weather like this. Essentially my current view is that the current Sunamp PCM34/58 lineup doesn't play well with heat pumps, and that this is a critical flaw if trying to combine them with the design of a low energy consumption house (note: not the same as a low energy import house). With that in mind, my order of preference would be: Heat pump with PCM43 Sunamp for hot water. Heat pump with unvented cylinder for hot water. Heat pump with standalone PCM58 Sunamp for hot water. I would expect quite a significant performance gap between the three options. A Uniq rHW fitted with PCM43 would I suspect be an extremely good solution for a low energy house. Should be fairly straightforward - the PCM is a bit like water in that changing phase from solid to liquid needs a lot of heat while the material stays at the same temperature, while heating it up after that still takes a fair amount of heat. However, planning on heating it much above the phase change temperature turns it into what is essentially an expensive hot water cylinder.

Are you sure that the sizes aren't standard? Taking a look at say https://www.appliancesdirect.co.uk/st/single-ovens standard height single ovens are all 595 x 595 (sometimes rounded) for the cutout. What changes a bit is the depth of the oven, but if the one you end up getting is a bit shallower then since it will just sit at the front of the cabinet who cares? Likewise for microwaves - going by https://www.appliancesdirect.co.uk/st/integrated-microwaves they're all 595 x 388mm or x450mm but of varying depth. They're pretty unlikely to change as there isn't any incentive to do so - and a lot of retrofits to lose.

Unless you are planning a very large house and the stove is one which can shift most of the heat to water, yes - at least if you plan to insulate to a decent standard. Controlling it so you don't bake is likely to be a major pain in the posterior. One other thing to consider is that a wood-burning stove needs a free-flowing chimney and an air supply. While there are solutions out there which attempt to address this problem (e.g. http://www.poujoulat.co.uk/catalogues/domestic/Leaflet_efficience_PF_2014.pdf), you're left with the issue that the air column forms a thermal bridge through the structure and the stove has to be designed to give a good thermal link between the gas inside it and the room - leaving you with what is essentially a log-burning fridge when you don't have a fire going in winter.

Umm... not really that convinced (although the inverter shutting down because the grid is limited would be one of the cases where I have no problem with it). The subsidy regime is one of the aspects which has contributed to your decision to use a heat pump for heating and resistance heat for hot water (albeit with a preheat tank from the heat pump). Over the course of the year, the net energy flow from grid to your house will be greater than if you had used the heat pump for water heating, even with the defrost issues you have, since you would only be export-limited for a small fraction of the time. The net result is that the subsidy forms a perverse incentive - it's a relatively modest impact in your case, but is still there. Most people with Immersuns or similar would have otherwise used mains gas to heat their water, and there the difference is rather bigger. The theory is that you'll be able to download some sort of annual energy consumption profile which you can then use to shop around on comparison sites and get the best deal for your circumstances. I think it'll be a while before that happens, however. This is one of the problems I have with the higher FIT rates (i.e. those which give a pretty decent rate of return on investment) - it's essentially a transfer of money from those who can't afford the capital cost of a PV system to those who can. There is some justification for the idea - the private sector has a huge amount of capital available which has been unleashed to increase the uptake of PV, which is IMHO a good thing and might never have happened without subsidy support - but balancing the two is essential. Either works.

I'm a lot more relaxed about it than I once was now that the rates are far lower and there is less coal on the grid, but I still really don't like immersion diversion devices except in very specific circumstances (as a dump load when you would otherwise exceed the DNO export limit). In most cases people are shifting from heat generated by burning gas at 90% efficiency in a domestic boiler to burning electricity which is generated by burning gas at maybe 50% efficiency in a CCGT, and getting paid to do so. I'm very curious to know how close we are to smart meters being able to do this. I know the capability is in the SMETS specification, but given how long it is taking everybody to meet this spec in full I'm guessing it isn't available yet. Time of use tariffs are starting to become available (e.g. https://octopus.energy/agile/), and I suspect eventually we'll also see dynamic pricing for export - most likely once batteries/V2G starts to become more widespread as there is a lot of potential commercial value to an aggregator being able to turn on a lot of export to the grid all of a sudden, and they'll need evidence that they've done this.

Insufficient information, unless you're planning on installing a huge battery system. Because electricity (unlike heat) isn't easily stored, then if you want to avoid exporting any power to the grid you need to have the smallest panel which can deal with the always-on loads (standby on the TV, clocks, MVHR, etc.) on a bright, cold day - probably about 50-100W. If you want to claim that you're "net zero" that's actually pretty easy - south facing panels in the UK make about 1000 kWh/kWp so for 3,000 kWh you'd need a 3kW system. If you're feeling a little more adventurous with a spreadsheet and have a decent idea of how your plug loads will vary by time of day and day of the week, take a look at Sheffield Solar (https://www.solar.sheffield.ac.uk/pvlive/) and download the data on it for several years (I think the complete dataset starts in 2015). That provides half hourly PV generation figures along with up to date grid-connected capacity - divide one by the other and you have the output per kW of installed capacity. Mix in your predicted plug loads, have some fun with macros, and you can see how your savings would vary with different sized PV systems and plot system size against ROI. Again, this doesn't actually answer your question but is probably as close as you're likely to get I suspect.

SMA have a pretty good online tool called Sunny Design (https://www.sunnydesignweb.com/sdweb/#/Home - registration required but free) which lets you specify a system and will then calculate production, self consumption and the like for given load profiles, heat pumps, etc. I suspect it's a little pessimistic for a well insulated house, but otherwise seems pretty reasonable. You'll need to put in your own electricity cost and subsidy estimates however - that's the one part of the programme where I get the strong feeling that the sales department got involved and the figures look very fishy to me. To give an example, for our (potential) build there is space for an ~8kW system. With electric heating and hot water plus SMA's system for increasing self-consumption, they're predicting ~1,700 kWh would be self-consumed (with 3,000 kWh still imported - this for a 40m2 "passive house" as for anything bigger the heating loads start to look silly) with 5,500 kWh exported. On that basis the system would be worth £288 in export tariffs, another £288 in FITs and ~£255 in avoided electricity bills for an annual value of ~£830, or about £100/kW. I think I could probably do a little bit better by playing with the smart grid functionality on a heat pump (e.g. turning up the thermostat a degree whenever the PV system is exporting and using the heat capacity of the house to store energy), but it won't change the value much. At ~£100/kW/year it's worth doing for us, but only within limits - we'd do it if we had the cash, but other aspects of the build would take priority.

Bear in mind that I was once described by a crusty old RAF flight sergeant as "being able to tell you the square root of a jar of pickled eggs but unable to get the f***ing lid off". I like to think that I've improved a bit in the years since, but being as I haven't yet built anything all my comments need to be treated as theoretical rather than real. 90% of them are probably right, but... I think trackers are a solution to a problem that no longer exists for terrestrial systems - when they were first a thing PV was hideously expensive and they added 1% to the price of the system for a 30% increase in capacity. Now PV panels and inverters are cheap, it adds >100% to the price of the system for a 30% increase in capacity - no longer such a great deal. I think Touchwood do something similar going by their website, and they also have a few happy customers on here (if fewer than MBC). There are probably a number of other companies capable of the same sort of thing. One of the things that it's really difficult to teach new engineers to actually do is to write a solution-neutral problem statement. That is, a description of what you are trying to achieve which does not indicate in any way how you are going to do it. It's a very valuable exercise because it often gets you thinking of new ways to attack the problem, but people instinctively think of ways to solve a problem as soon as they start working on it and that leads to particular approaches being adopted, whether or not they are appropriate. In this case, for instance, the problem statement was "we want a house built with an oak frame and SIPs shell" rather than "we want a house with oak beams visible in rooms X, Y and Z". The latter statement invites a far wider choice of potential solutions to the same problem, and the more solutions are considered the better your chances of finding the best one for your requirements.

As I understand it, most oak frame/SIPs combinations now have the oak frame inside and the panels wrapped around the outside of it to avoid cold bridging through the oak. The thing is, SIPs are themselves a structural option, which makes the oak frame largely redundant. If you do follow this route, then I would suggest thinking exactly where in the house you actually want to be able to see the oak - it may for instance be possible to work some oak timbers into the structure of a timber-framed building without needing an entire oak frame, saving £££££. More importantly, if the SIPs are on the outside then the only real limitation on thickness is the internal and external dimensions of the building. That means any insulation level up to Passivhaus is possible with a full structural timber frame - see https://www.oakwrights.co.uk/siteI/cfbc0416c099a39a8bd816131025b2d8/press/39935ffd63b49558c7873a6fea6f430e.pdf - but the best way of doing it may not necessarily be with SIPs (the couple in that example didn't use them, for instance). If I'm understanding things correctly, the UK building regulations at the moment should limit consumption to about 50 kWh/m2/year for space heating - about 9,000 kWh/year for your proposed buildings. For comparison Passivhaus is defined at 15 KWh/m2/year with a rather more rigorous modelling system to back it up - about a 70% saving over building regulations even before you start looking at heat pumps. By the time the heat demand gets that low even electric resistance heating (which is really cheap to install) is economical to run. Don't feel forced to go down the heat pump route - I happen to think it's a pretty good one, but the better insulated your house the better it works. If you're planning on only meeting the minimum PP target, then I would think seriously about alternatives like LPG or Oil: they're well understood, cheap to install and deal well with increasing the output temperature a bit because say your radiators were a bit too small. Personally I think heat pumps are the best solution for a low-energy house, which is why I want to go down that route, but don't make the assumption that they're the best option for every house. Good performance is critically dependent on them supplying heat at a low temperature - the worse the insulation, the harder this is to actually achieve. Assuming total annual consumption is about 14 MWh (plug loads, hot water, etc.) then you need about 1.5 MWh/year from PV to meet the PP specification. As a rule of thumb, a 1kW panel facing south in the UK will give about 1 MWh/year of generation - so you could meet it with a 6 x 250W panel system costing maybe £2k for a ground-mount system. That's almost certainly the cheapest way to meet the target, and will be why the PP suggested it. I would suggest taking a step back and thinking through (in order): What your energy target is going to be - the minimum to meet PP, minimum cost to live in afterwards, or something else. This will define the insulation levels in the structure, which will make clear what else is practicable. If you want to have a play with that, @JSHarris has written a pretty decent heat loss calculator at http://www.mayfly.eu/wp-content/uploads/2017/01/Fabric-and-ventilation-heat-loss-calculator-Master.xls When you know this, work out how you want to deliver the heat - radiators, underfloor heating, warm air, something else? Underfloor will work with just about anything, but warm air and radiators don't play well with heat pumps. Then you can do a meaningful trade-off between ASHP, GSHP, electric resistance, LPG, etc. - knowing the annual heat consumption and peak heating load is critical here because it changes the design of a heat pump system radically, while a traditional boiler system really doesn't change a lot. If you want building regs minimum and radiators, then you should probably look at LPG or oil - you need quite a bit of heat delivered at a high temperature. Building regs minimum plus underfloor might well be the sweet spot for a GSHP as demand will be high enough for the COP gain to be worthwhile. It's worth running some calculations on this though - the SCOP figures published by the manufacturers should be good enough for this, divide heat load by SCOP to give electrical consumption and work out if it ever pays back over an ASHP. If you want Passivhaus or close to it - particularly with underfloor heating - then you should probably look at an ASHP like @jack as the heat load will be so small that there is no point to a GSHP. Resistance heating is feasible at this point, however, and gives you low installation cost with bombproof reliability: at 15 kWh/m2/year and 15p/kWh then your heating bill for a 180m2 house is only going to be £400. @TerryE has gone down this route and it has worked very well for him. When you've done all that, then is the time to think about renewables (PV, realistically) - more insulation or underfloor heating is an utter nightmare to add by retrofitting, while a ground-mount PV system is incredibly easy. Make sure you've got the things that are locked in done early, and worry about the rest later.

My cousin lives in a Passivhaus in Hamburg, fitted with a log-burning stove. It's usable, but you've got to be careful - you burn one small log (sticks are no good - too much surface area so they burn too fast: they burn short fat logs) at a time, keep the doors open to spread the heat around the house and it's fitted with some hefty soapstone slabs on the top and sides. From memory the room ended up at 27 or 28C after a few hours - actually quite nice in that part of the world in winter, but definitely not a routine occurrence.

Worth looking into how small a stove you can practically run - they really don't like being run smouldering (lots of smoke and creosote), and when run hard most will put out at least 5kW which will be 2-3 times what you might need for the whole house. Again, a lot depends on how well insulated you're looking to go for - at building regulations minimum then you'll be fine with a couple of stoves, if you're anywhere near Passivhaus then you'll have overheating problems.

Not yet - I'm hoping to demolish & rebuild our existing house, just getting the money sorted at the moment (we bought it planning to refurbish but by the time we did everything it's cheaper to start again). One thing I've been looking at quite a lot is heat pumps - my wife is from the US and every year for the week or so when the weather is hot and humid I have to listen to her bitching and moaning incessantly about the fact that "this goddamn country doesn't have air conditioning". This means that - despite the fact that we have mains gas available on site - we're going to be doing a heat pump because it can also provide cooling. I'm also a chartered engineer working in R&D so doing things like setting up a spreadsheet to work out actual COP based in predicted heating/hot water loads and historical air temperatures comes pretty naturally. Instinctively I'd prefer to have a GSHP, but I just can't get the numbers to work out even though I've got a garden big enough for it. Based on 2016 data and published COP curves for a Samsung heat pump I'm predicting something like a COP of 4.4 for heat at 25C and 2.8 for hot water at 55C over the course of a year - and total electrical consumption of 1000-1500 kWh over the course of a year. By the time you get to that point there really isn't very much scope at all to reduce electrical consumption by going from an ASHP to GSHP - and the amount of extra money you need for a GSHP install is enough to make a big reduction in heat demand. You end up with the situation where a GSHP install costs maybe £10k extra including ground works, etc. in return for a modest reduction in sound levels. The logical conclusion to this is that alternative ways to reduce noise from an ASHP should be considered, and that given the cost differential it should be possible to throw a shedload of cash at this problem and still come out ahead. Looking at the noise ratings for split heat pumps is instructive here - the indoors units are very quiet indeed, quieter than an equivalent GSHP in fact, and the noise comes from the outdoor units. This appears to be predominantly flow noise from the air - and silencing air flow noise is something that is done all the time with commercial air conditioning systems. Segregating air flow should also be pretty straightforward, since the blows air out of the fan hole at the front and is sucked in everywhere else. So I'm thinking of maybe putting the outside unit in the loft of an attached garage, with louvres along the lines of http://www.wakefieldacoustics.co.uk/products/acoustic-louvres/ at either end wall, and the ASHP unit outlet directly ducted onto one of the louvres with some draft excluder, using the garage loft as essentially a giant air duct. Not a perfect solution, but looking at the sound curves at maximum power it would be about 35 dBa, mostly in the low frequencies which I can tolerate slightly better. Given that this will be outside the house and the house itself will further attenuate any noise (and ideally it would be at the front of the house so further shielded from the back garden), that should be good enough. So far as brands and reliability goes, any of the big brands should be fine - the main thing to be wary of is that the MCS installers tack a lot onto the price for the privilege of claiming the RHI payments, so it probably isn't worth going down that route. Monobloc units are probably easier to fit (there are a number of people on the forum fitting them as a DIY or quasi-DIY job), while split units need someone with an F-gas license to fit. That isn't too hard though - you'll have any number of small air conditioning companies local to you who could fit it, so it shouldn't cost too much. I'm personally leaning that direction, mostly because I think it should make the plumbing design a lot simpler.

If you're worried about noise (I am - continuous fan noise in particular makes me really tense) you can look into acoustic louvers and either fit it into a standalone box or integrated into something like an attached garage (which I'm thinking about doing). That should knock the noise down by about 10 dB(A), making them quieter than a GSHP anywhere you are likely to spend any time. Noise levels will also scale with heat demand (more heat needed = more air flow), so again better insulated houses should be quieter.

With regard to ASHP/GSHP, how highly insulated are you planning to go? A Passivhaus is 10W/m2 at the design low temperature condition, so at that heat load the biggest house would need 1.7kW of heat on a cold day in Winter (50 kWh/day). Assuming a GSHP has a COP of 4 and an ASHP a COP of 3, that's 12.5 kWh of electricity for the GSHP and 16.5 kWh for the ASHP, for a cost difference of about 50p on an unusually cold day. ASHPs are actually better than GSHPs for producing hot water in summer, so over the course of a year then for a low energy house then the running cost difference is pretty trivial while the installation cost difference is potentially very large in favour of the ASHP. Things are different if you're just designing to meet the minimum building regs requirement - there your heat loads are far higher and a GSHP starts to make a lot more sense. For me personally that's the wrong way to go though - I suspect in 90% of cases the cheapest way to a particular fuel bill is an ASHP plus more insulation rather than a GSHP. There will be exceptions of course (acres of north-facing glass with a lot of shading, hard to insulate retrofits, etc.) but I suspect it's probably a pretty good general rule.

From what I've read, their evidence so far can be summed up as "this was a really complicated design which should have been really expensive to build, so we're very upset with these people for failing to build it cheaply," I can't help but suspect giving the example of their TV room with three curved walls and one straight wall will not have helped their case...

You've also got quite a lot of what appears to be detail for the sake of detail - it's quite a complicated shape for what appears to be no real reason inside, presumably purely to add external interest - you should be able to break the building up into smaller sections in other ways that are cheaper and give a better airtightness & insulation result. Having upper level floors & roof offset from the ground level floor is going to give you structural headaches - if you can line them up it will save a load of cash. Likewise unless you really need one deleting the basement will save you a lot more money - delete it and you can just use an insulated slab which is a hell of a lot cheaper unless you're on a major slope. In some ways the current design reminds me a bit of something found on http://mcmansionhell.com/ , at least externally. On the insides I agree with @PeterW that it looks like it has an awful lot of house for not all that much space: the hall for instance is taking up a huge amount of internal volume, and unless you already have plans for the space the gym, most of the storage and possibly even the cinema would be much better off in the garage roof space than where they currently are in the basement.

Not really a problem - essentially you just need to size the boiler for the hot water load plus probably the standing losses of the cylinder, and your heating losses will probably come out in the wash. I'm assuming the thermal store is sized such that everyone can have a shower/bath at the same time before it's recharged? 500 litres translates to about 25 kWh to heat it up, so your maximum hot water heating power required is more or less 25 divided by the minimum number of hours between everyone using up all the hot water (8 hours for a 3kW load?). DHW is only the dominant heat consumer in a Passiv or near-Passiv house because the heating demand is so low - in absolute terms it isn't actually all that big. If you go for a big storage tank plus a large boiler, you're essentially ensuring that you can have unlimited hot water twice - a big tank and small boiler or small tank and big boiler would both be absolutely fine to do what you want. No reason that the heat load formulae wouldn't work for sizing the heating fraction of the total demand - you still need the same amount of heat to keep the place warm, it's just being delivered by a different route at a lower temperature.

Passivhaus has a heating load 10W (0.01 kW) per square metre at the design heating condition, so if you're a bit off then you probably need 0.02-0.03 kW per square metre for heating. As for hot water, you get about 20 litres of hot water for a kWh of heat (a bit less in winter, a bit more in summer - depends on the starting temperature) so a 35kW boiler should be able to give you about 700 litres per hour of hot water. As a cross-check, a 35kW combi is rated at about 15 litres per minute (870 litres per hour). So unless you actually want to get rid of the thermal store and run the place off a combi, 35kW is absurdly oversized - you want the smallest boiler you can get your hands on (probably 12kW) as otherwise you'll end up with short-cycling nightmares.

Heat Capacity is the term you want (Specific Heat Capacity x Mass), which has units of Joules per Kelvin (i.e. the amount of energy absorbed by the material for every degree increase in temperature). It isn't a terribly useful term however - the rate at which that heat is released again (a function of surface area, thermal conductivity, emissivity of the surface, etc.) is critically important. Combined that gives you the time constant of the system, which should be matched to the time constant of the building as a whole (too short and you'll get overheating followed by no heat at all, too long and it won't do any good).

Concrete is seriously strong in compression. Take https://stowellconcrete.co.uk/dense-concrete-blocksbricks-10-4nmm2/ : they have a compressive strength of 10.4 N/mm2. So each block can take 10.4 x 100 x 440 N = 457,600 N where 9.81N is the load exerted by gravity on a 1kg weight. That means each block can support a load of 46.6 tonnes without disintegrating - with each block weighing about 20kg, that's one hell of a tall wall. The reality is that a long thin wall will fail by buckling rather than the material failing in compression, but even so you're going to have a huge margin of safety here for what sounds like a very lightly loaded wall, particularly as you shouldn't have any sideways loads on the wall.

We're also used to the grid being based around fossil fuels with marginal load being picked up by coal or gas, so any additional renewable generation will reduce emissions. In the case of Eigg, 90% of the time there is excess renewable capacity and the other 10% of the time they're reliant on diesel but with very limited renewable resources. That suggests to me that expanding generation won't really help (essentially it's likely to reduce the output of the existing grid renewables), but that demand shifting would be helpful to some extent. The main issue is that most generation is hydro-based, so periods of low/no generation are likely to come in large blocks rather than over a few hours which makes the storage task rather more demanding. As a very rough approximation, and assuming from the earlier discussion on PV that reducing emissions is a priority: Reduce demand. Given the large percentage of Hydro on the grid, concentration should probably be more on reducing demand for hot water rather than heating since a lack of water is unlikely to be a problem during winter in the Hebrides. Shower heat exchangers are a good place to start, as is thinking about how the hot water is generated - some sort of heat pump is probably a good idea, and given that the diesels are likely to kick in at times of good weather which will give the best COP. Assuming that the main system isn't electric resistance heat (immersion heaters) then at least think about hot fill for appliances - for dishwashers it's pretty much a no-brainer (they clean a lot better on hot fill in my experience), not nearly so clear for washing machines however. Then again, washing machines are straightforward to program to run overnight when demand will otherwise be low. Reducing heating is a good thing from the point of view of comfort (draughts, etc.), but won't actually have much if any environmental impact beyond a certain point as the source will be pretty much 100% renewable with spare capacity. It does make using some sort of smart grid based system to avoid exceeding the 5kW limit much easier however - the smaller the draw from the system, the easier getting everything to work together is. Increase heat/power storage in the design. I'm actually not totally sure how far this can be practically taken given that times when the diesel plant is running will tend to be quite extended. Hot water - storing big volumes of it is actually quite easy, but to make a meaningful difference compared to a standard cylinder it needs to be really, really big and that might well not be practical given the fact it would need to be shipped to a very remote location. Heat - storing low-grade heat within the structure is relatively easy, e.g. by building a thicker slab with lots of rock in it but doesn't get you very much of a benefit as it's unlikely to be needed much when generation is low. Batteries work better at a grid scale than the individual scale, since demand can be averaged at grid scale and so they are used when needed and only when needed at a grid scale, something difficult to implement at a domestic scale (they have to be integrated into the grid control system). Again, if the problem is rare periods of extended diesel running, batteries don't help very much. Increased microgeneration - normally a good option, but doesn't do much in this special case. PV is the best of bad lot since it is easy to turn the wick down on it and there are ways (Sunny Island, etc.) to stop it affecting the grid - but given the unusual circumstances it's quite a lot of money to pay for not much improvement. Solar Thermal might actually be appropriate here for a change since integrating PV is more expensive than usual and hot water is likely to be a major demand at times when the diesels are running. For all that, it's quite expensive and maintenance intensive compared to some of the alternatives. Small wind, micro hydro, etc. have the same problems, plus when they're generating the grid should be just fine so you aren't saving much. Personally in the circumstances I'd probably follow the US "pretty good house" concept - push the insulation up towards but not reaching Passivhaus, then spend a bit on some things to shift load about (heat pump time programmer, boiling water tap, that sort of thing) and invest the budget otherwise assigned to PV with the island power co-op if that's possible, since it would do far more good there. Obviously, a lot depends on what people are comfortable with and what will make them happy - quite a few people out there want to generate all their own power even when on grid where it doesn't make financial or ecological sense.

Apologies for the slight thread necromancy, but I started reading this while looking up something else and started to wonder if what would normally be the right approach really isn't here. Electricity looks to be slightly expensive (http://euanmearns.com/eigg-a-model-for-a-sustainable-energy-future/ gives 20p/kWh - probably gone up a little since then) but not ridiculously so, and emissions are very low (85-90% renewables with the rest from diesel) so PV will only save you a bit of cash and not much else. Given that 5kW is actually quite a bit of power and well insulated houses don't need much heat (Passivhaus is 10W/m2 at design conditions - so ~3kW of thermal power, ~1kW of electrical power from a heat pump) then some sort of automated load shedding system is probably enough when combined with good insulation. There are a number of off-the-shelf items (mostly from Germany) that can do this, and cooking up something with Smart Grid Ready appliances should be quite easy (e.g. SG-ready heat pumps have an "inhibit all" contact which you could use whenever power consumption goes over 4kW).

This isn't going to work as a homebrew system - it would have to come from someone like Nest who could afford to pay for met data (incidentally, XCWeather.co.uk has got all the most recent met data plus forcasts on it so the data must be out there somewhere). I don't think accuracy is all that critical though - the heat pump would continue to provide heating and hot water no matter how good the data is, what it does is improve the COP significantly by reducing defrost time and the dT it has to pump heat across. IoT is one of those things that is actually really useful, but nobody really knows how best to use it yet - 3D printing is another example where we're probably only seeing 5-10% of the ultimate capability being exploited. Security for this sort of thing is also probably a lot easier than for other applications - unless you're doing demand response type applications, you set it up as essentially a broadcast system that the heat pump uses as an advisory to work out the most efficient profile for the day.