30kW PV, All electric heating, is this mad?

[TonyT]

Solar PV and solar thermal given the size of the roof

Unvented cylinder with solar coil for solar thermal, immersions off PV diverter, bottom immersion for off peak heating

 

GSHP, ASHP or A2A heating cooling.

 

Prior to all of that a heat loss calculation to work out heating demand.

[DevonBarn]

@SteamyTea 16MWh would definitely be preferable, I'll put that into the PVGIS later.  I guess everything is 'off grid' down there.

 

Thanks @Conor and @Thorfun I need to reassess annual demand, especially if 30kW system isn't possible and we need to make do with single phase. 

 

 

[SteamyTea]

23 minutes ago, DevonBarn said:

16MWh would definitely be preferable, I'll put that into the PVGIS later.

That is just space heat average (guessed).

Some houses can easily use the same amount for everything else.

25 minutes ago, DevonBarn said:

I guess everything is 'off grid' down there

It is when you lot bust the railway line.

26 minutes ago, DevonBarn said:

reassess annual demand

I assume that you already have a place, so you could use your current usage as a starting point. 

To get a rough idea if what the space heating loads are, just model the place on floor, roof and wall/window/door areas, then use the TMY figures from PVGIS to get your local temperatures.

[Marvin]

1 hour ago, DevonBarn said:

Also do you know how to find @Marvin's AIM APE? Also what's AIM? And what's APE?!😅

Sent you a message direct...

[Simon R]

2 hours ago, Thorfun said:

rather than guesstimating use the heat loss spreadsheet that @Jeremy Harris wrote. you should find it here

 

Absolutely fundamental to your planning. Only once you have the Watts per degree  figure for the building will you be able to look at options. Living in a well insulated and airtight structure is very different from a conventional build.  

Getting the place air tight/well insulated will reduce inputs and pay huge dividends.

Also in winter solar gain from east and south facing windows can easily keep a well insulated house warm, but you do need to makes sure you can shade them to prevent overheating in summer.

Using split air-conditioning units rather than radiators is a simple alternative and much better in energy and cost terms. Low cost Midea units have a SCOP of 4.9.

 

[HughF]

Multi head mini-split, job done.... Cheap as chips, easily controllable, easy to fit.

[Rob99]

2 hours ago, DevonBarn said:

and we need to make do with single phase

I would definitely pursue having a 3 phase supply, even if it costs you a bit more, as it will future proof your supply. It's becoming the norm now to have 3 phase.

[AliG]

I think the idea that summer excess PV pays for winter use is not the right way to think about things. The main reason being is that it doesn't take not consideration the £30,000 spent on the system. You can clearly get to a point where you have no actual bills to pay, but if you spend an excessive amount of money to get there you haven't really "saved" anything.

 

I also would not base any calculation on the current Octopus 15p SEG payment. Wholesale prices continue to fall, the current spot price of electricity i around 10p/kWh. The price cap will fall in July and I would not be surprised to see the export price fall also. Once we are through the current Ukraine inflated period I would expect export prices to settle into the 5-10p range. The more PV there is installed, the more likely it is that there is an excess of PV in the summer and less is paid for it.

 

You need to heat hot water, heat the house and obtain electricity whether by buying it or generating it yourself. In general you want to size a PV system so that you can self use as much of the output as possible, batteries can alter this calculation, but tend to currently be quite expensive.

 

You will probably need around 5kWh per person per day for hot water. Assuming 4 people in the house that is around 7000kWh per year.

 

Without knowing your exact insulation levels, using U-values of 0.15 for the walls, and 1ACH with 100sq metres of windows with 1.1 U-value I get around 22000kWh of heating requirement per year.

 

With a temperature of 0 outside you would need an instantaneous heat input of 6kW and at -5C you would need just over 7kW. This plus hot water requirements would drive the size of your heating system (ASHP probably).

 

So a few calculations. Let's keep it very simple. You need 30kWh a year of heating. Assuming that the average cost of this is 20p/kWh (e.g. with some time shifting and falling prices) then this would cost £6000 a year using direct electrical heating. If you used an ASHP with a COP of 3 it would cost £2000. ASHP pricing I think is often overstated. If you installed an electric boiler you would still need pipework, hot water tank etc. The extra cost of an ASHP compared to a gas or direct electric system is probably around £8000. An ASHP based wet heating and hot water system will cost maybe £15000. An electrical system with hot water tank (I'd say house is too large for a combo) might cost around £7-8000. These ar very rough guesses. So for an extra £8000, you would save £4000 a year at current prices, a great investment. The saving might fall to £2-3000 as electricity prices normalise. It is an absolute no brainer.

 

So how much PV would be reasonable?

 

You are going to need 7kWh per day for hot water via the ASHP, maybe a bit more with tank losses etc.

 

I don't know your background use of electricity for fridges etc, but assume around 15kWh day of general electricity usage.

 

These total to 8kWh/year. Now where you live an 8kW system might generate that much electricity but it is very skewed to the summer. Using @Alan Ambrose

numbers above an 8kW system would generate around 25-30kW per day in the summer, so already more than you would be using, and you would need a battery to use all of that as some of the use would be at night.

 

But say with this size of system you could utilise 70% of your yearly generation, very little would be wasted as peak production is not much more than normal use. You would be saving around £1680 a year at July 1 prices (5600kWh at 30p/kWh) plus receiving around £360 in payments (likely to fall soon). So a £10,000ish system would generate around £2040 a year in benefits.

 

Now let's double the size of the system,, say 16kWh, costing £18,000 (assume some scale economy). You generate 16000kWh. You self use 50% of the electricity at 30p and export 50% at 15p. That is around £3600 on a £18000 investment (Maybe more if you have to install 3-phase).

 

However, this is all based on recent electricity prices. Pre Ukraine war I was paying just over 12p/kWh and export prices were around 4p. Current spot electricity prices are around 10p (you have to add quite a bit to this to get the retail electricity price). Based on the current futures market I believe that a good price to use is 20p/kWh in the long run and 10p for exports. However, I think there is a good chance of a massive slump in prices if the Ukraine war ends and we can buy Russian gas again. There will be massive oversupply and prices will collapse. Using these numbers would reduce the returns of your system by around 1/3. Still good double digit returns. If however, you installed such a large system that you exported 70% of your generation I would be worried. In reality I doubt you would get permission for such a large system, so fill your boots and install as big a system as you can assuming that you are paying a reasonable price. (say £1200/kW installed). I was quoted more like £1500/kW for my parents' house on a small system that would generate less so it was not worth it. They also hardly use any electricity during daylight hours, so self consumption would be low.

[JohnMo]

Some more numbers

If you build it right your heat demand should be circa 6kW or better on the coldest day.

 

An A2W or A2A installed and designed correctly should give a SCoP of 4ish. But CoP would reduce as temperature drops. So reducing the electric input down to let's say 2-3kW (by using a heat pump), so you need 48 to 72kWh per day approx on coldest day. Otherwise you will need circa x4 the kWh with storage heaters or direct electric.

 

So assume you get an optimistic 6 hours PV generation, you would need 8kW or more output averaged over the 6 hours in December. 

 

So make use of that you need a storage medium, big battery or concrete floor via UFH.

 

You need circa 90kW array for that, I believe, based on a heat pump.

[SteamyTea]

4 minutes ago, JohnMo said:

concrete floor via UFH.

Highly insulated concrete floor.

[DevonBarn]

Thanks @AliG so this getting into the detail - that link in my first post is really good, goes into the details re: location/angles/temps everything so I'm pretty confident on what output we could expect on any given kW system.

 

You're right it's entirely dependent on import/export prices, but like you say even if there's a large change, it seems to be about 50% export to import price, so you still need to generate the same in the summer to cover the winter, probably.

 

If we're installing PV and all the gubbins really we should be installing as many panels as we're allowed to.. so @Rob99 I'll find out about 3 phase costs.

 

I need to get more quotes in so we're not shooting in the dark, (just had a quote in for GSHP supply only £25k, and that's without digging the 1.2m trench?!). I'm speaking to a ASHP company tomorrow I'll update the thread. Also @HughF thanks for the suggestion, Multi head mini-split does look like a good and easy additional option

 

 

[Alan Ambrose]

>>> I would definitely pursue having a 3 phase supply, even if it costs you a bit more, as it will future proof your supply. It's becoming the norm now to have 3 phase.

 

Yeah, I respectfully disagree with that. The object of the exercise is to use less energy, no? 3-phase let's you use loads of energy (and pump loads back, but you're probably not making any money on that). It's a bit like having a 8L engine Bugatti Veyron - great fun I'm sure, but it won't help reduce your petrol consumption much . Yes, on 3-phase you'll be able to run 3 fast car chargers, 3 ovens and ASHP on full blast - all at the same time, and you won't be using any fossil fuels (at least directly), great! Just like doing 267mph in your Veyron - theoretically you can, but you never will.

 

Add a bit of extra complexity for monitoring, PV feed-in etc, a bit of extra danger for having 400V around, and I don't get the hankering for 3-phase (and I do have 3-phase).

 

 

 

 

[SteamyTea]

Not all the PV needs to be grid connected.

You could set up some of it to heat water/Willis heater, small battery and A2AHP for shoulder season heating and summer cooling.

And if (never trust an if) you get an EV with V2G, you will have a huge battery system built in.

 

But as everyone says, get your heat load down as low as you can, that is the best return on your cash, and you are doing the work anyway. The labour to fit 150mm or 300mm of insulation is near enough the same.

[JohnMo]

1 hour ago, SteamyTea said:

Highly insulated concrete floor.

True

[JohnMo]

1 hour ago, DevonBarn said:

GSHP supply only £25k

Out of interest what size heat pump are they quoting?

 

Although that price is taking the Micky.

[DevonBarn]

@JohnMo that was for a 16.3kW GSHP. Yes floor will be well insulated, concrete base.

@Alan Ambrose actually with respect the aim isn't to use less energy, it's to generate as much as we use (in £), so 3-phase just makes things more flexible. But point taken about complexity.

[DougMLancs]

But if you can get the energy demand of the house down then you can do everything within the realms of single phase and save yourself £8k or (significantly) more on a three phase connection. 

Edited June 22, 2023 by DougMLancs

[JohnMo]

17 minutes ago, DevonBarn said:

16.3kW GSHP

That is huge, my house is basically 200m2, so half yours, but average ceiling hieght is well over 3m. I just installed a 6kW ASHP and it's really nearly double the size I could get away with on the coldest day (-9). But it cost £1300 so happy to comprise. But sized nicely for cooling.

 

You really need to understand the heating required, otherwise suppliers will give you something fit for purpose.

[saveasteading]

36 minutes ago, DevonBarn said:

actually with respect the aim isn't to use less energy, it's to generate as much as we use (in £),

You've lost my interest now.

 

What is the point of making energy, af huge expense,  and of wasting it? It sounds as if someone is trying to sell you lots of stuff

More pv than you can use,  electric rads, and much bigger heat pumps than you need.

[AliG]

2 hours ago, DevonBarn said:

I need to get more quotes in so we're not shooting in the dark, (just had a quote in for GSHP supply only £25k, and that's without digging the 1.2m trench?!). I'm speaking to a ASHP company tomorrow I'll update the thread.

I ldon't think anyone on the forum has a GSHP, maybe someone does. When I looked into it for my house it was way too expensive, although this was due to the cost of digging.

 

If you are getting prices, i would think a 12kW ASHP would be the right size for you as this would allow for heating hot water plus a maximum heat input of 7kW to the house (also allows for the as built performance to be a bit worse than the calculated performance).

 

My parents with a 200sq metre house have a 9kW Panasonic heat pump which has proven extremely good and absurdly cheap to run, we run it in Intelligent Octopus so try to keep most use overnight.

 

The pump itself costs around £4500, there is more work involved in installing an ASHP versus a boiler and you need a buffer tank usually, but some places clearly take advantage and massively bump up the cost. My parents were £11500 for the ASHP, Unvented cylinder, buffer tank and all installation. The heating system and connections to this would be on top.

 

 

[JohnMo]

12 minutes ago, AliG said:

there is more work involved in installing an ASHP versus a boiler

Why is there, my gas boiler required a buffer, my ASHP doesn't. A single thermostat, a 3 way valve and some pipe and insulation. Cylinder pretty much the same in both cases, if a system boiler.

 

My ASHP cost about £2000 installed, the gas boiler cost a similar amount

Edited June 22, 2023 by JohnMo
Miss details

[AliG]

Just a little bit more, assuming that you need pipework from the ASHP outside into the house. Certainly nothing like justifying some of the £10k+ estimates I see for just an ASHP installed.

 

Of course there is a considerable saving in not installing gas to the house.

[Thorfun]

ASHP is a buzzword at the moment. So unscrupulous people are cashing in on the fomo. 
 

but there are good installers out there who won’t take the piss. 

[saveasteading]

29 minutes ago, AliG said:

l don't think anyone on the forum has a GSHP,

Not me either but i have helped some unfortunate tenants of a housing association to get them replaced by ashp.

 

So I am predisposed against them in most cases.. I know they are sold where they are no good.

Ok into hot rock like norway, or a warmly flowing aquifer.

 

The last time i went to a seminar, the industry was admitting that the ground has to be heated in the summer.

[Alan Ambrose]

>>> actually with respect the aim isn't to use less energy, it's to generate as much as we use (in £)

 

OK I see - so the grid power you use in the winter, you're planning to pay for with exported power in the summer. Makes a certain amount of sense. So you'll be 'net zero' electricity over the year.

 

Probably not the most economic set-up though ... you can take a stab at calculating the benefit of each incremental panel - either as actual calculations/estimates or just in concept. The first panels get great payback, the last panels get fairly marginal payback.

 

Panels are not that expensive though - so if that's want you want, there's no reason why not.