Beelbeebub

If you used 31kwh yesterday (20th) that's pretty good. The 20th was pretty cold (dunno where you are but it was sub zero across the Midlands). If we assume your house uses something like 6kwh a day for non heating stuff like fridges and lights, then you used 24kwh in a day, basically 1kw continuous heating load. If we assume a cop of about 2.5, that's a heat demand of around 2.5kwh. Not unreasonable for a house kept at 22-24C during a subs zero cold snap. If I read your graphs correctly on the 20th you used 31kwh and paid £6.52. That's 21p per kwh. Coukd you post some pairs of graphs for the kwh in a given day, then the £for the same day. I suspect you are losing out by using some energy during the agile peak period. You could save a fair bit by making sure your hot water heating doesn't occur then and also maybe turning your heating off for those hours. Agile is best used with some sort of automatic system that adapts your use to the variable daily price curves. You migbt get a better price using Cosy.

Just for perspective our gas heating has shot up during this cold snap (shock!) We were sub 50kwh a day then the cold snap hit and we are more like 100kwh plus - and that's with a log burner going to take the edge off things.

Could the Op post the kwh usage graph from. Octopus. Thry say they are on agile which has a massive spike between 4-7pm (eg today 21st Nov it's neatly 50p/kwh) Agile is for people with a battery (mostly) For a HP you need a Cosy tariff.

Yes inverters have efficiencies, depending on the operating range it can range from 90-97%. Even if you take the absolute worst cases (90% in, 90%out and 95% battery) you're still over 75% round trip. But at more general levels my system can beat 90% round trip efficencies. I think the drop in price of hybrid systems has made them a "no-brainer" for the majority of installations - not least because even without PV you can save by time shifting your demand. Obviously there will be edge cases where it doesn't make sense - but the majority of those will be the result of tarrifs that may not hang around. If the ratio of import/export prices widens or the price of imported electricity rises batteries will make even more sense.

i am assuming he is talking about round trip efficiency ie how many kwh it takes to charge a battery so that you can discharge 1kwh. For lead acid batteries it can be 60% round trip ie you only get 6kwh out after putting 10kwh in. LFP batteries are very good in these circumstances with efficiencies in the 95% plus range. This is independent* of the nominal and usable batter capacities * nearly - i believe you get worse round trip efficiencies if you use all of the usable capacity vs a smaller subsection (eg 20-80%) but it's not a huge effect and i believe most data sheets assume a higher depth of discharge 10-100%

The grid is already sized and set up for all the houses on it. If you have enough PV and battery that you never export or import you essentially dissappear from the grid in the same way as if you disconnected. Yes your neighbours can't use your solar but there is now more headroom in the infrastructure for increaced demands from them. Your contribution to the local grid will not be as stable, when thry are charging an EV and your panels are cutting in and out because of clouds the grid is having to take up the slack. They may not even be charging at that time and your lower might be effectively powering a factory across town - which requires the grid to carry the current. If I installed a 2Mw solar farm, more than enough to run all the houses in thr area, the grid would still prob be overloaded because it couldn't carry the current from me out. Imagine a block of flats with water pipes, a big water main splitting into smaller pipes for each floor and then smaller ones for each flat before even smaller ones for individual taps. If I had a magic water fountain, that produced free water from nothing, I still couldn't provide water for the entire block just by plugging the fountain into my kitchen tap. On the other hand, if I (and some other tenants) used our magic water fountains to satisfy all our individual water needs, the remaining tenants without magic water fountains would have better flow and pressure.

You're right, 30p saving by using a battery. Given the sun doesn't sine 24h a day at some point we cannot use the panels directly. Our options are use the grid or use a battery. The viability of using the grid as your battery (ie exporting when you have excess and then importing when you have shortfall ) is critically dependant on the price ratio between import and export. Without a battery the best you are likely to get is 15p all day, and the best import you can get is 25p because you can't time shift without a battery. This is equivalent to a storage efficiency of 60% and is unlikely to stay as good. The SEG rate is a better long term metric which is around 5p IIRC (could be more but it's nowhere near 15p). This drops the efficency really low. For a battery, your estimate of 6 to 7kwh in to get 5kwh out implies a round trip efficency of 70-80% which I think is too low it's more likely 80-90% or better. To be fair panels are so cheap now that *if you could get a really high export limit*, like 10kw or something you could have a huge array, big enough to export so much elecreicty in the day and summer months that you could build up enough credit to import "off sun" and still end up ahead. But you would need the space for the panel and the high export limit.

But your import rate is likely to be higher - at least outside of night rate. So exporting at 15p and importing at 25p makes the financial efficency 60%. Say you produce 10kwh during day and over the same 24h use 10kwh During the generation period you use 5kwh and export 5kwh (earning 75p) Then after suunset you use 5kwh, buying it all from the grid at a cost of 125p Net loss of 50p If you stored the 5kwh you would forego 75p but not spend 125p. And lose 20p in depreciation. Overall a difference of 5p - chicken feed really, though it does widen if your power requirement is higher or if the imoort/export gap widens (eg the 15p rate is cut or price of day rate elec Rises) *but* the battery option has zero impact on the grid - there is no import or export

But batteries allow you power your air con in the evening/night with solar from thr day atow cost (4p/kwh assuming 4.5k cost for 14kwh usable over 8k cycles) And crucially at zero extra load on the grid.

Batteries.

Sadly, people will always try to diddle the system, whether it be a."free" air conditioner or a £120 million contract for faulty PPE. Which is why subsidising the consumer's bills for a period would be alot better - yes people could and would try to fiddle things by bypassing meters etc, but thry do that anyway.

The whole grant situation, especially with the closed shop MCS, invites grant harvesting. I still maintain using the subsidy to subsidise the heating costs of the HP system to guarentee a HP is always cheaper than gas would be a better way forward

True enough. That's a win anyway. Switching the country to Heatpumps that can perform better than 2.5 over the winter season reduces our gas consumption even if we have to build some more gas plants.

But why would you buy a 2nd hand unit when you could get a new one for free via the grant? And, sure a householder could do that but then they would have used their grant and wouldn't have any system. Removing an A2A unit intact is nearly as big a job as installing one in the first place.

With the prevalence of smart meters now, it will be alot easier to see which system is cheaper to run. I can imagine householders running with thr A2A, the maybe switching to gas every now and again and looking at the costs at the end of the day and switching back to A2A (or not). Over time they would gain confidence and experience as to which days are worth switching to gas. As I said, hopefully at some point the gas boiler woikd pack up and they'd decide it wasn't worth fixing.

It does seem, in hindsight, that a better approach would have been to allow the old system to be left in place and provide grants for fitting a HP as a hybrid/piggyback system much as JohnMo did. Then for (say) £750 you could get a HP fitted to the side of your house and a box inside to connect it to the existing system. If that box controlled the HP and boiler (the boiler often being a simple on/off switch) it could run each system depending on outside air temp. Over time thr owner would see the HP system kept them warm and was cheaper most of the time. Maybe upgrading the odd radiator every now and again would improve things until one day the boiler packed up and was never replaced

This seems like a good path because it removes the twin doubts about HPs - that it might not be able to keep you warm and that it might cost more than gas over the year. Having the original system in place is very much the best of both worlds. The user k ow that - at worst - they will have their original system performance and cost. The only disadvantage with this approach with A2W is that it has to share the emitter with the orginal system and if they are undersized the HP won't perform. A2A does sidestep this issue by having its own emitters, but has the downside of not providing DHW. Both approaches have their benefits but neither are brig supported under the current system which mandates removing the old system - making for a much bigger leap of faith.

I would say the leveling off of the renewables after 2030 and slow rise of Gas & imports thereafter may be an artifact of the "horizon" for renewables onky extending out that far and then making the conservative assumption that nothing is added after that therefore the forecast demand increace will be met by thr default of gas and imports. I suspect that leveling off exists in earlier versions and will exist in future ones. I can't see a physical reason why that growth would plateau so abruptly. Slow maybe but just suddenly stop?

I was wondering if installing A2A alongside the existing gas heating and DHW is a good step to moving off gas. The systems are entirely separate. The homeowner can always drop back to gas if they struggle to stay warm or it's costing too much and thr DHW is already taken care of - this is especially true for houses with combi. Then, at a later date the DHW can be replaced with a heat pump cylinder led to the existing outside unit or off peak immersion or whatever. Plus cooling in the summer will be useful.

A quick glance at gridwatch for this summer says we're a long way off not needing gas in the summer so best dialling down AC for a good while. NESO say for 2024 we had 37% green generation- 30% wind, 5% solar and 2% hydro. Looking at the hour to hour figures it seems like we quite often have around 2-3Gw of gas going even when the solar and wind are shooting up. The "margin" stays suspiciously constant so I suspect it's a spinning reserve issue.

There was a mhrv system that used a heat pump instead of the heat exchanger and could provide a kw or so of heating. Can't remember the name.

I wondered if having some sort of grant availible for A2A models as long as they have the minimum (maximum?) cooling temp firmware locked at (say) 26C. It should be a fairly simple software change - they already have different firmwares (or at least options) for things like language, units etc. The grant to make A2A really affordable doesn't need to be much maybe a cash back voucher for the customer or something. Though, to be honest just making the planning and red tape around A2A easier would help - if thry could get a refrigerant that didn't require an f-gas cert (and wasn't flammable or toxic) so thry could be diy'd it would be a massive step forward

There are some HP systems that are entirely self contained and just require a pair of 150mm inlet/outlet ducts. The unit itself isn't much bigger than a storage heater. They are more expensive than a 2kw panel heater but they do have much lower power requirements. They could be integrated with a ventilation system to provide fresh air at the same time. We really should be looking to removenresitive heating from all domestic space heating requirements.

I think the point of having open architecture is not that consumers have to (or even can) fiddle with things. It's to allow 3rd party integration - so don't like the controller that comes with your heatpump? Just use another one. No proprietary lock in via API/protocol. No having to reverse engineer comms protocols and registers - just look up the published and freely available document. And most of all, no legal bars (DRM, copyright etc) to 3rd party integration. Look at what is happening to some cars now - you can't fit a pattern or second hand part because the Ecu won't recognise them without an online activation. Imagine boiler or HP companies being able to require that any work be done by an "authorised" technician who has had to pay for all the software tools to activate the boiler /hp

My worry abiut all these intelligent optimisation services with a subscription is the often (always?) are "in the cloud". Which not only opens them up to outages and malicious actions but also puts the user at the mercy of subscription hikes (Buy our premium service to have set temps over 16C!) and plain old services shutting down. Nest thermostats are now crippled because Google decided it couldn't be arsed supporting them and they are only 10 years old. All that said, it's entirely possible to build a learning thermostat for a HP that is self contained and doesn't need any Internet connection. Which is waht we should be looking for.