Big battery for leccy and hot water. A2A for space heating.

[AGT]

I wouldn’t use a battery power to power the HW;
I would use the off peak electricity to charge it direct.

[Adokforme]

Hi Gareth, I'm no expert, but like your thinking, in fact, apart from inclusion of a thermal store, did precisely what you are planning and it's worked fine for us this last five years or so. We live in a two bed bungalow circa 100 sq mtrs, with separate 2kW A2A units in lounge/diner and lobby(I understood at the time that separates were more efficient than a single split system!) They cost £1600 each to have installed and once settled into their cycle draw between 300 & 400W's averaging out around the upper mark overall.
Lounge/diner (25 sq mtrs) unit covers this admirably with excess bleeding into entrance hall via door ajar, naturally kept at a lower temp. That in the Lobby keeps it and bathroom cosy with excess into bedrooms as required.
I had great pleasure in removing all the unsightly radiators and related pipework with a nice little payday when taking them to scrap metal dealer afterwards.
We are all electric with two EV's, total consumption for 2023 incl 9k EV miles was 11.6 kWh's.
Hope this helps.
Oh, we have 34 kWh's battery storage which covers our daily needs even in the coldest darkest days.

[Moxi]

Gareth,

I think the 12kW DHW boiler is the thing thats skewing your proposal - its a massive instant demand on the batteries and inverter, if you ditched that for alternatives then you are probably closer to your preferred solution.

For the sinks could you have small under sink immersion units ? That then just leaves bathing either shower or bath, the bath could be inline and if you don't mind a slow drawing bath, the flow can be throttled back to allow a smaller inline heater, electric showers are many and varied and its a personal preference but again should be well under the 12kW figure mentioned earlier.

I still think for DHW a very small ASHP and a 50 to 90 litre system would work, some DHW ASHP have a small cylinder built in to them and you could consider a large HWC located outside in a lean to or outbuilding with insulation (this is what I am looking in to as I also have no space in the cottage for a HWC and my batteries already live outside in a "shed". You should get a cop of at least 2 and more sensibly 3 for a suitable combined DHW system linked to a small HWC so your 6kWh per day could become at least 3kWh or even less than 2kWh per day which considering the cheap rate price you hope to import electric at should make the system comparable with gas when the ASHP has a cop of 2 and cheaper if its above a cop of 3 (you need to check your exact prices to confirm that). If you add in the heating then you will require a moderately larger ASHP BUT if its sized correctly and your flow temp is set to the right level with seasonal control then you could easily get up to 4 or more cop.

Moxi

[AE-NMidlands]

Using resistive heating means a cop of 1 so your battery is bigger and more costly but doesn’t give you access to high value electricity just expensive hot water.

Well, off peak electricity isn't that much more expensive than gas, and a gas combi boiler is less efficient at DHW than space heating. Needs more analysis on round trip and electric boiler efficiencies, but don't think the DHW costs should be -much- more expensive than gas, and carbon intensity (we're in Scotland) should be lower.

Really? My gas is 6.01 p/kWh, Flux electricity import is 17.74, so even with gas "inefficiencies" (much of which leak into the house anyway) I shan't be doing resisitive electric water heating anytime soon. I think the general impression is that Go tariffs are too good to be true, and can't last forever...

[Stinsy]

Apologies to start a thread which is a variation of a prior post, but would greatly appreciate if you guys could sense check. I'm wondering about the following:

- Install 60kWh home battery (as large as I realistically can find space for)
- Get 12kW inverter
- Use this to provide for all of my electricity usage via an off peak tarrif (thereby reducing my electricity bill and carbon emissions)
- Also use it to provide my hot water...
- Install (probably multi split) A2A heat pump for heating, probably just doing 'main' rooms (living room, kitchen and bedrooms).

Would it be doable to provide my DHW off a battery like this? I've checked my annual usage, and a 60kWh battery would easily cover my electricity and hot water needs.

Wondering if I could just replace my gas combi with an electric boiler which would just do hot water (and maaaaaybe the 'non main' room radiators - bathrooms etc - as ASHP would do rest).

The rationale is, if I want to move to a conventional air to water heat pump, I need to find space for an expensive hot water tank. Not sure if I could find space for a battery AND hot water cylinder/thermal store, so wondering if can do away with the latter.

Sorry if the above is a bit unclear. Partially just thinking 'out loud' as it were.

EDIT: Actually, checked my usage again, and the above battery would cover ALL of my energy needs, even in winter, if the ASHP manages a COP of 3+, which it should do easily.

Basically all good except that you’ll need a DHW tank heated on the off peak rather than trying to use batteries and instant DHW for that.

I’d also like to see your calculations re battery size. I have a 12kWh battery and make that work for a 5-bed house with 2x adults, 3x kids, and a dog. I use a genuinely gargantuan amount of electric but a bit of effort gets most of it into the cheap period.

Here is my latest bill:




A few other things to add:

Consider storage heaters to augment your A2A. An £800 SH can store c. 23kWh of energy. It’d cost £5750 in batteries to store that much (@ £250/kWh) even with the CoP of an A2A this is £1500 worth of batteries against an £800 SH for the same heat output.

You don’t need more than two A2A units. 1-up 1-down. I’d go hallway upstairs and living room downstairs, but depends on your layout/lifestyle. My setup is far from optimal, the single A2A unit on my upstairs landing does a great job, not quite on its own I have resistive heaters that are used sparingly to augment the A2A. I’d also suggest going small on the A2A units to avoid cycling (maybe 2x 2.5kW).

[GarethC]

Gareth,

I think the 12kW DHW boiler is the thing thats skewing your proposal - its a massive instant demand on the batteries and inverter, if you ditched that for alternatives then you are probably closer to your preferred solution.

For the sinks could you have small under sink immersion units ? That then just leaves bathing either shower or bath, the bath could be inline and if you don't mind a slow drawing bath, the flow can be throttled back to allow a smaller inline heater, electric showers are many and varied and its a personal preference but again should be well under the 12kW figure mentioned earlier.

Thanks all for the thoughts/points. Consensus that running combi electric purely off battery a big ask. Haven't completely discounted yet as EVs use LFP batteries with large current draws, so they might be OK for it, and 20kW hybrid inverters are available and not that much more expensive than smaller ones?

Do think worthwhile trying to avoid needing space and plumbing (and therefore expense) for a large tank perhaps worthwhile if possible. The cost of doing so vs. the benefit just seems large. A 7kW (so lower demand on inverter) electric boiler with an inbuilt small (say 50 litre) might also be a solution? Have found https://www.daxom.co.uk/product/electri ... -cylinder/, but 12kW which wouldn't solve the excess demand problem.

@Stinsy, good point re: battery size. Might get away with smaller.

Re: ToU tarrifs. Octopus EV is only 7.5p kWh. I know it's an outlier, and far and away better than anything else available currently, but with increased renewables, surely ToU is expected to become more, not less, common?

[AE-NMidlands]

Re: ToU tarrifs. Octopus EV is only 7.5p kWh. I know it's an outlier, and far and away better than anything else available currently, but with increased renewables, surely ToU is expected to become more, not less, common?

I agree about likely increasing availability of TOU tariffs, but I can't understand how Octopus can give power away so cheaply to EV people, so I wouldn't expact that to go on for long. In fact, as it is effectively subsidised by us non-EV people I don't mind how soon it ends!

[Stinsy]

Re: ToU tarrifs. Octopus EV is only 7.5p kWh. I know it's an outlier, and far and away better than anything else available currently, but with increased renewables, surely ToU is expected to become more, not less, common?

Octopus IOG is by far the best ToU tariff available. In fact, it is silly good, you can "game" extra cheap slots anytime you need them. There are a few hoops to jump through and conditions to comply with, but doing whatever is needed is very worthwhile. How long can the silliness last is anyone's guess.

On the other hand, Octopus is no longer the outlier they once were. There are other decent ToU tariffs now available from other suppliers including Scottish Power at 7.45p/kWh off-peak (wonder how they came up with that price?).

[sharpener]

I have looked at all this in some detail in the past. My sig shows what I have at present, working well, am planning to add a 12kW ASHP this year.

Capital cost of fitting batteries is much higher than investing in a saving of 3x by using an ASHP for HW, which will reduce yr running costs by 3x as well.

Space required for such a large battery is much more than that taken up by a HW cylinder too.

12kW inverter also expensive and yr DNO may not like such a big unit on yr single phase supply.

Another possibility which might suit is the kind of dedicated ASHP which extracts damp air from bathroom and uses it to produce DHW all in the one unit, have you looked at these? Would combine well with A2A for space heating.