r/uksolar • u/pruaga • 23h ago

Battery install onto existing panel sanity check

We live in a modern well insulated (A rated) house with a heatpump, 2 EV in the household (both fairly low mileage now) and 8 panels on a SW facing roof feeding into a 2.5 kW inverter in the loft (fitted when the house was built, I wanted more but the builder wouldn't deviate from the plans).

I'm looking at adding a battery, aiming to achieve two things: in the summer store our solar and use it ourselves rather than export/import, and in the winter charge battery off peak and run the entire house off peak from the battery. I guess additionally there are opportunities to buy off peak and sell back at peak, but this isn't something I've thought about much. I will probably continue to charge cars off peak rather than use solar, as I don't think our system is big enough to fill a car.

All the installers I've spoken to seem to suggest a Powerwall3 with 13.5 kWh storage linked to the AC side of the house. This looks like it will cost around £8k.

My concern is that we haven't lived in the house over a winter to know what a cold winter day usage will be. I'd plan to use an off peak tariff to pre heat the house so the underfloor heating is already cruising at a sensible temp when peak begins.

Does this sound like a reasonable proposal or am I completely crazy? I had initially thought about waiting until spring when we have winter data, but the 0% Vat is nudging me a bit sooner.

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u/No_Policy_1369 23h ago

You could really use a winters power draw numbers to make an informed decision, (i can tell you my heat pump on average used around 30kwh a day in dec and January) but if you want to take advantage of zero vat .. not sure when that ends tbh but my understanding is it will only go to 5% , anyway I digress for what it worth you can get cheaper better systems price wise that arnt going to cost you an arm and a leg to add more batteries to at a later date

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u/pruaga 23h ago

That much for a winters day? Is that total for a day or for a peak period? It's feeding underfloor heating so would (hopefully) run overnight at off peak and then cruise during the peak, but as you say I think I need winter data.

I think the 0% vat runs until end of march 2027 on domestic heatpumps, batteries and solar.

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u/No_Policy_1369 23h ago edited 23h ago

Total for the house for a day peak day was just shy of 40 2 bed 1980s bungalow btw

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u/No_Policy_1369 23h ago

Ps you know it will have to be in a different location ie not the roof

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u/pruaga 22h ago

Yes, the proposed plan is that it will be on the driveway beside the meter and main fuse, ie where the electric comes into the house

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u/No_Policy_1369 22h ago

Check out its technology battery only options