Which Solar PV direction to go

juneberry

Hello! I'm getting a 3.7kw solar system installed, with a 3.3kw SolaX inverter, and I've asked the installers about adding battery storage. They've quoted £3k price difference to add a 5.12kw battery and upgrade to a hybrid inverter. I'm wondering whether to install this.

The current situation is:

Property: The property is only 50 square metres, one bedroom, 400mm solid stone walls, and I'm renovating the whole thing. There's no gas.

Hot Water: Currently there's an immersion heater in the tiny bedroom, which I'd like to remove to save space (which is also the reason I'm not keen on an ashp) and add an instantaneous water heater.

Insulation: The EPC is very poor and insulation will be my first priority, but given the tiny dimensions and property age I'm thinking eg wood fibre insulation rather than battening thick kingspan etc

Heating: I'd like to add electric ufh, expensive to run but keep it on eg 16°C, then a log burner and high retention storage heaters or ir heaters for the rest of the heating.

I'm thinking my options are:
1) Add the battery now so that it's a DC coupled battery and hybrid inverter, as that wouldn't be something you'd retrofit. That said, a  5kw battery seems like a drop in the ocean for all-electric energy usage in the winter

2) No battery and see how it goes, with the daytime solar production contributing to the ufh when overall consumption is lower. Again, not a huge contribution, but better than feeding back to the grid at a terrible tariff. I could then add an AC coupled battery at a later date if required.

With AC coupled Vs DC coupled batteries, are there any big benefits to one over the other? AC coupled are less efficient but only by a few percent. I've seen stuff about differences in being able to charge from the grid, but I can't imagine that's right. There's also a question of adding to a system if manufacturers change systems.

Any thoughts very much welcome - my head's spinning with usage and production estimates!

Thank you!

wrf12345

I know it is small but Celotex insulation all round internally if you want to save serious heat loss, assuming you can't do it externally and weatherboard over then maybe get away with a single air-to-air air-conditioner type heat pump (under two grand fitted with good make or £600 DIY with eBay non-brand), and downsizing all non-south facing windows. The solar will help with the running cost but not so much in winter but if you tie that into one of the low-rate off-peak tariffs (Octopus have more than most) and decent insulation it should work out cheaper than gas - air-air is pretty instant heating and suited to turning on and off to match the low price electric rates (say half price and then a COP of 3 from the heat pump in worst case). The battery will even out solar for use during the whole day and can also be charged at the cheap rate and either used at peak electric rates or exported back into the grid but whether it justifies the cost is another matter.

juneberry

Thank you very much for your reply - that's really useful.
Air to air heat pumps sound like a great idea, saving the internal space through not needing a cylinder, but still gaining the COP of a heat pump. The only other consideration which I'd need to look into is the noise level of the outside unit, as it's a very quiet village!
A battery will mean it can run on low rate electricity, as you mention(i suppose the only other way would be timing its usage to off peak times, which would be more limiting), but I might wait and see how the system runs then add an AC coupled battery later. This might also avoid any issues with a hybrid inverter no longer being compatible with DC batteries in the future.
You're right about celotex. Perhaps I will have to add that in the ceiling and then decide room by room for the walls. 
Thank you again for your advice.

Ectophile

Bear in mind that solar works really well in the pring and summer, when the days are long and the sun is high in the sky.  It's a lot less effective in the winter as days are short and the sun is low.

So if you get a battery, you are likely to have as much free electricity as you want in the spring and summer.  And much less in the winter when you need it most for heating.