Battery prices continue to fall so their time will come and with enhanced export rates from ‘Axle’ it’s becoming tempting. If you are installing solar now, you’ll have to choose between micro-inverters, a direct to AC inverter or a battery based inverter/charger. For what it’s worth I’d go for Enphase micro-inverters and leave batteries for later.

Why not batteries now? I suspect these sums will seal the debate.

Let’s work around a hypothetical 10kWh battery costing £3,000. We’re assuming no losses so everything will be worse than shown by this basic maths.

Solar storage

Base line – use the free battery provided by the grid; export for 12p and buy back for 25.5p Here the free battery effectively costs you 13.5p per kWh. Think of it as a rental charge for an infinitely large battery that holds your inputs for as long as you like. Zero capital cost to you though so £3,000 in pocket; stick that in premium bonds for 10 years instead? Whatever you do, your battery project has to compare as an improvement over this virtual battery that is ready to use whenever you like.

Solar + Battery; input free, profit £25.5p so 12p better than the free grid battery solution above. For our 10kWh battery that’s £1.20 saving per day, so payback takes just under 7 years of surplus energy every perfect sunny day. So, in reality, about 15 years then. Is that fair? What about storing free energy and getting 25.5p worth later for a 3 year payback. Nice idea but that assumes 10kWh every day of the year, even the 200 days of winter. You’d also be directly consuming some of your solar production (practically all of it in winter) so you can more than double this estimate. 7 years? Possible but unlikely. And, sorry to rub this in, there’s still the grid battery available to you so these optimistic sums are based on a fragile platform.

Other interesting ideas

Off-peak storage sold to grid –   7.5p in 12p out – profit of 4.5p. For 10kwh that’s 45p/day. To pay back£3,000 would take 18 years but then more really because the batteries would die before that.

Off-peak storage used next day –   7.5p in 25.5p worth out – a saving over the grid of 18p. For 10kwh that’s £1.80/day. To pay back £3,000 would take 5 years. After that, the next 5 years drops £3,000 into your pocket but then the batteries need changing so it’s gone again. Close! But not overly compelling.

A bit of solar and a bit of off-peak

There’s a seasonal split here for sure. In Summer there is little demand for electricity and in practice, even with no battery at all, bills are pretty much matched by exports. In winter there is significantly reduced solar so the story is all about off-peak storage and we’ve covered that above.

Unfortunately, we have failed yet again to make a good case for battery storage. If that leaves you with £3,000 burning a hole in your pocket just consider that the payback time on solar panels is hovering around 3 years so make sure you have enough of them. Your money will do much more for you here.

Here’s a rough example for PV evaluation. Scale up to suit.

£1,400 buys 4 panels with microinverters. (Includes £500 for fitting)

Total 2000Wp

This makes 1,600 kWhrs a year

That’s worth £400

Payback is 3.5 years.

So, there you go. That £3,000 would buy you a nice solar rig and not a battery in sight.

While thinking about off-peak energy remember this. A heat pump multiplies input energy by about 3 times so if it was run off-peak and water was used for storage there would be different type of energy store to radically reduce heating bills.

See Absolute ultimate heating system to see how that works along with some other ideas too.