Costing batteries – RoI

I have one, However they are very easy to build yourself - it would probably take the same amount of time.

Essentially you just need a few simple figures

• Total cost of the system (£) - what you want to return on
• Cost of your power (£/kw)  - your figure you are measuring value against
• Discharged power from the battery (kw) - the actual benefit you are seeing

From those three you can make a simple excel in a few minutes. If you wanted to be extra fancy you could also have a price to charge from the grid and track that (if applicable)

For example if you spent £6,000 on a battery system with 10kw and your power cost 0.2/kw you can store £2 per day, but with depth of discharge this will be 80% of that, so £1.60 is useable. If you used that every day, you are looking at a 10 year payback - £6000 / (£1.6 x 365 days) = 10.27 years

The best thing to do is look at your cost per Wh of storage. In the example your cost is £0.6/Wh or of £600/kWh. For me, this is WAY too much, you wont see the benefit. Unless prices stay crazy. If you are paying £0.4/kw the payback is halved to 5 years.

I know I can build a 15kW system for around £4,300 or £0.28/Wh, or I can build a 30kW system for £6,400 (£0.21/Wh). At these figures the cost and payback become a few years; 4.7 years for the 15kWh system and 2.4 years for the 30kWh system (using £0.2/kW as a price).

As you can see HOW you build your system is critical. Also you cant expect to produce all that with solar over the winter (when you need it). Arguably you will need some kind of feed tariff which will have offset costs to consider. You might want to double those paybacks if you can't have a charging tariff as it might be 6 months of the year its useful.

As an aside this is how I would build my system

Goodwe - GW5000S-BP - 5kW battery charger - HERE - £1,679 - Charging / Discharging output is 5kw (100amps @ 52V); the highest you'll find, so perfect for ASHP

Mason DIY battery Box - HERE - £500

302ah cells x 16 - HERE - £2,144

Total cost of 15.4kwh (12.3kwh @ 80% DoD) system = £4,323 + install (£0.28/Wh)

Total cost of 31kwh (24.7kwh @ 80% DoD) system = £6,967 + install (£0.23/Wh); for this you just buy double the cells and battery box, but you only need one charger. You could also probably get the price down more for buying more cells

Charge time @ 5kw;

• 15.4kwh system = 2.4hrs
• 31kwh system = 4.9 hrs

@batalto It looks like the Goodwe - GW5000S-BP is pretty rare (and correspondingly expensive) nowadays; retailers quoting a price are few and far between (just had a conversation with Waxman Battery Storage who said they sold their last one a fortnight ago, and aren't planning on re-stocking).

I was looking at going down the Victron route (Victron Multiplus-II 48/5000/70-50 for £1460, plus accessories like a DIY VenusOS computer and current sensor for anti-islanding) since there is wider-spread BMS support (e.g. here to bridge RS485 to Victron D-Bus) as well as coming from a trusted (at least in boating circles) company. Victron ratings are complicated so this model supports 4kW continuous at 25 degrees C air temperature, where energy efficiency is down to 86.7%. However I guess the same limitations apply to all other inverters even if they are not advertised.