How the calculation works
This is a calculator for tariff arbitrage: buying electricity when it is cheap and using it when it is expensive. It does not include solar panels.
Usable capacity. Most lithium iron phosphate (LiFePO4) systems are run to around 90% depth of discharge, so a 16 kWh battery gives about 14.4 kWh of usable energy each cycle.
Can it fill in the window? Your inverter's charge power times the length of the cheap window sets the most you can put in each night. If that is less than the usable capacity, some of the battery never gets used, and you paid for it anyway.
Losses. Energy is lost charging and discharging. Round-trip efficiency of 85–92% is typical for a hybrid inverter with LiFePO4. That means stored energy costs a bit more than the off-peak rate:
Daily saving. For each kWh the battery delivers in place of peak-rate electricity, you save the peak rate minus the effective cost. The calculator caps this at your daytime use, since you can't save on electricity you don't use.
Sizing a battery
Size the battery to your peak-time usage, not to the biggest battery you can afford. A useful order of thinking:
- Find your average daytime usage from your smart meter data or supplier app. Use winter figures if you have a heat pump.
- Pick a battery whose usable capacity roughly covers that.
- Choose an inverter with enough charge power to fill it in your cheap window, allowing for losses.
- Check your supply can take that charge current on top of the rest of the house.
The alert box under the results flags it when the battery can't fill in the window, when it's bigger than your usage, or when the charge current looks high for a single-phase supply.
Cost per kWh over the battery's life
The "cost per kWh over battery life" figure spreads the whole system cost over every kWh the battery will deliver during its rated cycle life. If that figure plus the effective off-peak cost is below your peak rate, the battery pays its way over its life. Rated cycle life is usually quoted to 70–80% remaining capacity, so the battery will still work after that point, just with less capacity.
Safety and installation
Connecting a battery and inverter to your home is notifiable electrical work in the UK and must meet the Wiring Regulations. Your network operator also needs to be told about, and sometimes needs to approve, grid-connected inverters. Use a qualified electrician for the connection and sign-off.
Frequently asked questions
Is a home battery worth it without solar panels?
It can be, if your tariff has a big gap between the cheap and peak rates and you use a lot of electricity at peak times. The wider the gap and the higher your daytime use, the faster the payback. Enter your figures above to see your case.
How big a battery do I need?
Roughly enough usable capacity to cover the electricity you use outside your cheap window on a typical day. Capacity beyond that only helps if you can export the surplus at a good rate.
What size inverter do I need to charge a battery overnight?
Divide the usable battery capacity by the length of your cheap window, then add about 5% for charging losses. For example, 43 kWh usable in a 5.5-hour window needs roughly 8 kW of charge power.
What is round-trip efficiency?
The share of energy you get back out of the battery compared with what went in. At 90%, every 10 kWh taken from the grid gives about 9 kWh to the house.
How long do LiFePO4 batteries last?
Manufacturers commonly rate them for several thousand cycles. At one cycle a day, 6,000 cycles is over 16 years. Check the warranty terms for your specific battery.