Defaults last checked: October 29, 2026. Results are estimates.
A home battery earns money in one of two ways. It stores solar power you would otherwise export cheaply and lets you use it in the evening, or it charges when electricity is cheap and covers you when it’s expensive. Either way, the saving per kWh is the gap between two prices, minus what’s lost in the battery. This calculator works that out.
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How to use it
- Choose your country and a mode. “Store surplus solar” compares your import price with the export payment you give up. “Time-of-use” compares your peak rate with your cheap rate.
- Enter the usable capacity. The default of 13.5 kWh is Tesla’s published figure for the Powerwall 3, used only as an example size.
- Check the round-trip efficiency. We use the 89% solar-to-battery-to-home figure from the Powerwall 3 datasheet. Other batteries publish their own.
- Set the number of full cycles per year. We start at 365, which is a best case: one full charge and discharge every day. In winter a solar battery often won’t fill, so a lower number is usually more realistic.
- Enter your prices and the installed price. For Australia we prefill Victoria’s default offer time-of-use rates. Elsewhere, enter the rates from your own tariff.
The formula
- Saving per cycle = usable kWh × round-trip efficiency × price you avoid − usable kWh × price of the energy you put in
- For solar: price you avoid = import price, energy put in = export rate you give up
- For time-of-use: price you avoid = peak rate, energy put in = cheap rate
- Yearly saving = saving per cycle × cycles per year
- Simple payback = (installed price − grants) ÷ yearly saving
Worked example 1: time-of-use in Melbourne
Victoria’s Essential Services Commission sets the 2026–27 default offer for CitiPower customers at 38.31 cents per kWh in the 4–9 pm peak and 16.59 cents in the 11 am–4 pm “solar soak” window (both including GST).
A 13.5 kWh battery at 89% efficiency, charged in the solar soak and used at peak:
- Per cycle: 13.5 × 0.89 × $0.3831 − 13.5 × $0.1659 = $2.36
- Over 365 cycles: about $863 a year
Divide your installed price, after any discount from the federal Cheaper Home Batteries Program, by that figure to get the payback.
Worked example 2: storing solar in the UK
With Ofgem’s October–December 2026 cap rate of 26.32p and an export rate of, say, 15p:
- Per cycle: 13.5 × 0.89 × £0.2632 − 13.5 × £0.15 = £1.14
- At 365 cycles: £415 a year. At a more realistic 250 full cycles: £284 a year
A wide gap between your import and export rates is what makes a solar battery pay. If your export rate is close to your import rate, the battery saves very little. Our guide to the best home batteries in the UK explains why the 0% VAT rate on batteries until March 2027 also matters.
What the result doesn’t tell you
- Battery wear. Usable capacity falls over the years. Check the warranty’s throughput (MWh) as well as its years.
- Tariff changes. Time-of-use rates and export payments are changed by suppliers far more often than standard rates.
- Backup value. If the battery keeps your lights on in an outage, that’s worth something, but it isn’t in the payback.
- Combined strategies. Some households charge from solar in summer and cheap-rate grid power in winter. Run both modes and use a blend.
FAQ
Is a home battery worth it without solar?
It can be, if your tariff has a large gap between peak and cheap rates. Use the time-of-use mode to check.
How many cycles per year should I enter?
365 is the ceiling for one cycle a day. A battery charged on a nightly cheap rate can get close to it. A solar-only battery usually won’t, because winter days rarely fill it. Try a lower number, such as 250, to see how sensitive your payback is, then check your battery app after a year and update the figure.
Why is round-trip efficiency important?
Every kWh you put in returns a bit less. At 89%, 10 kWh in gives about 8.9 kWh out. That loss is paid for at the price of the energy you charge with.
Does a bigger battery always save more?
Only if you can fill and use it most days. A battery that only half-fills effectively does fewer full cycles, so the saving per pound, dollar or euro spent goes down.
Related
- Solar panel payback calculator
- Best home batteries for solar in the UK
- Australia’s Cheaper Home Batteries Program explained
- Is solar still worth it without the tax credit?