Tesla Powerwall Savings Calculator

Will a home battery pay for itself? Model time-of-use shifting, storing your own solar instead of exporting it cheaply, battery wear and rising ratesprices — then weigh the result against what backup power is worth to you.

System & cost
13.5 kWh usable · 11.5 kW continuous
Installed prices are for Powerwall 3 including fitting. Domestic batteries currently carry 0% VAT in Great Britain, so there's no VAT to add. There's no national grant for home batteries — enter any installer or local-scheme discount. Verify current terms. The 30% federal residential clean-energy credit ended for spending after Dec 31, 2025. Some states and utilities still offer battery rebates (California's SGIP, for example) — verify current terms.
Utility planElectricity tariff
Evenings are typically 30–40% of a household's daily use. On an overnight EV or battery tariff (cheap for about 6 hours), most daytime and evening use is at the peak rate — typically 70–85%. Don't count EV charging you already do overnight.
Solar
Under net-billing plans like California's NEM 3.0, exported solar often earns only a few cents per kWh.Smart Export Guarantee (SEG) rates run from about 4p to 15p per kWh depending on the tariff. About 9 kWh/day is a typical yearly average for a 4 kWp system.
Backup Power cuts are rare in most of the UK — typically less than one a year, lasting an hour or two.
Average draw with HVAC, cooking and everything else running.Average draw with cooking, the kettle, washing and everything else running (most UK homes heat with a boiler rather than electricity).
Assumptions
Savings, year 1
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Simple payback
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15-year net value
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Return (IRR)
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Cumulative cash flow

Net cost up front, then yearly savings (incl. backup value)

Where a day's value comes from

Backup on a full charge

Annual savings by year

Rates rise, capacity fades

What if you installed more or fewer?

Same home, rates and solar

Year by year

How this is calculated

Why doesn't solar make the battery pay back faster?
In the UK, overnight tariffs are so cheap (around 7–8p) that filling the battery from the grid is often better than storing your own solar, especially when a good SEG rate pays you 12–15p for exports. Solar still saves you money on its own; it just doesn't add much to the battery's case. On a low export rate (4–5p) stored solar becomes worth more — try it in the export field.
What about export tariffs that pay more in the evening?
Some suppliers offer tariffs that pay a high rate for exporting in the late afternoon and evening, which lets a battery earn money by selling stored energy. This calculator doesn't model battery export trading, so treat it as a conservative estimate if you plan to use one of those tariffs.
Why do the numbers look so different with and without solar?
Without solar, a battery earns only the gap between peak and off-peak prices, minus round-trip losses — often 10–20¢ per kWh. With solar on a low export credit, every stored kWh avoids buying power at the full retail price while giving up only a few cents of export credit. That gap is what makes batteries pencil out under net-billing plans.
Does this model daily cycling realistically?
It assumes the battery runs one full shifting cycle every day of the year. Cloudy weeks, battery reserve settings (many people keep 20–30% for backup) and plans with weekday-only peaks reduce real savings — lower the solar production or peak share to be conservative.
How should I value backup?
Think about what a power cut costs you: freezer contents, a day's lost work at home, medical equipment, a boiler that needs electricity to run. UK power cuts are infrequent, so for most homes backup adds only a little to the case — set outages to zero to see the pure tariff savings.
How should I value backup?
Think of what an outage costs you: spoiled food, a hotel night, lost work, medical devices, a sump pump. If you'd otherwise buy a standby generator, its installed cost is a useful comparison for the backup-only value of a battery.