How much does a home battery actually save in the UK?
A home battery earns its keep in three distinct ways, and how much each one is worth depends far more on your tariff than on the battery itself. By the end you will be able to work out your own saving from your own data, rather than trusting a calculator.
The three ways a battery earns
Every pound a battery saves comes from one of three mechanisms. It helps to keep them apart, because a tariff can make one of them worth a lot and another worth nothing.
1. Using your own solar after dark
Without a battery, solar you don't use the moment it is made goes to the grid. You are paid the export rate for it, then buy energy back in the evening at the import rate. A battery stores the surplus and gives it back after sunset. Each kWh that takes that route saves you the difference between what you would have paid to import and what you would have been paid to export. On most tariffs the export rate is a fraction of the import rate, so the difference is most of the import price.
2. Shifting cheap import to expensive hours
On a tariff with a cheap window, the battery can fill from the grid overnight and run the house through the dearer hours. The saving per kWh is the gap between the two rates, less the energy lost in the round trip. On a flat tariff this mechanism is worth nothing at all, because there is no gap to exploit.
3. Timing your exports
On an export tariff that pays more at some times than others, the battery can hold midday solar back and sell it into the evening, when export rates are highest. The gain is the difference between the peak export rate and the midday one. On a flat export tariff this is worth nothing, and the battery is better spent on your own house. UK export tariffs explained covers the options.
Why the tariff matters more than the battery
Two identical Powerwalls on identical roofs can save very different amounts, purely because of the tariffs behind them. A battery on a flat import tariff with a flat export tariff has only the first mechanism available. Add a cheap overnight window and the second appears, and it works every day of the year, sun or no sun. Add a peak export rate and the third appears too.
That is why "how much does a battery save" has no single answer. The honest answer is a range, and where you land in it is mostly a tariff decision. The battery's size and power set a ceiling; the tariff decides how much of that ceiling you reach. Sections below show what that looks like in kWh.
A worked example: one bright day
Take a common set-up: a 4 kWp south-facing array, a 13.5 kWh Powerwall, and a home using 10 kWh a day, of which about 3 kWh falls during daylight and 7 kWh in the evening and overnight. On a bright day in May the array might make 20 kWh.
| Without a battery | With a battery | |
|---|---|---|
| Solar used directly | 3 kWh | 3 kWh |
| Solar stored for later | 0 | about 7.8 kWh in, 7 kWh out |
| Imported from the grid | 7 kWh | 0 |
| Exported to the grid | 17 kWh | about 9 kWh |
| Share of the home's use covered by solar | 30 % | 100 % |
| Share of generation used at home | 15 % | about 50 % |
The saving on that day is the 7 kWh you didn't import, valued at the import rate, minus the roughly 8 kWh of export you gave up to store it. If export pays about a third of the import rate, each shifted kWh saves roughly two-thirds of an imported kWh's price. Notice that the battery only ever needed 8 kWh of the 17 kWh surplus. On a summer day, a bigger battery would have saved nothing extra; the rest still had to be exported.
The same home on a grey winter day
Now the array makes 2 kWh, all of it used as it is made. The home still needs 8 kWh more. On a flat tariff, the battery has nothing to store and saves nothing. On a tariff with a cheap overnight window at roughly a third of the day rate, it charges about 9 kWh overnight, gives back 8 kWh through the day, and the day's electricity costs about 40 % of what it would have at the day rate. The same battery, the same weather, and the difference between nothing and a substantial saving is the tariff.
What eats into it
- Round-trip losses. Roughly a tenth of what goes into a battery does not come back out, lost in the inverter and the cells. Every mechanism above is discounted by it. Round-trip efficiency explains where it goes.
- The standing charge does not move. A battery can cut your unit consumption to near zero on a good day and your bill still carries the daily fixed charge. On a low-use home that fixed part can be a large share of the bill, and no battery touches it.
- Winter. December generation is typically a sixth to an eighth of June's. For months, mechanism one nearly disappears. If your tariff has no cheap window, so does most of your saving. A home battery in winter sets expectations.
- The backup reserve. Energy held back for a power cut is capacity you paid for but cannot cycle. A 20 % reserve removes 2.7 kWh from every day's arithmetic.
- Doing the wrong thing. Charging from the grid the night before a sunny day, or running the battery flat before the evening peak, gives back part of the gain. The saving is not automatic; it depends on the battery being steered well.
Measuring your own real saving
Online calculators guess your usage, your solar and your tariff, then multiply. You have the real figures. Most battery systems record, for every day, how much solar you made, how much you used, and how much you imported and exported; the Tesla app shows these on its energy graphs. From those four numbers and your tariff you can work out the day's saving:
- What the day actually cost. Import kWh multiplied by the rate in force when it was imported, plus the standing charge, minus export kWh multiplied by the export rate.
- What it would have cost with no solar and no battery. Your total home use, all of it imported at the rate in force at the time.
- The difference is what solar and battery together saved. To separate the battery's share, note how much energy passed through it; on a flat tariff that kWh figure times the gap between import and export rates is close enough.
Do this for a week in summer and a week in winter and you will have a better estimate than any calculator, because it is built on your roof, your habits and your tariff. It also shows which mechanism is doing the work, which tells you whether a different tariff would do more.
In SunHarvest
SunHarvest does that sum for every day automatically. Once your real tariff is entered, the Money page and the Today so far tiles on Home show Spent, Earned and Net from your own metered half-hours, and on Pro a Saved figure that compares each day with what the same usage would have cost with no solar and no battery. The tariff comparison replays up to a year of your own history against every Octopus tariff with the battery run the best way for each, which answers the question this article keeps coming back to: on your data, which tariff makes the battery earn most. You can see it on a made-up home at /app/demo.
General information, not financial or electrical advice. Tariff terms, prices, warranties and connection rules change and differ by supplier, region and installation; check the current documents for yours. SunHarvest is not affiliated with Tesla or Octopus Energy.