Vicaya GlobalCalculator · Solar · battery sizing
What size battery, and does it ever pay back?
Every hour of a year, simulated: generation from the physics engine, a household drawing power, a battery charging on surplus and discharging on demand, with real round-trip losses and a state of charge that can run out.
Read this before the numbers.
The generation on this page is modelled and measured against a reference tool. The household load is not. A plausible domestic shape with a morning and an evening peak, repeated identically every day of the year. It is not measured, not sourced, and not seasonal.
A battery's value depends almost entirely on when you use electricity, so that gap matters more here than on any other page in this section. Treat the shape of these results as informative and the exact payback year as illustrative. If you have half-hourly data from your own meter, it would change these figures more than any refinement to the solar model could.
What do kWp and kWh mean?
- kWh — kilowatt-hour
- A unit of energy, and the thing your electricity bill charges you for. A 1,000-watt heater running for one hour uses one kWh. A typical home uses somewhere between 2,000 and 10,000 kWh a year depending on where it is and how it heats.
- kWp — kilowatt-peak
- A unit of capacity — how big the solar array is, not how much it makes. It is the output the panels would produce under standard test conditions: bright, cold and perfectly aimed. Real roofs rarely see those conditions, which is why a 4 kWp array does not generate 4 kW for most of the day. One modern panel is roughly 0.4 kWp, so 4 kWp is about ten panels.
- Putting them together
- kWp is the size of the system; kWh is what it produces over time. The ratio between them — kWh generated per kWp installed, per year — is the honest way to compare locations, because it strips out how big the system happens to be. It runs from roughly 700 in cloudy high latitudes to over 1,800 in sunny deserts.
At these prices and rates, no battery size on this chart pays for itself within any plausible lifetime.
That is the number that matters, and it is not a long payback so much as a payback arriving after the hardware is likely to have been replaced. On a flat tariff, at these prices, a battery bought purely as an investment does not currently make its money back. Bought for backup power during outages, or paired with a time-of-use tariff where the import-export spread is far wider, the arithmetic is different — and this page does not model either of those.
Quadrupling the battery from 5 kWh to 20 kWh multiplies the annual saving by only 1.2×.
This is the finding that survives every uncertainty in the load profile, because it comes from the shape of a solar day rather than from the details of a household. There is only so much surplus to store: once a battery is big enough to absorb a typical sunny afternoon, additional capacity spends most of the year empty. Beyond that point you are paying for storage that never gets used.
How much does this depend on what was assumed?
The figures above are one run through one set of assumptions. This is the same calculation run 200 times with the five inputs this page knows least about drawn from their own distributions — how much the household actually uses, what the battery costs, how well it holds charge, how fast it fades, and the gap between import and export prices.
Not one of the 200 modelled scenarios leaves this battery ahead. The best of them still ends £783 down over 25 years. That matters more than the base case does: a single negative figure invites the thought that the assumptions were harsh, and this says the conclusion survives the assumptions being generous.
The size answer is far steadier than the money answer. Across the same scenarios the best capacity stays between 4 and 6 kWh. That is the useful thing to take from this page: how big a battery should be is a question the model answers with some confidence, and whether to buy one at all is not.
In 100% of modelled scenarios the battery never repays its cost within 25 years. Those runs are counted rather than dropped; discarding them would remove the worst outcomes and leave an interval that looked tighter and kinder than the model produced.
| Measure | Base case | Favourable end | Middle | Unfavourable end |
|---|---|---|---|---|
| Discounted payback | never | — | — | — |
| Net present value | −£1,722 | −£783 | −£1,757 | −£2,864 |
| Best size | 5 kWh | 4 kWh | 5 kWh | 6 kWh |
What this range is, and what it is not
This is a sensitivity interval, not a forecast. It says that if the assumed distributions are right, 80% of sampled outcomes land between those figures. It does not say those distributions match reality, because they have never been checked against observed installations. That is why nothing here calls any outcome probable, expected or typical.
- The load profile is scaled, not reshaped — and shape is what a battery trades on. Every scenario above uses the same placeholder shape of day at a different size. A household that cooks at seven rather than at noon changes this answer in a way no amount of scaling can explore, and that limitation is the largest one on the page.
- Generation is held at its base case. The weather is not sampled here, because re-simulating 8,760 hours for every draw would turn this into a visible stall. So this interval covers the battery decision rather than the whole answer, and the true spread is wider than what is drawn above. The payback page publishes the generation range.
- Tariffs are flat. A time-of-use tariff is the single change that would most readily move a battery from never paying back to paying back, and none of these scenarios models one.
- No degradation of the finding itself. Battery prices have fallen fast, and a conclusion drawn at today's prices is a conclusion about today.
Uncertainty model 0.1.0 · 200 samples, Latin hypercube with a Gaussian copula · seed 1073733709, derived from the scenario rather than a clock, so the same inputs always produce the same range.
What ageing and waiting actually cost
Ignoring capacity fade and the time value of money, this battery pays back in 20 years. With 2% fade a year and a 5% discount rate, it never pays back, holding 60% of its original capacity after 25 years. Net present value over the whole period: −£1,722.
Where the returns stop
| Battery | Self-consumption | Extra saving | Installed cost | Simple payback | Cycles a year |
|---|---|---|---|---|---|
| No battery | 32% | — | — | — | — |
| 2 kWh | 46% | £114 | £2,600 | 23 yrs | 365 |
| 4 kWh | 58% | £206 | £4,000 | 19 yrs | 330 |
| 5 kWh | 63% | £246 | £4,700 | 19 yrs | 316 |
| 6 kWh | 66% | £270 | £5,400 | 20 yrs | 289 |
| 8 kWh | 68% | £282 | £6,800 | 24 yrs | 226 |
| 10 kWh | 68% | £282 | £8,200 | 29 yrs | 181 |
| 12 kWh | 68% | £282 | £9,600 | 34 yrs | 151 |
| 15 kWh | 68% | £283 | £11,700 | 41 yrs | 121 |
| 20 kWh | 68% | £284 | £15,200 | 54 yrs | 91 |
Why the payback is so long
A battery does not earn you the 27p/kWh you pay for grid electricity. It earns the spread: 20p/kWh, the difference between what you pay to import and the 7p/kWh you would have received for exporting that unit instead. Every stored kilowatt-hour was already worth something.
Valuing stored energy at the full import rate is the most common way a battery payback figure is inflated, and at the rates set above it roughly quadruples the apparent benefit. Then subtract the round-trip loss — a 90%-efficient battery gives back nine units for ten — and the real margin is thinner again.
None of which means a battery is a bad purchase. It means the case for one is usually about resilience during outages, or about a time-of-use tariff where the spread is far wider than the one used here, rather than about arbitraging a standard flat tariff.
What this assumed
| Assumption | Value | Note |
|---|---|---|
| Dispatch level | L3, hourly | Meets the standard required to publish a battery figure |
| Load profile | Placeholder | Not measured, not sourced, identical every day |
| Annual demand | 3500 kWh | Set it to your own from your bill |
| Dispatch strategy | Self-consumption | What an unconfigured battery does; a tariff-aware controller does better |
| Round-trip efficiency | 90% | Split evenly across charging and discharging |
| Usable capacity | 90% of nameplate | Batteries are not run flat |
| Import rate | 27p/kWh | Flat tariff |
| Export rate | 7p/kWh | Varies enormously by country and supplier |
| Battery cost | £1,200 + £700/kWh | Retrofit pricing varies widely; use a local quote |
| Degradation | Not modelled | Declared rather than silently ignored; it would lengthen payback |
Frequently asked questions
What size solar battery or battery bank do I need?
Enough to soak up the solar you generate but do not use during the day, without paying for capacity that sits empty. This calculator runs hour-by-hour dispatch across a full year, so you can see where extra capacity stops adding savings — the returns flatten sharply once the battery can absorb a typical sunny afternoon.
That is how to size a solar battery bank: the capacity beyond which a bigger battery stops adding much saving.
Is a solar battery worth it?
It hinges on the gap between what you pay to import power in the evening and the lower rate you are paid to export at midday, and on how much surplus you have. The tool shows what a battery actually saves and its payback, so you can judge whether the saving covers the cost within the battery’s life.
How much can a solar battery save?
A battery is worth the difference between buying power in the evening and the low rate you would have been paid for exporting it at midday. Enter your import and export rates and your usage, and the calculator computes the annual saving from that spread.
How many kWh of battery do I need for a 4 kW solar system?
There is no fixed ratio between array size and battery size — the right capacity follows your evening usage and export rate, not the kilowatts of panels. Size it from your own load with the calculator above rather than from a rule of thumb.