Solar battery size calculator
Use this solar battery size calculator to size a home battery against the three things that actually decide it in the UK: how much electricity you use after 4pm, how much your panels can really refill, and which tariff you are on. It runs on PVGIS irradiance data for Scotland’s Central Belt, so the winter answer is honest — in December a south-facing 4 kWp array near Livingston makes about 2.4 kWh a day.
Size your battery
—
| Month | Solar per day | Left after daytime use | Battery filled by solar |
|---|
How this calculator sizes a battery
Most battery calculators multiply your daily use by a fudge factor. That produces big numbers, and big numbers sell big batteries. We work it the other way round, from what a battery can actually do for you on each day of the year.
- Your evening need. Daily use is your annual kWh divided by 365. The share after 4pm (40% unless you change it) is what a battery can move from daytime into the evening.
- What your panels can refill. PVGIS gives the average daily output of a south-facing kilowatt-peak near Livingston for every month. What is left after your daytime use is the most solar you can store that day. There is no point buying capacity your panels can never fill.
- Your tariff. A time-of-use tariff lets the grid fill the battery cheaply, so the solar ceiling stops applying. Each tariff moves the recommendation differently — see below.
- A real product size. We round up to the nearest SigenStor build, which stacks in 5.2 kWh and 8.0 kWh modules up to roughly 48 kWh per stack. The kWh figure above it is brand-neutral.
Worked example: a 3-bed semi in West Lothian
Take a home using 3,500 kWh a year with a 4 kWp south-facing array. That is 9.6 kWh a day, of which 3.8 kWh falls after 4pm and 5.8 kWh in daylight hours.
- On a flat tariff: the evening need is 3.8 kWh. On an average May day the array makes 15.0 kWh, leaving 9.3 kWh after daytime use — so the panels can easily refill the battery, and the evening need is the ceiling. Recommendation: about 3.8 kWh usable, which rounds up to one 5.2 kWh module.
- On Octopus Flux: the battery also exports in the 16:00–19:00 peak and refills at 02:00–05:00, so we add up to 5 kWh on top of the evening need, capped at 15 kWh because the three-hour windows limit how much a home inverter can move. Recommendation: 8.8 kWh, which rounds up to two 5.2 kWh modules (10.4 kWh).
- On Intelligent Octopus Go: the battery carries the house through the 18 hours outside the 8p window. Recommendation: 80% of 9.6 kWh, or 7.7 kWh, which rounds up to one 8.0 kWh module.
Now look at December. The same array makes about 2.4 kWh a day — less than half the 5.8 kWh the house uses in daylight — so the panels put nothing into the battery at all. From November to February a battery in Scotland earns its keep through the tariff, not the roof. That single fact is why the right size depends more on your tariff than on your panels.
Why winter decides the answer in Scotland
Solar output in the Central Belt swings about sixfold between May and December. These are PVGIS averages for one kilowatt-peak facing due south at 35° near Livingston, with 14% system losses — the same numbers the calculator uses.
| Month | kWh per kWp per day | 4 kWp array per day | 4 kWp array per month |
|---|---|---|---|
| Jan | 0.77 | 3.1 kWh | 96 kWh |
| Feb | 1.45 | 5.8 kWh | 162 kWh |
| Mar | 2.33 | 9.3 kWh | 289 kWh |
| Apr | 3.45 | 13.8 kWh | 414 kWh |
| May | 3.76 | 15.0 kWh | 466 kWh |
| Jun | 3.68 | 14.7 kWh | 442 kWh |
| Jul | 3.56 | 14.2 kWh | 441 kWh |
| Aug | 3.18 | 12.7 kWh | 395 kWh |
| Sep | 2.59 | 10.4 kWh | 311 kWh |
| Oct | 1.69 | 6.8 kWh | 210 kWh |
| Nov | 1.17 | 4.7 kWh | 141 kWh |
| Dec | 0.61 | 2.4 kWh | 76 kWh |
| Year | 860 per kWp | — | 3,440 kWh |
Source: PVGIS v5.3 (European Commission Joint Research Centre), SARAH3 radiation data 2005–2023, retrieved 16 September 2026. East- or west-facing roofs produce less, and shading changes everything — a survey measures yours.
How your tariff changes the right size
Flat import tariff with an export tariff. Every kWh you store and use in the evening is a kWh you don’t export for 12p (Outgoing Octopus) and then buy back. Flexible Octopus in South Scotland costs 25.85p/kWh on Direct Debit to 30 September 2026 and 26.11p from 1 October, when VAT comes off household electricity — so each stored kWh saves roughly 14p. Size to your evening need, and no bigger than your summer surplus.
Octopus Flux. Built for solar-and-battery homes: cheap import from 02:00 to 05:00 and a high export rate from 16:00 to 19:00. Octopus’s published South Scotland rates for 16 September 2026 were 14.98p/kWh import off-peak, 24.96p standard, and 34.94p import with 27.19p export in the peak. You need solar, a home battery, a smart meter and an Octopus account. Octopus paused Flux to new customers in March 2026; its Flux page is taking sign-ups again as of 16 September 2026. More detail in our Octopus Flux guide.
Intelligent Octopus Go. An 8p/kWh rate from 23:30 to 05:30 for households with a compatible EV or charger, and the whole home gets the cheap rate in that window. A battery charged overnight can run the house for most of the next day, which is why EV households on this tariff often justify more storage than their solar alone would.
Cosy Octopus. For homes with a heat pump, electric boiler or electric radiators: three cheap windows (04:00–07:00, 13:00–16:00 and 22:00–00:00) and a peak from 16:00 to 19:00. A battery that fills in the afternoon window and empties through the peak is the natural fit, so size to your evening use.
kWh is not the whole specification
Power (kW) caps the speed. A battery’s inverter rating limits how fast it charges and discharges. A large battery behind a small inverter can’t empty itself in a three-hour peak or fill itself in a three-hour cheap window, so check the kW rating as well as the kWh.
Usable is not nominal. Compare batteries on usable capacity, which is what you can actually draw, rather than the nominal figure on the box. The datasheet shows both.
Modular beats guessing. Because a SigenStor stack can grow to roughly 48 kWh, you can start at the size this page recommends and add a module once a year of real data shows you need one. See our SigenStor installer page for the full specification.
When a bigger battery doesn’t pay
- Flat tariff, small array. If your panels leave little after daytime use, extra capacity sits empty for most of the year.
- Flat tariff, no solar. Without a cheap rate to charge from, a battery has nothing worth storing — on these numbers it isn’t worth fitting.
- Evening use already covered. Past your evening need, stored solar can only be exported, and the 12p it earns rarely justifies the extra cost.
If the numbers here point to a battery, our step-by-step battery sizing guide shows how to do the same sum from your own half-hourly smart-meter data, and our battery storage installation page covers what an install involves.
The figures behind this page
| Figure | Value | Source, checked |
|---|---|---|
| Annual solar yield, Livingston, 35° south, 14% losses | 860 kWh per kWp | PVGIS v5.3, SARAH3 2005–2023 — 16 Sep 2026 |
| Daily yield, December and May | 0.61 and 3.76 kWh per kWp | PVGIS v5.3 — 16 Sep 2026 |
| Octopus Flux, South Scotland | 14.98p import 02:00–05:00 · 24.96p standard · 34.94p import / 27.19p export 16:00–19:00 | Octopus Energy published rates for 16 Sep 2026 |
| Intelligent Octopus Go | 8p/kWh, 23:30–05:30 | octopus.energy — 16 Sep 2026 |
| Cosy Octopus cheap windows | 04:00–07:00 · 13:00–16:00 · 22:00–00:00 | octopus.energy — 16 Sep 2026 |
| Flexible Octopus, South Scotland, Direct Debit | 25.85p (to 30 Sep 2026) · 26.11p (from 1 Oct 2026) | Octopus Energy published rates — 16 Sep 2026 |
| Outgoing Octopus export | 12p/kWh | octopus.energy — 16 Sep 2026 |
| VAT on household electricity | 0% from 1 Oct 2026 to 31 Mar 2027 | GOV.UK announcement; Ofgem price cap notice for Oct–Dec 2026 |
| VAT on home batteries | 0% until 31 Mar 2027, including standalone batteries | HMRC VAT Notice 708/6 |
| SigenStor module sizes | 5.2 and 8.0 kWh, up to ~48 kWh per stack | SigEnergy product data held by Ecoaim |
| Evening share of use; EV miles per kWh | 40%; 3.5 (both editable) | Ecoaim assumptions — replaced by your meter data at survey |
Battery sizing questions
What size solar battery do I need for a 4kW solar system?
For a typical 3,500 kWh-a-year home in Scotland on a flat tariff, about 4–5 kWh of usable storage covers the evening, and a south-facing 4 kWp array can refill that on a good May day. Size bigger only if a time-of-use tariff such as Octopus Flux or Intelligent Octopus Go will fill the battery cheaply from the grid — that is when 8–10 kWh starts to earn its keep.
Will my battery fill from solar in winter in Scotland?
Mostly not between November and February. PVGIS data for Livingston puts a south-facing 4 kWp array at about 2.4 kWh a day in December and 3.1 kWh in January — usually less than a home uses in daylight hours — so in those months a battery earns its keep by charging on a cheap overnight rate, not from the panels.
Is a 10kWh battery too big for my house?
On a flat tariff it often is, because a typical home only moves 3–5 kWh into the evening and the panels cannot refill more than their daytime surplus. On Octopus Flux, Intelligent Octopus Go or Cosy Octopus a 10 kWh battery can be charged cheaply from the grid, which is where the extra capacity pays.
What size battery do I need for Intelligent Octopus Go?
Enough to carry the house through the 18 hours outside the 8p window, which runs from 23:30 to 05:30. We size it to about 80% of your daily household use, with the EV left out because the car charges directly at the off-peak rate. For a 3,500 kWh home that is about 7.7 kWh.
Can I add more battery capacity later?
With a modular system, yes. SigenStor batteries stack in 5.2 kWh and 8.0 kWh modules up to roughly 48 kWh per stack, so you can start with one or two modules and add more if your usage or tariff changes. Check any other brand’s expansion limits before you buy.
Do I pay VAT on a home battery?
No, until 31 March 2027. HMRC VAT Notice 708/6 zero-rates batteries installed in homes — with solar, added to existing solar, or on their own (standalone batteries have qualified since 1 February 2024). After 31 March 2027 the rate is due to revert to 5%.
Does a heat pump change the battery size I need?
Yes, because it raises your use in winter, exactly when solar is weakest. Heat pump homes can join Cosy Octopus, whose cheap windows are 04:00–07:00, 13:00–16:00 and 22:00–00:00, so a battery charged in the afternoon window can carry the house through the 16:00–19:00 peak.
How accurate is this calculator?
It is a sizing guide, not a design. It uses PVGIS averages for a south-facing roof near Livingston and an assumption about how much of your electricity you use after 4pm, which you can change. A survey replaces both with your half-hourly smart-meter data and your actual roof.
Get a battery sized from your real data
A calculator can’t see your roof or your meter. Send us your details and we’ll size the battery from your half-hourly usage and a proper survey, with a fixed price.
Free quote in 24 hours.
No high-pressure sales. No commission-driven scripts. Just a no-obligation survey + transparent quote — covering every postcode in Scotland's Central Belt.