Most UK homes on typical single-rate usage screen a solar battery between 3 and 10 kWh, and the right figure depends on the single job you size it for: storing your own solar, charging on a cheap overnight rate, or keeping essential circuits alive in a power cut. Homes that charge on a cheap overnight rate, or high multi-rate homes with an EV or electric heating, often screen larger. Enter your daily electricity use and the share you want to move out of expensive hours, and the solar battery size calculator below returns a screening capacity in kWh.
The result is a screening capacity, not a purchase recommendation. It gives you the rough size to take into an installer conversation, and it deliberately withholds savings and payback because a sizing form does not hold enough evidence to estimate money honestly.
What size solar battery do I need?
For a typical UK home on Ofgem’s 2026 medium usage of 2,500 kWh a year, about 6.8 kWh a day, a battery sized to shift half that load screens at around 5 kWh. Lower users land near 3 kWh and higher users near 8 to 10 kWh. Size for one goal first, then check the number against a real quote.
The table below reads across the Ofgem 2026 typical domestic consumption values, using the calculator’s neutral 50% screening share. That share is the real lever. If you plan to shift almost everything onto a cheap overnight rate, enter a higher figure and the size grows.
| Usage level (Ofgem 2026) | Annual electricity | Daily average | Screening size at 50% shifted |
|---|---|---|---|
| Low | 1,600 kWh | 4.4 kWh | 3 kWh |
| Medium | 2,500 kWh | 6.8 kWh | 5 kWh |
| High | 3,800 kWh | 10.4 kWh | 8 kWh |
| High multi-rate (Ofgem multi-rate high) | 6,100 kWh | 16.7 kWh | 12 kWh |
Size for one goal: self-consumption, cheap-rate charging or backup
A battery does three different jobs, and each one implies a different size. Decide which job matters most before you read the result, because sizing for all three at once buys more storage than you will use. The share you enter in the calculator is how you tell it which job you mean.
| Your goal | What the battery stores | Share to enter | What it means for size |
|---|---|---|---|
| Self-consumption from solar | Daytime solar you would otherwise export | The form asks for the share used outside your cheap window; enter your evening and overnight share there, capped by the spare solar you actually make | Smaller and seasonal; winter solar rarely fills a large battery |
| Cheap-rate charging | Grid electricity bought inside a cheap window | The share of daily use that falls outside the window, commonly three-quarters or more on a five-hour window | Larger; the battery carries most of a day |
| Backup during an outage | Only the essential circuits you choose | Not this tool; size by essential load power and the hours you want to cover | Depends on an inverter and wiring that support backup, not on daily kWh |
The cheap-rate case surprises people. On a five-hour window such as Octopus Go from 00:30 to 05:30, 19 of the day’s 24 hours fall outside it. If very little runs while you sleep, the share outside the window is commonly around three-quarters of the day or more, so use your own half-hourly data when you have it. To carry all of that on stored cheap electricity, the battery has to hold most of a full day, which is why cheap-rate sizing pushes toward the top of the range.
The self-consumption case pulls the other way. Here the battery only needs to hold the solar you cannot use as you generate it, and only as much as your array actually spills on a given day. Enter your evening and overnight share in the same field the calculator labels for use outside your cheap window. In summer the spare solar can be several kilowatt-hours. In winter it can be almost nothing.
How summer and winter change the size you actually use
Solar self-consumption sizing is seasonal because generation is. A south-facing UK array produces roughly four times more electricity in late spring than in December. PVGIS models a typical south-facing UK system at around four times the monthly yield in late spring that it reaches in December, so a battery you fill from spare solar every day in June can sit half empty for weeks in midwinter.
That gap changes what the right size means. Size purely for summer self-consumption and the battery is too big for the darker half of the year. Size for winter and you throw away summer solar you could have stored. Most homeowners settle in between, then lean on a cheap overnight rate to top up when solar cannot.
This is why the strongest year-round sizing usually blends two of the three goals: enough capacity to soak up summer solar, and enough to carry the expensive evening hours on cheap overnight charging through winter. The calculator sizes one goal at a time on purpose, so run it twice and compare. If you charge overnight, take the larger of the two numbers. If you do not, the smaller self-consumption figure is enough.
How is the battery size calculated?
The calculator multiplies daily electricity use by the share you choose to shift, the figure you enter in the field labelled for use outside your cheap window. It divides that energy by 90% usable depth of discharge, adds 15% sizing headroom, then rounds up to a commonly sold capacity. It does not use round-trip efficiency to inflate storage capacity.
For example, a home using 12 kWh a day with 50% outside the cheap window selects 6 kWh for shifting. Dividing by 0.90 and adding 15% gives 7.67 kWh, which rounds up to an 8 kWh screening size.
What does this result leave out?
- Half-hourly demand: use smart-meter data for the percentage outside the cheap window whenever possible.
- Battery and inverter power: capacity in kWh does not show whether the system can supply a kettle, heat pump or whole-home backup load.
- Solar production: the tool does not forecast solar generation, exports or seasonal self-consumption.
- Tariff economics: rates, standing charges, eligibility and cheap windows vary. The tool deliberately withholds savings and payback.
- Installed cost: electrical work, inverter choice, backup equipment and labour require a property-specific quotation.
A result above 30 kWh is flagged for professional design because normal domestic product ranges, inverter power and network constraints need closer attention.
What should you check before buying?
- Download at least four weeks of half-hourly smart-meter data, including a cold period if possible.
- Total the electricity used during the hours the battery would normally discharge.
- Check usable capacity, continuous power, round-trip efficiency and whether backup is included.
- Compare the screening result with at least two installed quotations.
- Ask each installer to show which demand data and equipment limits produced the proposed capacity.
How should you choose the cheap-window percentage?
Match the percentage to the tariff you can actually use. Octopus Energy’s smart-tariff terms specify a five-hour Go window from 00:30 to 05:30, while Economy 7 hours vary by supplier, meter and region. Your supplier must provide the rates and times that apply to your meter. To turn that into the share you enter, download a week of half-hourly readings from your supplier’s app or smart-meter export, total the kWh used outside the cheap hours, and divide by the week’s total.