UK HOME BATTERY GUIDE
What size solar battery do I need in the UK?
For existing solar, start with the surplus electricity you can store and the electricity you need later, not a battery size based on your annual bill alone. Compare usable capacity, charging and discharging power, and realistic year-round use. The aim is a justified size and an itemised quote, not simply the biggest battery.
Based on published sources and illustrative calculations, not a hands-on product test.
This guide considers moving otherwise-exported solar electricity to later use at home. Grid charging, tariff trading, the value of backup power and grid or reserve services need separate assessments; excluding them here does not mean they cannot be useful.
What should you check before choosing a size?
Gather solar generation, exported surplus and electricity bought from the grid, and look at when each occurs across seasons. Hourly or half-hourly records help if available; one sunny day does not represent a year. Energy Saving Trust's solar panels guide distinguishes direct use from export.
Grid purchases on a bill are not your home's total electricity use: solar used directly at home is excluded. Total solar generation is not all available to charge a battery once some has served daytime loads.
Annual consumption, panel count, kWp rating and bedroom count cannot settle battery size. Missing detailed records need not stop a quote discussion: ask which figures are measured or assumed, rather than accepting a precise-looking multiplier.
Usable kWh, power and annual use
Ask where each energy figure is measured; figures at different boundaries are not directly comparable.
| Quantity | Unit | Meaning |
|---|---|---|
| Usable capacity | kWh | Energy within the manufacturer's stated usable state-of-charge window, at its stated measurement boundary. |
| Charge and discharge power | kW | How quickly energy can be accepted or supplied. |
| Annual energy delivered to your home | kWh/year | Energy supplied through the battery to home loads instead of buying from the grid. |
Nominal capacity is the battery's nameplate total; usable capacity is the portion within the manufacturer's operating limits. Do not subtract the same manufacturer state-of-charge window again. A separate reserve you choose may reduce availability. Manufacturer-stated usable DC capacity does not automatically equal AC energy delivered to the home.
The panels' kWp is a peak power rating, not battery capacity in kWh or daily production. A battery's kWh figure does not tell you its annual delivery. Energy Saving Trust's battery storage guide says there is no exact formula for an individual home.
Is a 5 kWh or 10 kWh battery enough?
Either might be too small, sufficient or larger than you can use well. Compare usable capacity at its stated boundary, chargeable surplus, later demand and charge and discharge power. Neither is a default recommendation. Extra capacity needs extra charging energy and later use; doubling capacity does not double annual savings.
Here is a deliberately hypothetical energy check. Suppose a steady home AC load is 2 kW for three hours and direct solar does not cover it: the home needs 2 kW × 3 hours = 6 kWh. If, only for this example, whole-battery-path efficiency at the AC comparison boundary is assumed to be 90%, supplying that load needs 6 ÷ 0.90 ≈ 6.7 kWh of solar that would otherwise have been exported. The 90% is an invented assumption, neither a typical value nor a manufacturer's promise.
The 6 kWh is load energy; about 6.7 kWh is charging energy. Neither number directly recommends nominal or usable battery capacity, establishes any device's ability to run the 2 kW load, or predicts annual use.
Why timing and seasons matter
Solar used directly by day is no longer surplus for charging. Exports and grid purchases can fall on different days or seasons, so annual totals need not overlap. The Energy Saving Trust solar guide distinguishes direct use, export and storage.
Charge and discharge power, and the time available, can limit how much energy moves through the battery. More capacity does not itself generate more solar electricity.
If there is no usable stored energy at the start of a period, no solar surplus during it and no grid charging, adding capacity alone cannot deliver energy to the home in that period. This says nothing about a battery that was charged earlier and then used on a cloudy day.
An electric car, heat pump or planned load changes your use profile, but does not automatically justify particular extra capacity.
What should an installer show you?
Ask for a home-specific estimate and compare sizes on the same basis. An estimate is not a guarantee:
- Which generation, export, import and timing records, or named assumptions, support the proposed size?
- What are the exact configuration's usable capacity, measurement boundary, charging and discharging power, and compatibility with the current solar system?
- How much first-year AC energy is estimated to reach home loads through the battery in place of grid purchases, and how have losses been allowed for?
- How do a smaller and a larger option differ in complete installed price and expected yearly use under the same starting assumptions?
You could say: “Please recommend a size for our existing solar using our generation, export and import records. Show the exact configuration, a fully itemised installed price and your assumptions for annual energy delivered to our home.”
Check the quote, not just the battery size
The UK home battery cost guide covers complete installed prices. Also consider avoided purchase price, export income given up, storage losses and additional yearly costs. More energy delivered does not automatically improve the financial result.
First, Use the itemised quote checklist to clarify equipment, installation and annual-use assumptions. A smaller battery, more evidence or keeping solar without storage remain options.
With a quote, Check your assumptions in the calculator. Select GBP for a UK quote and use your own first-year AC energy delivered through the battery to home loads instead of grid purchases in kWh/year. Do not enter battery capacity as annual energy. The calculator does not size a battery or simulate a product; how the calculations work explains its boundary.
The link does not prefill this example, choose a currency or change stored data. Neither link requests a quote or sends information to an installer.
Quick questions
What size battery do I need for a 4 kW solar system?
Peak solar power does not reveal your surplus or later demand. Compare those records with usable battery capacity and power; ask for a size justified by your home's profile.
How long will a 10 kWh battery last?
There is no fixed runtime: usable energy, load and discharge power matter. Calculate the load's energy need in kWh from its kW and duration; a nominal 10 kWh is not guaranteed energy at your appliances.
Can I add more battery storage later?
Check the manufacturer's expansion terms for the exact configuration and compatibility with your inverter, controls and installation. Ask about extra equipment and work; expansion is not guaranteed to be simple.
Sources and checking date
External sources checked 27 September 2026. The sizing questions above apply published general guidance to an existing-solar home; the AC comparison boundary and the hypothetical 90% example are Battery Payback's own model choices, not Energy Saving Trust household findings.
- Energy Saving Trust, Battery storage — storing surplus solar for later use, the need for an individual installer assessment and the limits of a single sizing formula.
- Energy Saving Trust, Solar panels — direct solar use, export of unused generation and storage for later household use.
Start with the guide to adding a battery to existing solar for an itemised quote checklist.