How to size a home solar battery in the UK
A careful battery conversation covers usable capacity, power, cycling, seasons, tariffs, backup and real demand.
Reviewed 2026-09-24 · 6 min read
Capacity and power differ
Capacity is energy stored, usually kWh; power is the charge or discharge rate, usually kW. A large kWh figure does not automatically run every appliance, and a high kW rating does not mean more stored energy. Request usable capacity, continuous and peak power and the definitions used.
Size depends on demand timing, solar output, export and whether grid charging is allowed. Daytime users may need less storage than evening users. EVs, heat pumps and immersion heaters change the load profile.
Ask whether the quoted capacity is nominal or usable. A reserve may be held back to protect the battery or provide backup, and conversion losses mean the energy put in is not all available at the socket. The model should state these limits instead of treating the nameplate number as delivered energy.
Collect at least a year of bills and, where available, half-hourly or smart-meter data. Note changes expected soon: an electric vehicle, heat pump, home office or change in occupancy can make historical demand unrepresentative. A transparent model can show how those changes affect the recommended size.
Model seasons and degradation
A battery sized around a bright summer day can sit underused in winter. Request monthly scenarios, round-trip efficiency, minimum reserve and degradation. Warranty terms may specify end-of-term capacity, throughput or operating conditions.
Backup is separate design work. Ask what circuits are backed up, how long changeover takes and whether large loads are excluded. It is not automatic because a battery is present.
Read the warranty’s operating conditions. It may specify a minimum state of charge, temperature range, throughput, cycle count or end-of-warranty capacity. Ask who pays for labour and disposal if a replacement is required, and whether software or communications are needed to keep the warranty valid.
Battery controls can change over time. Clarify whether the owner can change schedules, whether a supplier or installer can remotely alter settings, and what happens if an app, internet connection or company ceases to operate. A useful handover includes local controls and a route to support.
Compare economics
Request solar generation, direct use, battery losses, export and import as separate values. Compare solar-only, smaller-storage and proposed-storage cases on identical assumptions. Ask about inverter type, expansion, controls and replacement costs.
Do not count the same unit of electricity twice. Energy used directly is different from energy routed through the battery, and both differ from export. Ask the installer to show the flow in a monthly table and explain how it calculated the value of each route.
A tariff that rewards time-shifting may alter the result, but eligibility, prices and controls can change. Model a case without a favourable future tariff and decide whether the battery still meets your objective. If the main reason is backup, assess it as resilience rather than only as a payback investment.
Common questions
How many kWh do I need?
There is no universal number; model interval demand, generation, tariff and backup objectives.
Does bigger save more?
Not always. Oversizing can increase cost while the battery remains underfilled or cycles less.
Does a battery guarantee blackout power?
No. Backup requires appropriate islanding design and clearly defined backed-up circuits.