What could a battery add to your solar savings?

Reviewed 20 October 2026. Model FB-hourly-1.0.0; profiles FB-synthetic-2026-10-20; averages FB-quick-2026-10-20. References inform the assumptions; none endorses this calculator. No MCS-derived equipment average is claimed.

• Panels: 375 watts each, derived from the Energy Saving Trust reference of around 12 panels for 4.5 kWp. This is a reference size, not the average of every panel sold.
• Batteries: 4–6 usable units of stored electricity per battery, around a 5 kWh reference discussed by Energy Saving Trust. The 4–6 range, 90% round-trip efficiency and shared 2.5 kW charging/discharging limit are Fairbeam assumptions. More batteries add storage, not unlimited power.
• Household: 2,500 units a year before habit adjustments. This uses the DESNZ 2026 report’s median for England and Wales in 2024 as a GB proxy, not a measured UK-wide household average. Example habit frequencies are Fairbeam defaults, not surveyed averages.
• Appliances: washing 0.5 units/load, dishwasher 0.9, dryer 3 and full kettle 0.2, from the Energy Saving Trust 2022 guide, page 5. We use 0.1 per half-full boil. These are examples; modern appliances and cycles differ. The guide’s old prices are not used.
• Lights assume 8W LEDs. Room heater assumes 2 kW for 120 winter-equivalent days; air conditioning assumes 1 kW electrical input for 60 summer-equivalent days. These are illustrative Fairbeam assumptions, not measured appliance averages. Typical included appliance energy is replaced when you refine habits, so it is not counted twice.
• Generation: 800 and 1,000 units a year per kWp are Fairbeam sensitivity assumptions, not verified UK averages or lower/upper bounds. Region, roof direction, weather and shading can put actual results outside them.
• Import price: 26.32p per unit, Ofgem GB average Direct Debit, October–December 2026. Estimates pause after that period until updated. Export scenarios 0p, 5p and 15p are illustrations, not average tariffs or guaranteed offers. GB export eligibility and supplier terms vary. NI estimates are withheld because these tariff assumptions do not apply.

Both installations use the same solar generation and household demand. An 8,760-hour model uses solar in the home first, charges surplus within battery capacity and power limits, then discharges to meet later demand. Remaining electricity is exported or imported. Export is assumed unrestricted; a real export limit may change results.

Storage is measured as charging energy before discharge losses. Efficiency is applied once at discharge. Repeated representative years settle the opening and closing storage balance, avoiding free starting energy. Extra battery benefit equals the difference between the two complete electricity bills, including exports. No grid charging or tariff arbitrage is added.

Hourly profiles are original Fairbeam synthetic assumptions (CC0-1.0), not MCS data, PVGIS weather or measured homes. They use a 365-day standard-time year without clock changes, summer-weighted solar and winter-weighted household demand. Appliance cycles are spread across days as expected usage. Heating peaks in winter; cooling peaks in summer. Smooth solar can overstate useful storage in cloudy spells; hourly averages miss brief appliance peaks.

Heavy shading, unusual roofs, large batteries, electric heating, heat pumps and EV charging need a tailored estimate. This is first-year guidance, with flat prices and no allowance for degradation, standby use, finance, maintenance or replacements. A cash payback needs your actual extra installed battery cost and tariff, so this quick tool does not give one.