Home battery payback calculator

By Daan Haket · researched and written with an AI assistant · updated 27 October 2026

Start with your yearly use, your installed quote and the rates you can actually get. The prefilled numbers only show how it works; the assumptions below say which ones come from a source.

An estimate for one household, not a quote and not a product test. It uses monthly averages, two import rates, one export rate for every hour and leaves out the standing charge, which can change if you switch tariff. What it leaves out.

Your home Electricity use per year (kWh)From your bill or supplier app. Prefilled with Ofgem’s typical (medium) use for a home on a single rate: 2,500 kWh. Your own bill is better. Share used in daylight hours (%)Your estimate; few people know it. 40 is only an example. Try 30 and 50 to see how much it moves the answer. Solar generation per year (kWh)From your installer's quote or your inverter app. Enter 0 if you have no solar panels. Example only: 3400. Your region (for the monthly solar pattern)Spreads your yearly generation over the months using the EU's PVGIS estimate for the nearest city: more in summer, less in winter. Or: system size (kWp)Only if you do not know your yearly generation. We then use PVGIS for your region (south-facing, 35° tilt). Leave empty to use the figure above.

Your tariff Standard or peak unit rate (p/kWh)Prefilled with the Ofgem price cap average for Direct Debit, 1 October to 31 December 2026: 26.32p. Cheap-hours unit rate (p/kWh)Only if your tariff has cheap hours; otherwise leave it equal to the standard rate. See smart tariffs for checked rates. Export rate (Smart Export Guarantee)Pick a checked flat rate or type your own below. Peak-only, “up to” and unclear rates are not in this list, because the calculator uses one export rate for every hour. Many of the highest rates need you to buy your electricity, or your battery, from that supplier: see export rates and their conditions. Export rate (p/kWh)

The battery Usable capacity (kWh)From the datasheet. Example: 13.5 (GivEnergy All in One, checked). Round-trip efficiency (%)From the datasheet if stated. Example only: 90. Installed price (£)Your quote. Example only: 6500, inside the Energy Saving Trust range of around £5,000 to £8,000.

More detail (optional): cheap window, load shifting, warranty, loan

Cheap hours Length of the cheap window (hours)For example 5 for 00:30–05:30, 3 for 02:00–05:00. Only used together with the charging power below: the two together cap how much the battery can fill at the cheap rate. Battery charging power (kW)From the datasheet. Only used together with the cheap window above. Share of your use already in cheap hours (%)For example an EV or timed appliances. Example only: 0. Share of your evening use you could move to cheap hours yourself (%)Running the dishwasher, washing machine or car charger on a timer instead of in the evening, without a battery. We then show what that alone saves, and count only what the battery adds on top. Example only: 0.

Warranty and wear Warranty (years)We then tell you whether it pays back within the warranty. Yearly loss of saving (%)Allowance for capacity fade or tariff changes. Example: 0.

Paying with a loan Interest rate (% a year)From the loan or finance offer. Leave empty if you pay cash. Loan term (years)

The assumptions behind the prefilled values

Standard unit rate: Ofgem's price cap average for Direct Debit across England, Scotland and Wales is 26.32p per kWh. Valid from 1 October 2026 to 31 December 2026.

“26.32 pence per kWh 54.83 pence daily standing charge”

Source: ofgem.gov.uk, read 19 October 2026; quote rechecked 26 October 2026

Energy Saving Trust states that battery storage tends to cost around £5,000 to £8,000.

“Battery storage tends to cost around £5,000 to £8,000.”

Source: energysavingtrust.org.uk, read 19 October 2026

Ofgem gives a medium standard single-rate electricity TDCV of 2,500 kWh per year.

“Standard single-rate meters, kWh per year) Usage Level 2023 TDCV 2026 TDCV Change Low 1,800 1,600 -200 Medium 2,700 2,500 -200”

Source: ofgem.gov.uk (PDF, page 7), read 20 October 2026

The updated TDCVs apply from 1 July 2026. Valid from 1 July 2026.

“We will implement the updated TDCV values from 1 July 2026.”

Source: ofgem.gov.uk, read 20 October 2026; quote rechecked 26 October 2026

Solar generation per kWp, by region

The EU Joint Research Centre’s PVGIS service estimates what 1 kWp of panels produces in a typical year. We use its monthly pattern to spread your yearly generation over the seasons. Settings: south-facing, 35° tilt, 14% system losses (the PVGIS default), satellite data 2005–2023.

Nearest city	kWh per kWp a year	December	June	Source
London	1,019	36	123	“"E_y":1018.84” — PVGIS, 22 October 2026
Cardiff	1,042	32	131	“"E_y":1041.7” — PVGIS, 22 October 2026
Birmingham	968	33	117	“"E_y":968.06” — PVGIS, 22 October 2026
Manchester	886	28	111	“"E_y":886.08” — PVGIS, 22 October 2026
Edinburgh	903	22	111	“"E_y":902.72” — PVGIS, 22 October 2026
Belfast	919	26	116	“"E_y":918.76” — PVGIS, 22 October 2026

Your roof’s direction, tilt and shading can change this a lot; your installer’s figure for your own roof is better than a regional estimate.

Round-trip efficiency is 90% in the example. What manufacturers state, where we could check it: Tesla Powerwall 3: 89% solar to battery to home/grid (includes the inverter) (source, 26 October 2026); Fox ESS EP11 (global datasheet v1.5, 24 November 2025): At least 95%, battery pack only; not whole-system AC efficiency (source, 26 October 2026); SolarEdge Home Battery 400V (BAT-10K1P): 94.5% (peak) (source, 26 October 2026); Enphase IQ Battery 5P with FlexPhase: 90% (AC round trip) (source, 22 October 2026).

The full method, and what the calculator leaves out, is on how we calculate. The same model is available as data at /api/v1/calculator.json.