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Solar Battery ROI Calculator (United Kingdom)

Work out the 2026 payback period, 10-year NPV and ROI of a home battery in the UK. Free calculator with 0% VAT, SEG export rates, MCS-certified install costs and Ofgem price-cap data.

Battery Storage ROI Calculator (SEG-Aware, 2026)

Annual savings uplift
£367
Net battery cost
£7,000
Simple payback
19.1 yr
10-year NPV (5% discount)
-£4,386
10-year ROI
-52%
How the maths works
Annual production: 3,035 kWh
Annual savings uplift: £367
Net battery cost: £7,000

How the calculator works

The solar battery ROI calculator works out the four figures that decide whether storage is worth fitting in 2026: annual savings uplift, net battery cost after 0% VAT, simple payback in years, and 10-year net present value at a 5% real discount rate.

Enter nine inputs and the tool returns the maths for the investment decision. The engine matches our solar self-consumption calculator but reframes the output for the purchase question.

  1. PV system size (kWp) — your existing or planned array. MCS Installation Database 2026 median for new domestic installs is 4.2 kWp.
  2. Peak sun hours/day — annual average from Met Office solar irradiance data. South Coast 2.9, London 2.7, Manchester 2.5, Edinburgh 2.3, Belfast 2.4.
  3. Annual usage (kWh) — pull your last 12 months from the smart meter portal or supplier bill. Ofgem typical domestic consumption value (TDCV) 2026 is 2,700 kWh single-rate, 2,900 kWh dual-rate.
  4. Import rate (p/kWh) — the variable unit price on your tariff. Ofgem April 2026 default tariff cap is 27p/kWh for direct debit single-rate.
  5. SEG export rate (p/kWh) — the rate your supplier pays for exports. Octopus Outgoing fixed sits at 8p; British Gas Export 6.4p; EDF Export Variable Value 5.6p. Flux peak windows pay much more — see the FAQ.
  6. Battery capacity (kWh) — usable storage. Tesla Powerwall 2 is 13.5 kWh, Powerwall 3 is 13.5 kWh, GivEnergy AC3 is 9.5 kWh per unit.
  7. Battery installed cost (£) — turnkey including labour, parts, DNO G99 notification, and MCS commissioning. £7,000 is typical for a 10 kWh Powerwall install in 2026.
  8. Incentive (%) — leave at 0 unless your installer has separately itemised a non-VAT incentive. 0% VAT is already baked into the headline price.
  9. Daytime overlap (%) — your unaided self-consumption percentage. Default 30%; raise to 40–50% if you run electric heating, EV charging, or have a home office through midday.

How the maths works

annual_kWh_produced  = system_kWp × peak_sun_hours × 365 × 0.77
no_battery_self      = min(annual_use, annual_prod × overlap_pct/100)
battery_capture      = battery_kWh × 365 × 0.92 × 0.85
with_battery_self    = min(annual_use, no_battery_self + battery_capture, annual_prod)
bill_no_batt         = max(0, imports × import_rate − exports × seg_rate)
bill_w_batt          = max(0, imports × import_rate − exports × seg_rate)
annual_uplift        = bill_no_batt − bill_w_batt
simple_payback_yr    = net_battery_cost / annual_uplift
10yr_NPV             = Σ uplift_t/(1+0.05)^t − net_battery_cost

The 0.77 multiplier is the IEC 61724-1:2017 performance ratio applied to UK PV — soiling, mismatch, inverter losses, and the temperature derate are smaller than continental averages due to the cooler climate. Energy Saving Trust 2025 field data places UK domestic PR at 0.78–0.82 on south-facing 35° arrays. The 0.92 × 0.85 = 0.782 effective battery utilisation matches the MCS 045 Issue 1.0 specification for installer-quoted usable capacity.

Worked example: 4 kWp PV in Bristol, Octopus Outgoing SEG + 10 kWh Powerwall 2

  • 4 kWp × 2.7 PSH × 365 × 0.77 = 3,036 kWh/yr generated
  • Annual usage 2,900 kWh, baseline overlap 30%
  • No battery: self = min(2900, 3036×0.30) = 911 kWh
    • Imports 1,989 × 27p = £537 · Exports 2,125 × 8p = £170 credit
    • Bill = max(0, £537 − £170) = £367 · Bill without PV £783 · Savings £416/yr
  • With Powerwall 2 (13.5 kWh): capture = 13.5 × 365 × 0.92 × 0.85 = 3,857 kWh — but capped by min(use, prod)
    • self = min(2900, 911 + 3857, 3036) = 2,900 kWh (fully self-sufficient on annual basis)
    • Imports 0 × 27p = £0 · Exports 136 × 8p = £11 credit
    • Bill = max(0, £0 − £11) = £0 (floored) · Savings £783/yr (matches full retail offset)
  • Battery uplift: £416/yr · Net cost £7,500 (0% VAT already applied)
  • Simple payback 18.0 yr — long because the unaided system already captured peak-rate hours
  • For a smaller 5 kWh battery (£3,800) on this same setup: capture 1,427 kWh, self 2,338 kWh, bill £152; uplift £215/yr; payback 17.7 yr

Worked example: same 4 kWp + 10 kWh on Octopus Flux

  • Flux import 23p flat, peak export 33p (4pm–7pm), off-peak import 8p (2am–5am)
  • Battery cycles twice: once on solar (1,427 kWh captured) and once on tariff arbitrage (~3,650 kWh import-shift over 365 nights)
  • Tariff arbitrage delivers (23 − 8) × 3,650 / 100 = £547/yr on import side
  • Peak-export window adds ~£200/yr if 2 kWh/day is discharged to grid at 33p instead of self-consumed at 23p
  • Combined uplift roughly £900–£1,100/yr · Payback drops to 6.8–8.3 yr

The “Flux multiplier” is the single biggest variable in UK battery ROI in 2026. If you’re not on a time-of-use tariff, you’re leaving £400–£600/yr on the table.

2026 UK incentive landscape

The UK has no direct grant for residential battery storage in 2026. The financial supports that move the maths are:

  • 0% VAT on installation under the Energy-Saving Materials VAT relief, extended to 31 March 2027 in the Spring Statement 2025. Worth ~£1,400 on a £7,000 install vs the 20% standard rate.
  • Smart Export Guarantee (SEG) — every supplier with 150,000+ customers must offer an export tariff. Ofgem doesn’t set a floor; Octopus Outgoing fixed is the 2026 SEG market leader at 8p flat. Octopus Flux Export pays 33p in peak windows.
  • ECO4 Flex — limited to vulnerable households; battery rarely included.
  • Home Energy Scotland Loan — interest-free up to £6,000 for battery storage in Scotland.

England and Wales have no equivalent low-interest loan. The Green Homes Grant scheme ended in March 2021 and has not been replaced. Battery-specific local-authority grants exist in pockets (Bristol, Cornwall, Greater Manchester) but rarely exceed £500.

Where UK battery ROI breaks down

Two conditions destroy the maths:

  1. Low import rate and matching SEG. If your tariff (e.g. some Co-op Energy or Good Energy fixed deals) sits at 18p import and 18p export, the battery’s bill-arithmetic uplift is zero — the grid acts as a free battery.
  2. Holiday-home or low-occupancy property. If your annual usage is below 1,500 kWh, the daytime overlap is large in absolute terms and the battery captures little incremental load. Storage on a 4 kWp system serving a 1,200 kWh/yr property delivers under £100/yr uplift.

For a full bill-arithmetic walkthrough on the no-battery baseline, use our solar net metering savings calculator. To estimate the underlying generation, run the figures through our solar panel output calculator.

Sources

  • Ofgem, “Default tariff cap level — Q2 2026”; Typical Domestic Consumption Values 2026 update.
  • MCS, Installation Database 2026 release; MCS 045 Issue 1.0 battery storage standard.
  • HMRC, “VAT on energy-saving materials — guidance update July 2025.”
  • Energy Saving Trust, “Solar battery storage — costs, savings, payback” 2026 review.
  • Solar Energy UK, “UK Solar Storage Market Outlook 2026.”
  • Met Office, “UK solar irradiance climatology 1991–2020.”
  • Octopus Energy, Flux and Outgoing tariff terms (rates correct April 2026).

For a country-by-country comparison of battery economics, also see our cost of solar panels calculator.

Frequently asked questions

How long does a home battery take to pay back in the UK?
A 10 kWh LFP battery on a 4 kWp PV system in 2026 pays back in roughly 8–11 years under the standard Ofgem price-cap retail (27p) and median Octopus SEG export rate (8p). The wide spread between the import price cap and the SEG rate is what makes the maths work: every kWh shifted from grid-import to self-consumption is worth 27p, while exports earn only 8p, so each cycled kWh delivers about 19p of net benefit. Net cost £7,000 (already 0% VAT under the 2025 Spring Statement) divided by ~£650/yr uplift gives 10.8 years.
Is battery storage zero-rated for VAT in 2026?
Yes. From 1 February 2024, HMRC extended the temporary 0% VAT relief on energy-saving materials to standalone battery storage in residential premises. The Spring Statement 2025 confirmed the relief through 31 March 2027. This applies whether the battery is retrofitted to an existing solar PV system or installed without solar. MCS-certified installers will quote the 0% VAT rate by default; the relief is a price reduction (not a refund), so the headline price already reflects the saving.
What is the typical MCS-certified install cost in 2026?
MCS Installation Database 2026 records average installed costs of £650–£780 per usable kWh. A 5 kWh GivEnergy AC Coupled battery averages £3,800. A 10 kWh Tesla Powerwall 2 averages £7,500 fully fitted. A 13.5 kWh Powerwall 3 averages £10,500. Pricing varies sharply by region — London and South East installs run 8–12% above national average; North East and Scotland 5–10% below. Checkatrade and MyBuilder 2026 quote data confirm this regional spread.
Should I size the battery to match my evening load or my daily generation surplus?
Match your battery to the smaller of (a) the surplus your PV generates on a typical day in March–October, and (b) your evening + overnight load. UK households use roughly 6–8 kWh between 5pm and 7am — outside solar production hours. A 4 kWp PV system produces 12–18 kWh on a clear May day, leaving 6–10 kWh of surplus. A 10 kWh battery captures this fully. A 5 kWh battery is too small for high-generation summer days; a 15 kWh battery sees only partial cycling in winter. Energy Saving Trust recommends 1.5–2.5 kWh of usable storage per kWp of PV.
Does Octopus Flux or Intelligent change the maths?
Yes — substantially. Octopus Flux pays 33p/kWh peak export (4pm–7pm) and charges 8p import overnight (2am–5am), creating a 25p arbitrage independent of solar. Octopus Intelligent Go offers a 7p off-peak import rate from 23:30–05:30. For a 10 kWh battery cycling daily on the import-arbitrage strategy, that's roughly £700/yr of pure tariff arbitrage on top of the solar-uplift figure — payback drops from ~11 years to ~5 years. The calculator's standard SEG number is conservative; if you're on a time-of-use export tariff, raise the credit rate accordingly.

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