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Solar Self-Consumption Calculator (UK)

Free 2026 calculator for UK solar self-consumption and SEG savings. See how a 5–10 kWh battery lifts self-use from 30% to 75% under Octopus Outgoing, British Gas SEG, and post-FIT export tariffs.

Solar Self-Consumption Calculator (SEG-Aware)

Self-consumption (no battery)
30%
Self-consumption (with battery)
77%
Self-sufficiency (no battery)
31.4%
Self-sufficiency (with battery)
80.6%
Annual savings (no battery)
£416
Annual savings (with battery)
£687
Battery uplift per year
£271
How the math works
Annual generation: 3,035 kWh
Self-consumption (no battery): 911 kWh · Self-consumption (with battery): 2,338 kWh

How the calculator works

The UK solar self-consumption calculator estimates two ratios — self-consumption (the share of generation used on site) and self-sufficiency (the share of household demand met by solar) — and shows the annual £ uplift a battery delivers under your Smart Export Guarantee (SEG) tariff.

Plug in seven figures and the tool returns annual generation, no-battery and with-battery self-consumption percentages, savings, and the battery uplift in £/year.

  1. System size (kWp) — DC nameplate. MCS 2024 installer survey median for new residential UK installs is 4.4 kWp.
  2. Peak sun hours/day — annual UK average ranges 2.4 (Glasgow) to 2.9 (Penzance). Energy Saving Trust uses 2.7 as the England-wide planning figure.
  3. Annual usage (kWh) — your 12-month total. Ofgem TDCV 2026 puts the medium-use household at 2,700 kWh electricity.
  4. Import rate (p/kWh) — your supplier’s unit rate. Ofgem April 2026 default tariff cap is 27.03p/kWh for a direct-debit dual-fuel customer.
  5. SEG export rate (p/kWh) — what your supplier pays for exports. Octopus Outgoing Fixed 15p, EDF Export Variable 5.6p, British Gas Export & Earn Flex 6.4p, OVO SEG 4p as of Q1 2026.
  6. Battery capacity (kWh) — usable. GivEnergy AC Coupled 5.2 kWh, Tesla Powerwall 3 13.5 kWh, Sonnen eco 9 (9.8 kWh).
  7. Daytime overlap (%) — your unaided self-consumption assumption. Use 30% default, 40% if one occupant works from home, 45%+ for retired households or heat-pump homes.

How the math 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)
self_consumption_pct = self_kWh / annual_prod × 100
self_sufficiency_pct = self_kWh / annual_use × 100
import_cost          = (annual_use − self_kWh) × import_rate
export_credit        = (annual_prod − self_kWh) × SEG_rate
bill_with_solar      = max(0, import_cost − export_credit)
annual_savings       = annual_use × import_rate − bill_with_solar
battery_uplift       = with_battery_savings − no_battery_savings

The 0.77 system performance ratio comes from IEC 61724 and matches MCS Yellow Book derating assumptions for residential UK PV. The 0.92 × 0.85 battery factor reflects lithium-iron-phosphate round-trip efficiency (92%) and usable depth-of-discharge (85%) — conservative figures applicable to GivEnergy, Tesla, and Sonnen LFP systems.

Worked example: 4 kWp PV in Manchester, Octopus Flux

  • System: 4 kWp, 2.6 PSH (NW England, MCS Yellow Book), 27p import, 15p Octopus Outgoing Fixed export
  • Annual generation: 4 × 2.6 × 365 × 0.77 = 2,923 kWh/yr
  • Annual usage: 3,500 kWh (medium-high UK household)
  • No battery: self_consumed = min(3500, 2923×0.30) = 877 kWh
    • Self-consumption 30.0% · Self-sufficiency 25.1%
    • Imports 2,623 × 27p = £708 cost · Exports 2,046 × 15p = £307 credit
    • Bill with solar = max(0, £708 − £307) = £401 · Bill without solar = £945
    • Savings £544/yr
  • With 5 kWh GivEnergy battery: battery_capture = 5 × 365 × 0.92 × 0.85 = 1,428
    • self_consumed = min(3500, 877 + 1428, 2923) = 2,305 kWh
    • Self-consumption 78.9% · Self-sufficiency 65.9%
    • Imports 1,195 × 27p = £323 · Exports 618 × 15p = £93
    • Bill with solar = max(0, £323 − £93) = £230
    • Savings £715/yr — battery uplift £171/yr

The GivEnergy 5.2 kWh battery at £4,200 installed (post-0% VAT relief) pays back in 24 years on Octopus Outgoing Fixed. On a 4.5p British Gas SEG tariff, where every shifted kWh is worth 22.5p, the same battery uplift jumps to £321/yr and payback drops to 13 years — still long, but within the 10-year warranty and 15-year practical battery life.

Worked example: 4 kWp PV in Bristol, EDF Export Variable

  • System: 4 kWp, 2.8 PSH, 27p import, 5.6p EDF SEG
  • Annual production: 3,148 kWh
  • Annual usage: 2,700 kWh (Ofgem TDCV medium)
  • No battery: self 944 kWh (30%), savings £505/yr
  • With 5 kWh battery: self 2,372 kWh (75.4%), savings £828/yr
  • Battery uplift £323/yr — payback 13 years on £4,200 install

The economic case for batteries in the UK depends almost entirely on the SEG export rate. A like-for-like 5 kWh battery is worth nearly twice as much on a 4-6p SEG (British Gas, OVO, EDF) as on a 15p Octopus Outgoing tariff — because the gap between import and export is what the battery monetises. Counter-intuitively, switching to a generous SEG tariff makes a battery less economically attractive.

Self-consumption levers that don’t require a battery

If a £4,000+ battery isn’t in the budget, four free or low-cost levers raise self-consumption by 5–15 percentage points each:

  1. Diverter to immersion — an iBoost+, Solar iBoost, or Eddi diverter sends surplus PV to your hot water cylinder instead of exporting it. Typical capture is 800–1,400 kWh/year for a 4 kWp system, worth £180–£300/yr on a 21.7p gap. Diverters cost £350–£550 fitted.
  2. Heat pump scheduling — modern ASHP controls (Mitsubishi MELCloud, Daikin Madoka, Vaillant Sensonet) allow PV-aware schedules. Setting the higher-temperature charge cycle to 11am–3pm overlaps fully with peak solar output.
  3. EV midday charging — Octopus Intelligent Go, Octopus Flux, and OVO Charge Anytime are tariffs that already optimise EV charging for low rates; for those on a flat rate, scheduling charging to start at 12pm shifts 8–10 kWh/day from grid to solar.
  4. Smart-plug load shifting — running dishwashers, washing machines, and tumble dryers on solar-aware delay timers (Hive Active Plug, Tapo P110) lifts self-consumption by 5–8 percentage points for the price of a £15 smart plug.

Model the bill impact in our solar feed-in tariff calculator (export-revenue focus) and the battery-specific uplift in the solar battery ROI calculator.

Self-consumption under future SEG changes

SEG, established by Ofgem in January 2020 to replace the closed Feed-in Tariff, has no mandatory minimum rate — only that licensed suppliers with >150,000 customers must offer one. Ofgem’s 2024–25 SEG review concluded that the SEG market is functioning competitively and no rate floor will be introduced. However, the regulator flagged two trends:

  • Time-of-export tariffs (Octopus Outgoing Agile) now offer half-hourly pricing pegged to wholesale day-ahead. These reward batteries that can discharge into the 4–7pm system peak rather than just shift self-consumption.
  • Export caps — several SEG tariffs now cap paid exports at 5,000–10,000 kWh/year, encouraging right-sizing rather than oversizing.

Both trends raise the economic value of self-consumption optimisation relative to passive 1:1-style export. The Energy Saving Trust’s 2025 storage guidance recommends sizing batteries at 1.0–1.5 × daily evening load (typically 5–8 kWh usable), and prioritising appliance scheduling before stretching for a larger battery.

Sources

  • Ofgem, Default Tariff Cap Decision April 2026 and Typical Domestic Consumption Values 2026.
  • Energy Saving Trust, The Clean Energy Revolution 2025 storage guidance.
  • MCS, Solar PV Yellow Book (Issue 4) — derating and self-consumption assumptions.
  • Department for Energy Security and Net Zero (DESNZ), Smart Export Guarantee Annual Report 2024–25.
  • HMRC VAT Notice 708/6, zero-rate energy-saving materials (Feb 2024 batteries inclusion).
  • Octopus Energy, British Gas, EDF, OVO published SEG tariff terms (Q1 2026).
  • IEC 61724-1:2017 Photovoltaic System Performance.

Need to model the full SEG export revenue or compare batteries vs no-battery payback head to head? Use our solar feed-in tariff calculator and solar panel payback calculator.

Frequently asked questions

What self-consumption percentage should a UK home expect without a battery?
Energy Saving Trust 2024 field data across 1,200 monitored UK PV installs shows unaided self-consumption averages 28–35% for a typical 4 kWp array on a household consuming 2,700–3,500 kWh/year. Households where one occupant works from home reach 35–40%; commuter households with empty daytime properties sit at 22–28%. The MCS Yellow Book gives 30% as the planning default for SAP and EPC self-consumption assumptions.
How much does a battery raise self-consumption in the UK?
Adding a 5 kWh battery typically lifts self-consumption from 30% to 55–65%, and a 10 kWh battery to 70–80%. The marginal value is sharpest under the Smart Export Guarantee (SEG): Octopus Outgoing pays 15p/kWh export against a 27p import (April 2026 Ofgem price cap), so every shifted kWh is worth 12p more. EDF, OVO, and British Gas SEG tariffs average 5–8p — there, every shifted kWh is worth 19–22p more, making battery economics significantly better.
Are batteries worth it under the current SEG export rate?
It depends entirely on which SEG tariff your supplier offers. On Octopus Outgoing Fixed at 15p/kWh, the gap between import and export is 12p — a 5 kWh battery cycling daily captures roughly 1,427 kWh/year of would-be exports, worth £171/year in extra savings. On a standard 4.5p British Gas SEG rate, the same 1,427 kWh shift is worth £321/year. At £4,500–£6,000 installed for a Tesla Powerwall or GivEnergy battery, payback runs 14–35 years depending on the SEG tariff and household load profile.
Does the 0% VAT relief still apply to batteries in 2026?
Yes. The HMRC zero-rate VAT relief on residential battery storage (introduced February 2024) runs until 31 March 2027. This applies to batteries installed alongside solar AND retrofitted standalone, but only for domestic premises. A typical 5 kWh GivEnergy or Tesla Powerwall install drops from £6,000 incl 20% VAT to £5,000 — about 17% off the headline price. After March 2027 the policy is scheduled to revert to 20% VAT pending HMRC review.
Self-consumption vs self-sufficiency — which matters more in the UK?
Self-consumption (production used onsite) matters more under SEG because the export credit is well below the import rate — every percentage point gained is worth 19–22p per kWh of generation. Self-sufficiency (load met by solar) matters more if you're considering going off-grid or installing a hybrid inverter for blackout backup. For typical UK households on SEG, self-consumption is the right metric to optimise; SAP 10 and the MCS standard both use self-consumption (not self-sufficiency) in their PV yield assumptions.

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