Off-Grid Solar System Calculator (UK)
Free UK off-grid solar calculator. Sizes PV array (kWp), battery bank (kWh), and inverter (kW) from daily kWh load using PVGIS irradiance data and MCS-aligned design factors.
Off-Grid Solar System Calculator
How to use this calculator
The UK off-grid calculator above takes your daily kWh load and PVGIS-derived peak sun hours and returns three numbers: PV array size in kWp DC, battery bank capacity in kWh nameplate, and inverter continuous kW rating. All values are in metric / SI units and align with MCS MIS 3002 sizing methodology.
- Daily energy use (kWh) — your average daily AC load. Pull it from a recent Ofgem-regulated supplier bill (monthly kWh ÷ 30) or sum nameplate watts × hours-on. Ofgem’s TDCV (Typical Domestic Consumption Values) puts the medium UK household at 2,700 kWh/year ≈ 7.4 kWh/day electric only. Off-grid UK cottages typically run 4-8 kWh/day because high-draw loads (oven, immersion, heat pump) get swapped for LPG, oil, or wood.
- Peak sun hours (h/day) — annual average kWh/m² hitting an optimally tilted module. PVGIS-SARAH3 averages: Cornwall 2.9, London 2.6, Manchester 2.5, Edinburgh 2.4, Inverness 2.3, Stornoway 2.2. Default 2.6 covers southern England; bump to 2.4 for Scotland.
- Days of autonomy — UK winter cyclonic weather routinely produces 5-7 days of <30% normal PV output. 3 days minimum for southern England; 4 days for Scotland and the Western Isles. Pair with a 5-7 kVA Honda/Stephill diesel generator for backup.
- Battery chemistry — LiFePO₄ (BYD, Pylontech, Fogstar, Solax) is now standard. AGM (Victron, Rolls) still appears in budget Welsh smallholdings but the £/kWh-cycled is poor.
- Peak instantaneous load (W) — usual UK suspects: kettle 3,000 W, electric shower (avoid off-grid!) 8,500 W, immersion 3,000 W, washing machine 2,200 W, microwave 850 W. Most UK off-grid systems run a 3-5 kW Victron Multiplus-II continuous.
How the math works
The UK calculator follows the methodology in MCS MIS 3002 (PV system design) extended for stand-alone systems per IEC 62124 and the Energy Saving Trust’s Solar Photovoltaic Systems for Stand-Alone Use guide:
PV array (kWp DC):
kWp = daily_kWh / (peak_sun_hours × derate)Derate 0.77 covers BS 7671 inverter losses, MPPT charge-controller losses, DC cable losses, soiling, and 2-3% module mismatch — the same Performance Ratio MCS uses for grid-tied UK systems but with the inverter loss applied at the battery side instead of the grid side. With UK defaults of 6 kWh/day at 2.6 PSH: kWp = 6 / (2.6 × 0.77) = 3.0 kWp DC. This typically maps to 7-8× 400 W panels (Trina Vertex S+, JA Solar, Longi Hi-MO X6).
Battery bank (kWh nameplate):
usable_kWh = daily_kWh × autonomy_days
nameplate_kWh = usable_kWh / (DoD × battery_round_trip_eff)6 kWh × 3 days = 18 kWh usable; nameplate = 18 / (0.90 × 0.95) = 21.1 kWh of LiFePO₄. That maps to about 4× Pylontech US5000 (4.8 kWh nameplate each) or 1× BYD HVS 22.0.
Inverter (kW continuous):
kW = peak_load_W × 1.25 / 1000A 3,000 W kettle plus 500 W background load = 3,500 W peak × 1.25 = 4.4 kW. The Victron Multiplus-II 5kVA/48V is the standard answer for UK residential off-grid.
What an off-grid system actually costs in the UK (Q1 2026)
Pulling installed-cost ranges from MyBuilder, Checkatrade, Solar Energy UK members, and Wind & Sun (Ledbury) and Mr Solar (Liphook) off-grid specialists. All prices include the 0% HMRC VAT relief running through March 2027:
| System size | PV kWp | Battery kWh | Total installed | Notes |
|---|---|---|---|---|
| Cottage / outbuilding | 1.5-2.5 | 8-14 (Li) | £8,500-13,500 | Pylontech + 3 kW Victron |
| Small full-time off-grid (6 kWh/day) | 3-4 | 18-25 (Li) | £14,000-22,000 | BYD HVS + Multiplus-II 5kVA |
| Full off-grid house (10 kWh/day) | 4-6 | 25-40 (Li) | £22,000-35,000 | + Honda 5 kVA generator backup |
| Croft / smallholding (Highlands & Islands) | 5-7 | 40-60 (Li) | £30,000-45,000 | Sized for 4-day autonomy |
Add a wood stove for primary heating and budget £1,200-2,500 for a Stephill or Honda dual-fuel generator (essential north of Manchester for December cover).
Where most UK off-grid systems get under-sized
Three common mistakes from Solar Energy UK and Wind & Sun customer post-mortems:
- PVGIS averages used in winter. The 2.6 PSH headline figure is a yearly average; December alone runs 0.8-1.0 PSH. A bank sized to year-round mean will run flat by 2 January. Use either 4-day autonomy at the annual PSH or recompute at the worst-month PSH.
- Electric showers and kettles in the load list. Electric showers at 8.5 kW will trip even a 5 kVA inverter on start. Switch to LPG instant-water, gas hob, and gas oven for off-grid; reserve electric for fridge, freezer, lights, electronics, and small AC tools.
- Heat pump on solar. UK ASHPs draw 6-8 MWh/year; an off-grid PV array large enough to cover that is wildly uneconomic. Heat with wood/oil/LPG and reserve PV for the small loads.
Pair this with the battery bank calculator, charge-time calculator, and wire-size calculator
This UK off-grid sizing calculator gives the three headline numbers. The battery bank calculator drills into Ah at your chosen system voltage (24 V or 48 V). The charge-time calculator confirms recovery time after a 3-day overcast stretch. The wire-size calculator picks BS 7671-compliant CSA in mm² for the DC runs between array, charge controller, bank, and inverter.
Sources
- PVGIS-SARAH3 (European Commission JRC) — UK peak-sun-hour and irradiance data by postcode
- MCS MIS 3002 — PV System Design — UK installer standard for PV (extended methodology used for off-grid)
- Energy Saving Trust — Solar PV Stand-Alone Systems guide — UK off-grid sizing guidance
- Solar Energy UK — installer and product directory
- HMRC VAT Notice 708/6 — 0% VAT on solar and storage to March 2027
- Ofgem Typical Domestic Consumption Values (TDCV) — UK household kWh benchmarks
- BS 7671 (IEE Wiring Regulations) and G98/G99 — UK electrical and grid-connection standards