Solar Generator Sizing Calculator (UK)
Size a portable solar generator for UK camping, caravanning and power-cut backup. Calculate battery Wh, inverter W and solar panel W with UK sun-hour data.
Prepared by the Solar Calculator HQ editorial team. Review our formula, source and limitation standards.
Solar Generator Sizing Calculator
How to use this calculator
The UK solar generator calculator above converts a load profile into the three figures every portable power station spec sheet quotes: battery capacity in Wh, inverter continuous power in W, and solar panel input in W. Enter your average load, daily runtime, peak surge load and UK peak sun hours — outputs recalculate instantly.
- Average load (W) — sum the running wattage of everything running at once. A laptop runs 50 W, a 12 V compressor fridge 50–80 W, LED lighting 5 W per bulb, a CPAP 30–60 W.
- Runtime per day (h) — actual run-time, not 24 h. A fridge cycles about 8 of 24 hours; CPAP runs the 8 hours you sleep.
- Days of autonomy — UK weather is rarely sunny for two consecutive days outside July–August. Plan on 2 days minimum, 3 in winter or for prepping.
- Peak surge load (W) — the largest single load. A 1,000 W kettle, 1,500 W hair dryer, fridge compressor start 1,500–2,000 W. The calculator multiplies by 1.25 to size the inverter.
- Peak sun hours — UK annual averages from PVGIS / Met Office: 2.6 for England/Wales, 2.3 for Scotland, 2.4 for Northern Ireland. Winter (Nov–Feb) drops to 0.8–1.5 across the country.
How the math works
Three first-principles formulas:
Daily energy:
daily_Wh = avg_W × hoursBattery capacity (Wh):
battery_Wh = daily_Wh × autonomy / (DoD × inverter_eff)For 60 W × 6 h × 2 days at 90% DoD and 90% inverter efficiency: 720 / 0.81 = 889 Wh. Round up to the nearest commercial size — 1,000 Wh class units (Jackery Explorer 1000 v2, EcoFlow Delta 2, Bluetti AC70) are the UK sweet spot.
Inverter continuous (W):
inverter_W = peak_W × 1.25Solar input (W):
solar_W = battery_Wh / (PSH × charge_eff)At UK average 2.6 PSH and 85% MPPT charge: 889 / 2.21 = 402 W of panel input. Plan a 400 W folding array (e.g. four 100 W panels or two 200 W).
What a solar generator costs in the UK (Q1 2026)
Pricing from Jackery UK, EcoFlow UK, Bluetti UK, Anker SOLIX UK and Argos / Halfords portable power categories:
| Use case | Battery Wh | Inverter W | Solar input W | Total kit |
|---|---|---|---|---|
| Weekend camping (phone + lights + CPAP) | 300–500 | 600–1,000 | 100–200 | £350–650 |
| Caravan / campervan (12 V fridge + appliances) | 500–1,200 | 1,000–1,800 | 200–400 | £600–1,400 |
| Power-cut essentials (fridge-freezer + lights + router) | 1,000–2,000 | 1,800–2,400 | 300–500 | £1,200–2,500 |
| Whole-home critical loads (fridge + central-heating pump + boiler controls) | 3,000–5,000 | 3,000–5,000 | 800–1,200 | £2,800–5,500 |
| Off-grid expandable (modular battery packs) | 6,000–15,000 | 4,000–7,200 | 1,200–2,800 | £5,500–15,000 |
VAT note: portable plug-and-play units are billed at 20% VAT. Fixed solar + battery installations via an MCS-certified installer qualify for 0% VAT under Schedule 8 Group 23 until 31 March 2027.
Where most UK solar generator buyers under-size
Three patterns from the UK Caravan and Motorhome Club, Camper UK and EcoFlow UK community forums:
- Sizing on running wattage, ignoring surge. A 1,000 W inverter handles a 700 W kettle but trips on a fridge compressor start. UK fridge-freezers commonly hit 1,500–2,000 W surge.
- Solar input sized for Spanish PSH, not British. A 200 W folding panel that fully recharges a 1 kWh unit on the Costa del Sol covers about 40% of that in a UK November.
- Mixing chemistries. LiFePO₄ packs (current Jackery v2, EcoFlow Delta 2/Pro, Bluetti AC range) tolerate 3,000–6,000 cycles at 90% DoD. Older NMC-based units cycle 500–1,000 times. For daily UK off-grid use, LiFePO₄ is the right pick.
Pair this with the off-grid system calculator, battery bank calculator, and campervan solar calculator
This calculator gives the headline three numbers for a portable solar generator. The off-grid calculator scales the same logic to a fixed cabin or summer house. The battery bank calculator drills into amp-hour capacity at 12 V / 24 V / 48 V for DIY builds. The campervan calculator picks roof-mount panel wattage for caravan and motorhome installs.
Sources
- Met Office UK climate averages — sunshine hours by region
- Energy Saving Trust — solar PV and batteries — UK solar advice for homeowners
- MCS — Microgeneration Certification Scheme — accredited installer scheme
- Solar Energy UK — industry data and standards
- Ofgem — Customer interruption data — average UK power cut duration
- PVGIS — JRC EU solar resource tool — UK peak sun hours by postcode
- HMRC VAT Notice 708/6 — Energy-saving materials — 0% VAT scheme details