Off-grid solar sizing for the British climate (no fluff)

The British solar reality

The UK is one of the least sunny populated places on earth. London averages around 1,500 sun hours a year; Manchester 1,300; Lerwick 1,100. The system you build must survive a fortnight of grey December rain with three hours of dim daylight a day. Get this right and you are genuinely off the grid; get it wrong and you are tied to an inverter generator and the same fuel suppliers you wanted to leave behind.

Region	Dec	Mar	Jun	Sep
South England (e.g. Devon)	0.9	2.8	5.4	3.2
Midlands	0.7	2.4	4.9	2.8
Wales / NW England	0.6	2.1	4.6	2.5
Central Scotland	0.5	2.0	4.7	2.4
Highlands / NI	0.4	1.8	4.5	2.2

Step 1. Honest load audit

Before you buy a single panel, list every device, its wattage, and the hours per day you actually use it. Off-grid living rewards ruthlessness. Every 100W of unnecessary draw means another £400 to 600 of panels and battery.

Load	Watts	Hours/day	Wh/day
LED lighting	30	6	180
Fridge (DC compressor)	40	8	320
Laptops + phones	60	4	240
Water pump	120	0.5	60
WiFi router + 4G	10	24	240
Misc (kettle on gas, no electric heat)			200
Total			1,240 Wh

Step 2. Panel sizing for December

Take your daily load (1,240 Wh in the example above), divide by your worst-month PSH (0.7 for the Midlands), divide again by system efficiency (0.75 is realistic for off-grid losses) to get array size:

Round up to standard panel sizes. Six 400W panels (2.4kW) is the sensible build for the example above. South Scotland or Wales might need 2.8 to 3kW. The same load in southern Spain would need just 900W.

Step 3. Battery sizing for British winters

Solar fights cloud, not just night. Plan for 3 days of autonomy in case of a winter weather front. With LiFePO4 chemistry you can safely use 80% of nominal capacity.

That 4,650Wh is a bare three-day minimum, so round up. Around 10kWh of usable battery is the sensible target for southern England, giving headroom for longer grey spells without cycling the pack too deeply, rising to 12 to 15kWh for Scotland, north Wales or anyone running a winter heat-pump-assisted load. LiFePO4 cells last 10+ years; lead-acid is half the price and a quarter of the life. False economy at this scale.

Step 4. Plan for the generator you don't want

What it costs in 2026 (UK)

System	Panels	Battery	Inverter	Total
Weekend cabin (300Wh/day)	£250	£600	£250	£1,100
2-person cabin (1.2kWh/day)	£900	£2,200	£700	£3,800
Small family home (4kWh/day)	£2,800	£6,500	£1,500	£10,800
Off-grid smallholding (8kWh/day)	£5,500	£12,000	£2,500	£20,000

UK regulation. What you can DIY

You can install your own off-grid solar (that is, a system with no connection to the National Grid) without an MCS certificate, G99 form or DNO notification. The moment you connect to the grid for export, full Smart Export Guarantee and DNO approval rules apply. Off-grid stays mercifully outside that system.

• Off-grid: no permit required, no MCS, no DNO notification.
• Standard Part P electrical rules apply for any 230V AC wiring inside the cabin.
• Battery storage should meet the PAS 63100:2024 fire-safety standard for home battery systems, and the electrical work falls under Part P; there is no simple kWh threshold that triggers building control notification on its own.
• Planning permission only if panels exceed permitted development limits on the roof (and most off-grid systems are ground-mounted).