Guide · Fundamentals
Solar sizing: how many panels do you need?
Start from what you use, not from what fits on the roof. The same method works for a house, a field, a narrowboat and a van.
Sizing a home system
1. Find your yearly use
Your annual statement shows it in kWh. Ofgem's typical household uses 2,500 kWh of electricity a year. A home with a heat pump or an electric car uses far more.
2. Estimate what a panel produces
In the UK, each 1 kWp of south-facing panels produces about 850 to 1,000 kWh a year: less in northern Scotland, more on the south coast. Modern panels are around 430 to 475 W each, so 4 kWp is nine or ten panels.
| Array | Panels | Yearly output | Suits a home using |
|---|---|---|---|
| 3 kWp | 7 | About 2,700 kWh | Up to 2,500 kWh |
| 4 kWp | 9 or 10 | About 3,600 kWh | 2,500 to 4,000 kWh |
| 6 kWp | 13 or 14 | About 5,400 kWh | 4,000 kWh and above |
Output at 900 kWh per kWp. Panel counts assume 450 W panels.
3. Allow for direction and shade
East or west-facing roofs produce roughly 15 to 20% less than south-facing ones, spread across more of the day. That often suits a household well. Shade from a chimney or tree matters more than direction, so ask the installer for a shading assessment.
4. Remember the seasons
A December day produces roughly a fifth of a June day. Solar will cover most of your summer use and a small part of your winter use, whatever the size.
5. Check the connection rules
Inverters up to 3.68 kW on a single-phase supply can be fitted first and notified to the network operator afterwards. Larger inverters need the operator's approval before installation. Your installer handles both.
Small ground-mounted arrays
If the roof is shaded, listed, thatched or facing the wrong way, panels on a frame in the garden or paddock can do better. You choose the direction and the tilt, and cleaning is easy.
- Tilt and direction. Due south at 30 to 40 degrees gives the most across a year. A steeper tilt gives more in winter.
- Cable run. The trench back to the house is often the largest extra cost. Keep the array as close as shade allows.
- Fixings. Ground screws or ballast frames avoid concrete and can be removed later.
- Planning. In England a first stand-alone array can usually be installed without planning permission if it is no more than 9 square metres with no side longer than 3 metres, no higher than 4 metres and at least 5 metres from the boundary. Conservation areas and listed buildings have tighter rules. Check with your council before you order.
Nine square metres is about four panels, or 1.8 kWp. That produces around 1,600 kWh a year, which is a useful share of a typical home's use.
Narrowboats and campervans
On a boat or van you are sizing a 12V system that has to refill a battery every day. Work in watt-hours.
1. Add up a day's use
A 12V compressor fridge, LED lights, the water pump and phone charging use about 650 Wh a day. A diesel heater and a laptop add about 850 Wh more. The system finder will add up your own list.
2. See what the roof can give
Panels on a boat or van lie flat, which costs output, most of all in winter.
| Flat-mounted panels | Summer day | Midwinter day |
|---|---|---|
| 100 W | 300 to 400 Wh | 30 to 60 Wh |
| 200 W | 600 to 800 Wh | 60 to 120 Wh |
| 400 W | 1,200 to 1,600 Wh | 120 to 240 Wh |
Rough UK figures for unshaded panels. Mooring under trees or parking in a wood can halve them.
So 200 W covers the essentials from spring to autumn. Nothing that fits on a roof covers them in December. In winter you will rely on the engine, a shoreline or a generator, and a battery large enough to go several days between charges.
3. Choose the controller and battery
- An MPPT charge controller gets noticeably more from panels than a basic PWM one. A Victron SmartSolar 100/30 handles up to 440 W of panels on a 12V system and costs about £95 to £130.
- A 280Ah lithium battery (3.6 kWh) runs the essentials for about five days with no charging.
- For mains sockets you need an inverter, sized for your largest appliance.
Off-grid cabins
With no grid to fall back on, size for winter and decide early what the generator is for.
- List your loads and total a winter day's use. Cook and heat with gas or wood if you can: electric heat empties a battery quickly.
- Size the battery for two or three days of that use, so a run of dull weather does not mean the generator every evening.
- Fit as much solar as the site allows. Panels are the cheapest part. Tilt them steeply, 50 to 60 degrees, to favour the low winter sun.
- Plan the top-up. An inverter-charger with a generator input can start charging automatically when the battery runs low.
A common core for a small cabin is a 48V inverter-charger of about 4 kW with a 16 kWh battery. At current prices those two parts cost about £2,400 before panels, controller and fitting.