A typical London home uses around 2,700 kWh of electricity a year and needs a 3 to 4 kW system to cover it, which is 8 to 10 panels at today's 400 to 450 W each. Each panel takes about 2 m² of roof, so that is 16 to 20 m² of unshaded pitch. Homes with an electric car or a heat pump use more and need 12 to 16 panels if the roof allows. Here is how to do the sums for your own roof.
Key takeaways
- A modern panel is 400 to 450 W, so 1 kW of system is roughly 2 to 2.5 panels and 4 kW is 9 to 10 panels.
- Each panel needs about 2 m² of roof, plus a margin at the edges, so 10 panels need roughly 20 to 22 m² of usable pitch.
- In London, 1 kW of south facing panels makes around 850 to 950 kWh a year; a 4 kW system makes 3,400 to 3,800 kWh.
- A typical household uses 2,700 kWh a year; an EV or a heat pump can add 2,000 to 4,000 kWh each.
- Typical counts: 6 to 10 panels on a terrace, 8 to 14 on a semi, 12 to 20 on a detached house.
- Size to what you can use at home. Export is paid at only 5p to 15p per kWh, so oversizing has a long payback.
How many panels make a kilowatt?
System size is quoted in kilowatts peak (kWp, usually shortened to kW): the output of all the panels together in full sun. Panels in 2026 are typically rated 400 to 450 W, so 1 kW is two to two and a half panels. The old rule of "16 panels for 4 kW" dates from 250 W panels.
| System size | Panels at 400 W | Panels at 450 W | Roof area needed | London output (south facing) |
|---|---|---|---|---|
| 2 kW | 5 | 4 to 5 | about 10 m² | 1,700 to 1,900 kWh |
| 3 kW | 7 to 8 | 7 | about 15 m² | 2,550 to 2,850 kWh |
| 4 kW | 10 | 9 | about 20 m² | 3,400 to 3,800 kWh |
| 5 kW | 12 to 13 | 11 | about 25 m² | 4,250 to 4,750 kWh |
| 6 kW | 15 | 13 to 14 | about 30 m² | 5,100 to 5,700 kWh |
Output figures assume a south facing, unshaded pitch. East or west facing arrays make roughly 15 to 20% less, so add a panel or two to reach the same annual figure.
How much roof does each panel take?
A residential panel is about 1.7 to 1.9 m long and 1.1 m wide, so call it 2 m² each once the gaps are counted. Installers also leave a margin of a few hundred millimetres from the ridge, eaves and verges for wind loading. Roof windows, chimneys, soil pipes and dormers all take space out of the usable area.
To estimate your own roof: measure the width of the house, then the distance from gutter to ridge up the slope. Multiply the two for the area of one pitch, take off obstructions and edge margins, divide by 2 and round down. That is the maximum panel count for that pitch.
How many panels fit on a London terrace, semi or detached house?
Indicative counts from the roof shapes we see across London, Surrey, Kent and Hertfordshire. Every roof is different, and a survey measures the real pitch, but these show where most homes land.
| Home | Usable pitch | Typical panels | System size | Notes |
|---|---|---|---|---|
| Victorian or Edwardian terrace | 15 to 22 m² on one pitch | 6 to 10 | 2.5 to 4 kW | Rear pitch often has a dormer or roof windows; front pitch may face the street |
| 1930s semi | 20 to 30 m² per pitch | 8 to 14 | 3.5 to 6 kW | Hipped roofs limit the count on each face; two faces can be used |
| 1960s to 1980s semi or link | 20 to 28 m² | 8 to 12 | 3.5 to 5 kW | Simple gable roofs are the easiest to fill |
| Detached | 30 to 50 m² across two faces | 12 to 20 | 5 to 8 kW | Room for a larger array if usage justifies it |
| Bungalow | 25 to 40 m² | 10 to 16 | 4 to 6.5 kW | Large roof for the floor area; low scaffold |
A terrace facing south has one pitch pointing the right way. A terrace running east to west has an east and a west pitch, and using both usually beats a single south array for the number of panels you can fit. Top floor flats can take a small array on their own section of roof with the freeholder's consent; see solar panels and planning permission.
How many panels do I need for my electricity use?
Start from your annual consumption, which is on your bill or in your supplier's app. A typical UK household uses about 2,700 kWh a year. Divide by the London output per kW, around 850 to 950 kWh per kW for a south facing roof, and you get the system size that would match your annual use.
- 2,700 kWh a year (typical 2 to 3 bed): about 3 kW, or 7 to 8 panels.
- 3,500 to 4,500 kWh (larger family, home in the day): about 4 to 5 kW, or 9 to 13 panels.
- Add an electric car: an EV doing average mileage adds roughly 2,000 to 2,500 kWh a year. Another 2 to 3 kW if the car can charge in the day.
- Add a heat pump: roughly 3,000 to 4,000 kWh a year for a 3 bed home, mostly in winter when panels produce least. Solar covers hot water and the shoulder months, not January heating.
Matching annual generation to annual use does not mean self-sufficiency. Panels make most of their power between April and September and at midday; you use most of yours in the evening and in winter. Without a battery a household typically uses 30 to 50% of what it generates and exports the rest at 5p to 15p per kWh.
Should I fill the roof or size to what I use?
Extra panels are cheap at the margin, because the scaffold, inverter and electrician are already paid for. But a unit exported is worth a fraction of a unit used, so the payback on the last few panels is long. Fill the roof when you expect your use to rise, for example a car or a heat pump in the next few years, or when you are adding a battery that can store the surplus. Size to usage when the household is settled and there is no storage.
One more limit: a system above 3.68 kW per phase needs prior approval from the network operator (G99) rather than a notification after the event (G98). The MCS certified installer handles it, but it can take several weeks, so it is worth knowing early.
Getting the count right for your roof
Stay Warm surveys the roof, measures the pitches, checks shading and the fuse board, and matches the array to your consumption data. Solar is arranged and project-managed by Stay Warm and installed by accredited, MCS certified installers. Prices are in solar panels cost in London, and whether the sums stack up is covered in are solar panels worth it.
If the loft is thin or the walls are solid and bare, the house is using more energy than it needs and the panel count will be wrong. A free heat loss survey fixes the order: insulate first, then size the solar to the home you will actually have.
Frequently asked questions
How many solar panels does a 3 bed house need?
Usually 8 to 12 panels, a 3.5 to 5 kW system, depending on how much electricity the household uses and whether there is an EV or a heat pump. A typical 2,700 kWh a year is covered by about 3 kW, or 7 to 8 panels, on a south facing roof.
How many panels is a 4 kW system?
Nine to ten panels at 400 to 450 W each. Older articles say 16 because they were written for 250 W panels, which are no longer fitted.
How much roof space do 10 solar panels need?
About 20 to 22 m² of unobstructed pitch, allowing for gaps between panels and the margin installers leave at the ridge, eaves and verges. A pitch 7 m wide and 3 m from gutter to ridge is enough.
How many panels do I need to charge an electric car?
An average mileage EV uses roughly 2,000 to 2,500 kWh a year, about the same as a whole household. Add 2 to 3 kW, or 5 to 7 panels, on top of the household system, and use a solar-aware charger so the car takes the surplus in the day.
Is a bigger solar system always better?
No. Panels beyond what you can use at home mostly export at 5p to 15p per kWh, so their payback is long. Fill the roof if you plan an EV, a heat pump or a battery; otherwise size to your consumption.