How much electricity do solar panels generate?
There's a simple rule of thumb that answers this for any UK home, and once you know it you can sanity-check any quote you're given. A solar system generates roughly 900 to 1,050 units (kWh) a year for every kWp installed. So a typical 4 kWp system makes around 3,400 units a year, which is a big chunk of a normal household's electricity. Here's how that breaks down, what moves it, and how it compares with what you actually use.
The rule: kWh per kWp
Solar systems are sized in kWp, which stands for kilowatts-peak, the output under ideal test conditions. The useful number is how many real units that translates to over a year. Across the UK that's about 900 to 1,050 kWh per kWp, varying mainly by region: sunnier in the South West, lower in northern Scotland. Multiply your system size by that figure and you have your annual generation.
| System size | Panels (approx) | Annual generation (approx) |
|---|---|---|
| 3 kWp | ~8 | ~2,700 kWh |
| 4 kWp | ~10 | ~3,400 kWh |
| 5 kWp | ~13 | ~4,300 kWh |
| 6 kWp | ~15 | ~5,100 kWh |
These assume a reasonable roof. A perfect south-facing array sits at the top of each range; a shaded or east-west roof sits lower.
How that compares to what you use
A typical UK household uses somewhere around 2,700 to 3,500 kWh of electricity a year, more with an EV or a heat pump. So a 4 kWp system generates roughly as much as a typical home uses across the year. The catch is timing: the panels make most of their power in the middle of summer days, while you use power year-round, mornings and evenings included. That mismatch is why you don't simply "go off bills", and why self-consumption matters so much in solar panel payback.
What changes your output
- System size. More kWp means more generation, within the limit of your roof space, covered in is my roof suitable for solar.
- Orientation and pitch. South-facing at around 35 degrees is the baseline; east-west produces 10 to 20% less, as in south vs east-west.
- Location. The South West gets meaningfully more sun over a year than northern Scotland.
- Shading. Even partial shade cuts output disproportionately, explained in solar panels and shading.
- Season. A summer month can produce close to ten times a December one, the pattern in solar in a UK winter.
Daily and seasonal shape
On a bright summer day a 4 kWp system might make 20 to 30 units; on a dull December day it might make one or two. Output peaks around midday and tapers toward morning and evening. This is why the annual total, not any single day, is the figure that decides whether solar pays, and why a battery or a good export tariff helps you make the most of the summer surplus.
The detail most people miss
Generation and savings are not the same number. A system that generates 3,400 units doesn't save you the value of all 3,400, because some is exported at the lower rate rather than used at the higher one. The headline "it generates as much as you use" is true and useful, but the money depends on how much of that generation you actually use yourself. That's the distinction at the heart of is solar worth it.
Frequently asked questions
About 3,400 kWh a year in a typical UK location, using the rule of roughly 850 to 950 kWh per kWp for a reasonable roof. A perfect south roof can exceed this; a shaded one falls below.
A single 400 to 450W panel produces very roughly 340 to 400 kWh a year in the UK, depending on orientation, shading and location.
Over a year, a 4 kWp system generates about as much as a typical home uses, but not at the same times, so you still draw from the grid at night and in winter. It offsets a large share of your usage rather than replacing the grid.
It varies hugely by season: roughly 20 to 30 units on a bright summer day for a 4 kWp system, down to one or two on a dull winter day. The annual total is the meaningful figure.