What is kWp and how is it different from kWh?
kWp (kilowatt-peak) is the size or maximum output of a solar system, while kWh (kilowatt-hour) is the amount of electricity it actually generates or you actually use over time. In the UK, each 1 kWp of panels produces roughly 850 kWh of electricity a year. So a 5 kWp system generates about 4,000 kWh annually.
A simple way to remember it
- kWp = the size of the system (how much it can produce at peak).
- kWh = the energy delivered over time (what shows up on your bill).
- kW = power at a moment in time (how fast energy is used or made).
- Rule of thumb: UK kWh per year ≈ kWp × 850.
Why it matters when buying solar
Installers quote system size in kWp, but your savings depend on kWh. To size a system, divide your annual electricity use (in kWh, on your bill) by about 850 to get the kWp you could justify. For example, 4,500 kWh a year divided by 850 is roughly 5 kWp.
Frequently asked questions
- How many kWh does a 4 kWp system produce?
- Around 3,400 kWh a year in the UK on average, varying with region, roof direction and shading.
- Is a bigger kWp always better?
- Only up to your usage and roof space. Oversizing means exporting more at the lower SEG rate rather than saving at the higher import rate.
Related questions
Indicative estimates based on UK average data. Last updated 27 May 2026.