Two panels can both be rated at 400 watts but be very different sizes. The smaller one is more efficient — it squeezes the same power out of less area. That's what efficiency measures: the share of the sunlight landing on the panel that it turns into electricity.
The formula
Efficiency compares the panel's output to the sunlight hitting it. At standard test conditions, full sun delivers 1000 watts per square metre. So a panel's efficiency is its rated watts divided by the sunlight on its area: efficiency (%) = W ÷ (area × 1000) × 100. A 400 W panel measuring 1.75 m × 1.10 m has an area of about 1.93 m², so it receives 1930 W of sunlight and converts 400 of them — about 21% efficient.
What counts as good
Most modern panels land between 17% and 22%. Premium monocrystalline panels reach the top of that range, mainstream panels sit around 18–20%, and older or thin-film types can be 10–15%. Higher isn't always necessary — it matters most when roof space is limited and you need maximum power per square metre.
Efficiency vs power density
This tool also shows power density (watts per square metre), which is just efficiency expressed as raw output per area. Both tell the same story: how much power you get from a given space. If your roof is the constraint, use the roof area to system size calculator to see how many watts will fit.
Related tools
Pair this with the solar panel calculator to size an array, the roof area to system size calculator to fit it to your roof, and the temperature derating calculator to see how heat cuts real output. To compare panel technologies, see mono vs poly vs thin-film.
Frequently asked questions
How do you calculate solar panel efficiency?
Divide rated watts by area × 1000, then × 100. The 1000 is standard-test sunlight in W/m². A 400 W panel of ~1.9 m² is about 21% efficient.
What is a good solar panel efficiency?
Most modern panels are 17–22%. Premium mono reaches 20–22%, mainstream ~18–20%, older or thin-film 10–15%.
Why does a smaller panel with the same wattage have higher efficiency?
Efficiency is power ÷ area. Same watts in a smaller panel means more power per square metre, so higher efficiency — which fits more power on a limited roof.