Solar panel efficiency, finally explained simply

You read a solar panel datasheet and spot "efficiency: 22.4%". Fine. But what does that actually mean? Is more always better? And why can two 440 Wp panels post such different efficiency figures?

Here is the full picture, clearly, with no shortcuts.

What exactly is efficiency?

A solar panel's efficiency measures the share of incoming sunlight it converts into electricity. If the sun delivers 1,000 watts per square metre (the standard test condition) and your panel turns 220 W of that into power, its efficiency is 22%.

In practice it mainly determines one thing: how much area you need to produce a given amount of electricity.

• A 22% efficiency panel takes up less space for the same power than a 19% model.
• If your roof is large and well exposed, lower efficiency can be perfectly fine.

Efficiency vs power rating: do not mix them up

This is the most common confusion. The peak power in watt-peak (Wp) says how much energy the panel produces under ideal conditions. Efficiency says how well it uses the available area.

A large 500 Wp panel can have lower efficiency than a compact 420 Wp one, simply because it is physically bigger.

What drags real-world efficiency down

In real conditions (your roof, your region), the rated efficiency is never reached at 100%. Several factors come into play:

• Heat: panels lose efficiency as their temperature rises. This is where the temperature coefficient comes in (see below).
• Shading: even a small shadow on a corner of a panel can cut output significantly, especially without optimisers.
• Orientation and tilt: a south-facing roof at 30 to 35 degrees is optimal in much of Europe. Facing east or west, you lose 15 to 20%.
• Dirt and soiling: a dirty panel produces less. An annual clean is usually enough.

The temperature coefficient, the hidden indicator

Rarely shown on consumer-facing datasheets, it is nonetheless an important criterion, especially in warmer regions. It is expressed in %/°C and tells you how much output drops per degree above 25 °C.

A good TOPCon panel typically posts -0.29%/°C. A standard PERC sits around -0.35%/°C. The difference can amount to several hundred kWh over 25 years on a ten-panel system.

How to compare intelligently

Here is the right method, step by step:

1. Estimate your available area: how many square metres of well-exposed roof?
2. Work out your target total power, usually 3 to 9 kWp for a single home.
3. Compare the total installed cost (hardware plus labour plus warranties), not just the price per panel.
4. Check the performance warranty: a good panel should retain at least 87% of its output after 25 years.

In short

Efficiency is a useful indicator, but it must be weighed against price, roof area and warranties. For the vast majority of residential systems, a panel at 21 to 23% efficiency with a solid 25-year warranty will do the job very well.

Want to compare real models side by side? Our solar panel comparison tool is built for exactly that.

FAQ

What efficiency is good for a residential panel? Between 20 and 23% in 2026. Below 19%, the panel is dated or entry level. Above 23%, you are in premium territory (HJT, IBC).

Does efficiency decline over time? Yes, slightly. This is annual degradation: expect 0.3 to 0.5% per year for a good panel. After 25 years it should still produce 85 to 90% of its original output.

Can you improve the efficiency of an existing panel? No, efficiency is fixed by the technology. But you can optimise production: regular cleaning, power optimisers where there is shading, and proper inverter sizing.