Solar panel power and yield: kWp, kWh explained

When comparing photovoltaic quotes, the units pile up: watt-peak, kilowatt-peak, kilowatt-hours, efficiency percentages… Yet behind this technical vocabulary lies a simple logic. Understanding what each figure means saves you from nasty surprises and helps you judge an installation on what really matters: the energy it will produce in your home, year after year.

Power, production, efficiency: three distinct things

These three notions are often confused, even though they describe different realities. Power tells you what a panel can deliver at most. Production tells you what it actually generates over the year. Efficiency tells you how effectively it turns light into electricity. A panel can be very powerful without being very efficient, and vice versa.

The kWp: peak power

The kilowatt-peak (kWp) measures the maximum power of an installation under standardised lab test conditions: irradiance of 1,000 W/m², a cell temperature of 25 °C and a reference light spectrum. It’s a comparison value, not a promise of permanent production: your roof only reaches these ideal conditions in brief moments. The kWp plays the role of an engine’s displacement — it states the potential, not the actual mileage.

At the scale of a single panel, we talk in watt-peak (Wp): a modern residential panel commonly shows between 400 and 450 Wp. Nine to ten panels of this range thus form an installation of about 4 kWp, a typical size for a family home.

The kWh: the energy actually produced

The kilowatt-hour (kWh) is the unit of energy, the one that appears on your electricity bill. It’s what your panels actually produce, hour after hour, depending on the light received. The useful question is therefore not only “how many kWp?” but “how many kWh per year?”.

This ratio of about 950 kWh per kWp per year is the reference figure in Belgium. It already factors in our climate, the alternation of seasons and normal operation. It’s what lets you turn an installed power into an estimate of concrete savings.

Cell efficiency: the conversion effectiveness

A panel’s efficiency is the percentage of the light energy received that it manages to convert into electricity. Current residential panels show a cell efficiency in the order of 20 to 22 %: of all the light that hits the panel, about one fifth becomes usable current. The rest is reflected or dissipated as heat.

A higher efficiency doesn’t necessarily mean “better” in absolute terms, but it becomes decisive when roof surface is limited. At equal power, a more efficient panel takes up less space: if your roof is small or partly shaded, aiming for good efficiency lets you install more kWp on the available surface, and therefore produce more.

What makes real production vary

Between the rated power and the energy actually produced, several factors come into play. Knowing them helps explain why two installations of the same power may not produce the same thing.

Orientation and tilt

This is the most important lever after power itself. In Belgium, the ideal is a roof facing due south, tilted at around 30 to 35°: that’s the configuration that captures the most energy over the year. An east-west orientation remains very relevant — it produces a little less in total but spreads production over the morning and end of day, which often matches a household’s consumption better. A north-facing exposure, on the other hand, clearly reduces annual yield.

Shading

A chimney, a tree, a neighbouring building or even an antenna can cast a shadow that penalises production well beyond the shaded surface. On some setups, shade on a single panel can drag down the yield of a whole string. That’s why shading analysis is part of a serious sizing, sometimes accompanied by power optimisers.

Temperature

Here’s the great photovoltaic paradox: the panel produces thanks to light, not heat. Beyond a cell temperature of 25 °C, its efficiency declines slightly. In practice, a bright but cool spring day is often more productive than a heatwave day in the middle of August. Good ventilation under the panels limits this effect.

Soiling and ageing

Dust, pollen, droppings or dead leaves slightly reduce production as they accumulate on the glass. At our latitudes, rain is most often enough to naturally clean tilted panels, and an occasional cleaning suffices. Finally, the cells lose a fraction of their efficiency each year: manufacturers generally still guarantee around 85 to 90 % of the original performance after 25 years.

How to read a panel’s datasheet

Faced with a quote, two figures deserve all your attention on the datasheet of the proposed panel.

• The power in Wp (for example 430 Wp): what the panel delivers under test conditions. Multiplied by the number of panels, it gives the total power in kWp.
• The efficiency as a percentage (for example 21.5 %): the conversion effectiveness, decisive when roof surface is counted.

Two indicators usefully complete the reading: the temperature coefficient, which tells how much the panel loses when it heats up (the lower, the better), and the performance warranty, which commits the manufacturer on the yield retained after 25 years.

Optimising your production in practice

The good news is that most of it is decided at the design stage. Careful sizing is worth far more than any later adjustment. To get the most from your installation:

• favour the best orientation and tilt available on your roof, due south or east-west;
• have the shading analysed beforehand, and consider optimisers if a zone is unavoidably shaded;
• choose a panel with good efficiency if your surface is limited, to maximise the kWp installed;
• ensure a ventilated mounting that limits cell heating in summer;
• adapt your consumption to production hours (washing machine, charging, hot water during the day) to make the most of every kWh produced.

The final calculation always depends on your actual roof. A technical visit lets us measure orientation, slope, usable surface and shading, then convert all of that into an estimated annual production in kWh — and therefore into savings. You can dig deeper into how it works and its profitability on the solar panels page, understand how a project unfolds on the solar panel installation page, or move on to sizing with the how many panels for my home guide.

The 30-second recap

Guide reviewed in May 2026 · updated every year