How much electricity do solar panels actually produce?

The number on the panel is a lie — a useful, standardised, internationally agreed lie, but still. A "400 Wp" panel produces 400 watts under Standard Test Conditions: 1,000 W/m² of irradiance, a cell temperature of exactly 25 °C, and a specific light spectrum. Your roof achieves that combination approximately never. On the sunniest July afternoon the light might be there, but your panels are then far hotter than 25 °C, and silicon gets less efficient as it warms up.

So what does a roof really make? There's a single rule of thumb worth memorising.

The one number: kWh per kWp per year

In Belgium and the Netherlands, a well-oriented roof produces roughly 900 to 1,000 kWh per kWp per year.

"kWp" is your installed peak power — add up the panel ratings. Ten 400 Wp panels is 4 kWp, so expect somewhere around 3,600 to 4,000 kWh per year. Further south the figure rises (Spain sits comfortably above 1,400); further north it drops. It's a boring number, but it beats every installer's brochure at predicting your first annual statement.

Note what this quietly includes: cloudy days, dirty panels, inverter losses, cable losses, and the fact that the sun sets every single night. It's a whole-year average, not a best-case.

Where the year actually happens

Annual totals hide the most important part: the distribution. Solar production in this part of the world is savagely seasonal. A rough shape of a Belgian year:

Period	Share of annual production	Feels like
April–September	≈ 70–75%	Surplus almost daily
March + October	≈ 12–15%	Good days, short days
November–February	≈ 10–15%	You are mostly a consumer

A December day might yield a fifth or a tenth of a matched June day. This is why "my panels cover my annual consumption" and "my panels power my house" are two very different statements — and why the gap between injection and offtake prices hurts: you overproduce exactly when everyone else does, and buy in the dark months when nobody's selling cheap.

Orientation: less dramatic than you'd think

People agonise over this. The honest numbers, relative to a perfect south-facing roof at about 35°:

• South, 30–40° tilt: 100% — the benchmark.
• South-east or south-west: roughly 95%. Barely worth a sigh.
• Straight east or west: roughly 80–85%.
• Flat roof, panels tilted: 90–95%, and you can choose your angle.
• North: 60% or less. This one's real.

And here's the twist worth knowing: an east–west split often beats south for self-consumption. South gives you a tall, narrow midday spike — exactly when nobody's home. East–west gives a broader, flatter curve with production at breakfast and dinner, which is when your house actually eats electricity. Fewer kilowatt-hours in total, more of them used at full retail value.

Shade, meanwhile, is the thing that genuinely deserves your anxiety: one chimney shadow crossing a string can cost far more than a suboptimal compass bearing. How many panels fit around those obstacles, and at what spacing, is what SolarFit is for.

What about ageing?

Panels degrade slowly — typically around 0.5% per year, which is why 25-year performance warranties are standard and unexciting. After two decades you're still looking at roughly 85–90% of day-one output. The inverter is the component that usually needs replacing first, often somewhere in year 10–15. Budget for that rather than for panel failure.

From averages to your roof, today

Annual rules of thumb are for sizing decisions. For daily decisions — should the laundry run now or at 14:00? — you want the actual curve for your location and weather, which is what a solar production forecast gives you (the mechanics are in how solar forecasts work, the practical routine in planning your day around your panels).

And once you own panels, the most instructive number is the one your meter reports right now: production minus what the house is consuming. Mine lives on an €8 LED strip in the hallway, and it taught me more about my own roof in a fortnight than a year of monthly statements did.

Figures here are typical ranges for Belgium and the Netherlands, meant for estimating — not a guarantee for your specific installation. Shade, roof pitch, panel quality and inverter sizing all move the result.