How much should your solar system produce?

The question looks simple, but any answer of the form "X kWh" without stated assumptions is a little dishonest. This explains where expected output comes from and how accurate that figure actually is.

A rough orientation

In Central Europe a well-built system delivers roughly 950–1,150 kWh per year for each installed kWp, given a south-facing roof at about 30–35°. An 8 kWp system therefore produces on the order of 7,600–9,200 kWh a year.

That figure is called specific yield, and it's useful precisely because it doesn't depend on system size — it lets you compare a small installation with a large one.

The band is wide, though, and it only holds for a southern orientation. An east–west roof commonly lands around 800–950 kWh/kWp and is still perfectly healthy.

Where a sharper number comes from

A general band is fine for orientation. Judging a specific system needs a model that knows your location — how much solar radiation actually reaches that spot, how it varies across the year, and how that translates to your particular tilt and orientation.

Slunora uses PVGIS, the European Commission's model built on a long series of satellite measurements. The calculation takes:

• geographic location,
• panel tilt,
• orientation (azimuth),
• installed capacity,
• assumed system losses.

Where an input is missing, the result isn't filled in with a guess — either a more cautious variant is used and the confidence of the conclusion drops, or that part of the assessment is skipped entirely.

How accurate is it

This is the most important section, and the one most often missing from marketing material.

The model carries an error of roughly ±5–10 %. On top of that comes year-to-year weather variation of about ±4–6 %. Combined, you land near a 7 % standard deviation.

The practical consequence: a gap of up to roughly 15 % from expectation is not a signal. It's noise. A system producing 92 % of modelled expectation is very probably entirely healthy.

That's why we only raise suspected underperformance below 85 % of expectation, and treat it as a serious problem below 75 %. These aren't round numbers picked by feel — they're derived from exactly the deviations above, so that a finding means a real anomaly rather than a bad year.

Why sources disagree

Ask three tools for your expected output and you'll get three numbers. The usual reasons:

• Different system losses. The gap between assuming 8 % and 14 % moves the result by over 6 percentage points.
• Different radiation database. Models build on different satellite series covering different periods.
• Optimal versus actual angle. Calculators often assume the optimal tilt even when your roof doesn't have it.
• With or without shading. Most calculators ignore near shading entirely.

So before comparing your output against someone else's number, check what went into that number.

What to do with this

To find out how your specific system is doing you need three things: location, installed capacity, and actual output for a year or by month. The quick check works out the rest.

Related reading: your solar is producing less than expected works through the explanations for low output in order of likelihood.