kWp

What are the conditions behind the peak rating?

The rating is measured at 1,000 watts of irradiance per square metre, a cell temperature of 25 degrees Celsius, and a defined light spectrum. Those are standard test conditions, applied identically to every panel sold.

That uniformity is the point. Kilowatt-peak exists so two panels can be compared against each other, not so a roof can be predicted from a datasheet.

Why does a real roof never reach its rating?

Because panels lose output as they heat up, and because sunlight rarely strikes a Dutch roof at the angle the test assumes. A panel baking on a July afternoon delivers less than its rating precisely when irradiance is highest.

Orientation compounds this. A south-facing pitch outperforms an east-west split, and partial shade costs more output than the shaded area alone would suggest.

How do I convert a rating into an annual yield?

Multiply the kilowatt-peak figure by the yield per kilowatt-peak for your orientation — roughly 850 to 950 kWh per kWp on an unshaded south-facing Dutch roof.

Treat that as a range and distrust any quote that collapses it into a single number. Year-to-year weather variation exceeds the difference between competing panel brands, so a precise annual figure claims certainty nobody has.

What should I actually compare between quotes?

Compare total array capacity rather than the rating of an individual panel. Twelve 400-watt-peak panels give 4.8 kWp; ten 450-watt-peak panels give 4.5 kWp, a smaller gap than the panel count implies.

Then ask whether that capacity suits the roof and the household. Capacity beyond what a home can absorb mostly produces exports, and exports are the least valuable kilowatt-hours after 2027.