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How solar actually performs
What an array physically does on a roof, and why it delivers less than the quote said. Every figure in this section is modelled in PVGIS and the script that produces it ships with the site.
Why your panels produce less than the quote said
A 4 kWp array does not deliver 4 kW. Between the irradiance landing on the glass and the kilowatt-hours reaching the meter sit five separate losses and one small gain, and they can be listed with numbers against them rather than described as inefficiency.
Read the guide- PerformanceDo solar panels work in shade, and how much do you losePanels keep producing in shade, from diffuse light. The expensive question is different: what one small opaque shadow does to the rest of the array. Bypass diodes decide the answer, and they work in thirds.9 min
- PerformanceHow much output solar panels lose after 10, 15 and 25 yearsManufacturers warrant a straight line. Field measurements across 11,000 recorded degradation rates show a median close to that line and a mean well below it. The gap between those two numbers is where warranty claims live.9 min
- PerformanceWhy solar panels produce less in a heatwaveA panel is rated at a cell temperature of 25°C. On a still, sunny day in the low thirties the cells sit near 60°C, and every degree above the rating costs output. The annual cost in Britain is modest. The cost at two in the afternoon in July is not.8 min
- PerformanceWhat 30° off south actually costs a solar arrayRoof direction is treated as the thing that decides whether solar is worth doing. Modelled properly, 30° off south costs between two and three per cent a year, and any tilt between 25° and 50° is within about two per cent of the best one.9 min
- Straight answersWhen cleaning solar panels actually pays for itselfCleaning is sold as maintenance. Treated as an investment, it has to recover its cost in extra kilowatt-hours, and in a climate with regular rain the number it has to recover is larger than the dirt is worth. There are three exceptions, and they are specific.8 min
- RoofsWhy an east–west roof can beat a south-facing oneSouth-facing solar panels produce the most electricity over a year. East–west arrays produce less but spread generation across morning and afternoon, which can be worth more once exported units are paid at a fraction of retail price.6 min