The basic formula
Estimated annual output is obtained by multiplying the installed power (kWp) by the specific yield, that is, the number of kWh produced in a year by each kWp installed. Correction factors are then applied for orientation, tilt, shading and losses in the system.
A worked example
Suppose, purely for the sake of example, a 5 kWp system and a specific yield of 1,100 kWh/kWp a year. The result is 5 × 1,100 = 5,500 kWh a year. In a real case, the figure of 1,100 may be lower or higher, depending on the area, the orientation and shading. That is why we do not present general figures as promises.
What affects output
- location: areas further south generally receive more radiation;
- orientation and tilt: facing south at a medium tilt is usually best;
- shading: even partial shade can reduce output considerably;
- temperature: heat slightly lowers efficiency;
- dirt and snow: deposits temporarily reduce the light that reaches the cells;
- equipment: the efficiency of the panels and of the inverter.
Variation through the year
Monthly output in June or July is often several times higher than in December. The annual average hides this difference, so sizing also has to take account of the months when consumption is high but output is low. More in the article on photovoltaic panels in winter.
How to check an estimate
Ask to see the assumptions behind the figures: the specific yield used, the orientation, the losses. There are public simulation tools, such as PVGIS, that can be used for comparison. We provide an indicative calculator, and in the quote we explain the assumptions we used.