Here you will learn how to calculate the annual energy output of a photovoltaic solar installation.

The global formula to estimate the electricity generated in output of a photovoltaic system is :

E = A * r * H * PR

E = Energy (kWh)
A = Total solar panel Area (m2)
r = solar panel yield or efficiency(%)
H = Annual average solar radiation on tilted panels (shadings not included)
PR = Performance ratio, coefficient for losses (range between 0.5 and 0.9, default value = 0.75)

r is the yield of the solar panel given by the ratio : electrical power (in kWp) of one solar panel divided by the area of one panel.
Example : the solar panel yield of a PV module of 250 Wp with an area of 1.6 m2 is 15.6%.
Be aware that this nominal ratio is given for standard test conditions (STC) : radiation=1000 W/m2, cell temperature=25 celcius degree, Wind speed=1 m/s, AM=1.5.
The unit of the nominal power of the photovoltaic panel in these conditions is called "Watt-peak" (Wp or kWp=1000 Wp or MWp=1000000 Wp).

H  is the annual average solar radiation on tilted panels. Between 200 kWh/m2.y (Norway) and 2600 kWh/m2.y (Saudi Arabia). You can find this global radiation value here :Solar radiation databases
You have to find the global annual radiation incident on your PV panels with your specific inclination (slope, tilt) and orientation (azimut).

PR : PR (Performance Ratio) is a very important value to evaluate the quality of a photovoltaic installation because it gives the performance of the installation independently of the orientation, inclination of the panel. It includes all losses.

Example of detailed losses that gives the PR value (depends on the site, the technology, and sizing of the system):
- Inverter losses (4% to 10 %)
- Temperature losses (5% to 20%)
- DC cables losses (1 to 3 %)
- AC cables losses (1 to 3 %)
- Shadings 0 % to 80% !!! (specific to each site)
- Losses at weak radiation 3% to 7%
- Losses due to dust, snow... (2%)
- Other Losses (?)