The difference between nominal and real PV power explained

Solar panels

The difference between nominal and real PV power explained

In order to perform proper calculations concerning PV system yield, it’s important to be aware of the meaning of concepts like ‘nominal power’ and ‘real power’, which are often incorrectly used.


The maximum electric or nominal power of your PV system can be defined as its ‘Peak Power’ (in Watt Peak). The Peak Power of solar panels is registered under the following Standard Test Conditions:

  • a light intensity of 1.000 W/m²
  • sunlight hitting the positioned solar cells perpendicularly
  • a standard airmass (AM) of 1,5. This is a measure for the relative length of the optical path through the atmosphere.
  • a temperature of 25°C at the solar cells

In other words, the Peak Power is a specific feature of your PV system, regardless of the location where it is tested. Under these standard conditions, the nominal power of solar panels can be compared with each other. A solar panel with a Peak Power of 5 kWp in Spain is for example exactly the same as such a panel in Belgium.


The real electric power of your solar panels is primarily dependent on the number of hours of sunshine to which they are exposed. Of course, this is dependent on the location. In Belgium for a proper PV system (i.e. good slope and good orientation, absence of shade,…), one can expect a yield of 0,95 kWh/Wp. In more sunny locations, one can enjoy of course a higher yield. For instance, in the Spanish Capital Madrid we can assume a yield of about 1.4 kWh/Wp.

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