What is the difference between energy (kWh) and peak power (kWp)?

The kiloWatt (kW) is used to express the power of an electrical appliance or generator.

The kiloWatt hour (kWh) is used to express the amount of electrical energy consumed by an electrical appliance or produced by an electrical generator.


1 kWh is the amount of electrical energy consumed or produced by an appliance or generator per unit of time, i.e. the power consumed or produced during one hour.


1 kWh = 1 kW x 1h
10 kWh = 10 kW x 1h but also 10 kWh = 1 kW x 10h


Orders of magnitude to remember:
1 kW = 1000 Watts
1 kWh = 1000 Watt-hours


What is 1 kWp?

The power of a solar panel is expressed in Wp (Watt peak).

Reminder of the dimensions: 1 kWp = 1000 Wp

We talk about watt peak because the electrical power produced by a solar panel depends on the power of the solar radiation received.


The power of the solar radiation depends on the angle of incidence between the solar rays and the panel. 

Therefore, it depends on :

  • the time of day and the season to determine the astral position of the Sun: the thinner the atmospheric layer through which the solar rays pass, the greater the irradiation power.
  • the geographical position, orientation and tilt angle of the solar panel to determine the angle of incidence between the solar rays and the surface of the panel: the more the solar rays arrive perpendicularly to the surface of the solar panel, the greater the transmitted power.


All these parameters are variable, so it is necessary to set a standard to define the power of a solar panel.

The watt-peak (Wp) is therefore the maximum electrical power that can be supplied by a photovoltaic panel under standard temperature and sunlight conditions.


Definition of standard conditions (STC):

  • Solar irradiation power = 1,000 Watts/m².
  • Ambient temperature = 25°C (above this value, the efficiency of the photovoltaic panels decreases, see panel data sheet for more details)
  • Air density = 1.5 kg/m3 (definition of a clear sky)
  • Incidence angle = 90° (perpendicular to the plane of the panel)

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