One consideration for solar energy systems is the seasonal nature of the availability of light. Changes in the hours of darkness throughout the year and prevailing weather conditions act to limit the light levels in winter compared to summer, at least in locations that are away from the equator.

The graph shows the average daily light energy incident on a horizontal surface for each month of a typical year in London, UK. It can be seen that the daily average in the summer months is six to seven times higher than in the winter months.

This big difference between summer and winter influences the sizing of building-mounted solar systems, where the demand for energy each day is limited. This is particularly the case for for solar thermal where a large excess of energy compared to the daily heat demand simply cannot be stored. For solar photovoltaics where any excess energy that cannot be stored can be exported to the grid, the sizing considerations differ.

The total light energy is made up of two component parts - direct irradiation (straight from the sun and shown in orange) and diffuse irradiation (light that has reflected off clouds, the sky and the surroundings, shown in blue).

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