In Section 1.3 we saw that the amount of light a surface receives depends on the direction it is facing - with a position that faces south and at an angle of roughly 35 degrees to the horizontal maximising the energy collected by a solar panel in the UK.

We also saw that the total energy collected in a year is not as strong a function of the direction in which the panel faces as you might expect, with a difference of around 20% in energy yield between facing due south and facing east or west with an inclination angle of 35 degrees (a typical roof slope). This insensitivity to the direction faced is even more marked for solar thermal panels heating domestic hot water, which are even less affected than PV panels by orientation. For a 35 degree inclination, the difference between facing east or west and facing due south is only around 12%, compared to a difference of 20% for the incident light energy.

The explanation for this is the same reason there are diminishing returns in the energy collected as a solar thermal system has higher panel area or higher panel efficiency. This is discussed in the next section.

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