How much energy a PV module delivers depends on several factors, such as local weather patterns, seasonal changes, and installation of modules. PV modules should be installed at the correct 'tilt-angle' in order to achieve best year-round performance.

When sizing a PV system, it is important to know whether it is expected to be used all-year round or only during a certain period of the year. The energy yield vary during the year; energy produced in winter is much less than yearly average and in the summer months the generated energy can be more that the average. PV array power output estimate is made based on the worst case scenario and follows two steps:

• The PV array power output estimate is obtained by dividing its energy output by the equivalent sun hours (ESH)
• To calculate the number of modules, the PV array output estimate is divided by the maximum power value of the module size selected for wiring the PV array.

In the PV language, 1 equivalent sun means the solar irradiance of 1000 W/m2. This value corresponds to the standard, at which the performance of solar cells and modules is determined. Therefore, the rated parameters of PV modules are determined at a solar irradiance of 1 sun.

When solar irradiation data are available for a particular location than the Equivalent Sun Hours can be determined.

The map in illustration bellow shows a rough estimation of the daily equivalent sun hours for an average annual solar irradiation.

Irradiation and Equivalent Sun Hours data are available from NASA and meteorological agencies websites.

The choice of modules is dictated by market availability and performance; modules performance are reflected by modules specifications sheets.