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The power output of a PV module depends on the number of cells in the module, the type of cells, and the total surface area of the cells.

All modules are rated by manufacturers in terms of their peak power (Wp) under standard test conditions: ie. 1000W/m² of sunlight (‘peak sun’); 25 ºC; and air mass of 1.5.

Modules nearly always produce less than their rated peak power in real-life conditions.

The output varies depending upon:

  • the amount of solar radiation;
  • the temperature of the module (output decreases as temperature rises);
  • the voltage at which the load (or battery) is drawing power from the module.

Please Note: shading a single cell of a module will considerably lower it’s output.

Sunlight data is given in terms of ‘mean daily peak sun hours’ at a site. ie. daily solar radiation is averaged out to give equivalent number of hours of ‘peak sun’. (or kWh/sq m).

For battery charging applications it is not accurate to simply multiply number of peak sun hours by peak watt rating of a module to calculate output. The peak current rating should be used instead since modules only produce their peak power at their peak voltage (not battery voltage).

Current @ peak output (A) x peak sun hours (h) = Expected output (Ah).
This can be multiplied by battery voltage to give energy output (Wh).

For grid-connected applications, inverters such as the ‘Sunny Boy’ can accept a wide range of input voltages and use ‘maximum power point tracking’ to let modules work at their optimum voltage. So multiplying peak watts by peak sun hours is accurate.

Peak power output (Wp) x peak sun hours (h) = Expected energy output (Wh)

For most of the UK there are approx. 4-5 peak sun hours in Summer reducing to 1 hour in Winter.1 sq metre of PV module ~ 115Wp 1 kWp of well sited PV array in the UK will produce 700-800 kWh of electricity per year.

Solar PV estimation tools

Solmetric Suneye 210 Annual Sun PathThe Solmetric SunEye provides an accurate and fast method for estimating the effect of shading on any area of land or building.

It combines a digital camera with calibrated fisheye lens, electronic tilt & compass sensors to provide easy, accurate measurements and instant feedback.


Solar Horizon EstimatorHandheld Solar Horizon Estimator. This enables the potential effect of shading by trees or buildings to be gauged and is useful for a pre-installation site assessment.

A simple method that will give you a rough idea of the effects and whether the site is viable.


JRC (Joint Research Centre) offer a very useful tool for estimating monthly output using sunlight data for anywhere in Europe and Africa.