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.
NB. 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.
A useful tool
for estimating monthly output using sunlight data for anywhere in Europe
and Africa is available
here.
