The capacity of a Solar PV Panel is as what is given in its name plate. The output will invariably be given in DC power output in watts. The watt output of an array of PV panels has to be converted to a practical unit which is kw (DC) and for this purpose the total watts (DC) of the PV module array is added up and divided by 1000.
Sadly this out put too does not give an indication of the ultimate power available to you for useful work due to several factors which derate the output.
Affect of location of the building and the name plate capacity
The name plate detail of generated DC power is measured at Standard Test Conditions (STC). Now this does not tally most of the time with the highly variable conditions of a particular locality of an average building.
The STC is laid down simply to arrange a common platform for the sake of convenient comparison. Strangely two different STCs can exist depending on the country and authority responsible. A widely used STC is the standards laid down by the NRFL laboratory of the US Department of Energy.