Determining the power output of a PV array is not a precise exercise
...For most field testing, the measurable quantities are irradiance and temperature.
Power is an instantaneous value; the PV array produces so many Watts (or kilowatts) of power at a specific time depending on existing conditions. Power output will change continuously throughout the day. Energy includes a time factor and is measured in Watt-hours or kilowatt-hours (kwH).