You can however undertake power measuring with basic equipment needing only an
ammeter, voltmeter and variable load. This method while slower than the previously
mentioned methods will give good accuracy within the limits imposed by panel heating.
During the time taken to gather and record data panel heating will have reduced the
power reading you obtain to a figure below that which would be obtained by the other
much faster methods mentioned above.
Remember panel power falls by about 0.5% per deg C temperature rise of the panel. A
power reading about 10% below that obtained at scrutineering is possible.
Test your panel in full sun about midday, preferably between October and March
(December is best) as the sun strength is lower during the winter period.
Set up the circuit as shown below, place panel in the sun with the sun striking it at right
angles and take readings of voltage and current as the variable resistor is changed from
zero ohms to about 40 ohms. The intervals between readings required to accurately
determine the maximum power vary depending on the voltage and current characteristics
of your panel. Initially aim to take readings at current intervals of about 1/10 of your
panels short circuit current. Calculate the power.
Power (Watts) = Volts multiplied by Amps.
Graph the results with Power on the Y axis and Volts on the X axis. Read maximum
power from the graph. Take extra readings if required to increase accuracy.
SOLAR PANEL
VARIABLE RESISTOR
0-80 OHMS 10 WATTS
OR DUMMY LOADAMPSVOLTSIf equipment is not available to you here is an approximate method of calculating panel
power. With your panel directly facing the sun at around midday preferably in December
or January measure the Open Circuit Voltage (OCV) and the Short Circuit Current (ISC)
you also need to know the panels Fill Factor (FF) for accurate power estimation. If FF is
unknown use 0.75 see below for an explanation of fill factor.
The approximate power is given by OCV x ISC x FF in watts.