To test a 18v solar panel voltage output directly put your solar panel in direct sunlight set your multi meter to the dc volts setting.
How to test a solar panel with a multimeter.
You will want to ensure that you place the positive to positive and negative to negative when attaching your multimeter to the connection on the back of your solar panel.
You want to choose a voltage range capable of displaying the maximum possible voltage of the panel in open circuit this means that if you have a panel rated for 20 volts you should set the multi meter to read up to 200 volts to ensure that you receive an accurate reading.
As most solar panels are 12 volts the solar panel test methods are basically the same.
Set multimeter to dc.
Tilt the solar panel in order for your solar panel to have full sunlight exposure.
Set the meter to above 200 vcd to ensure you get the most accurate reading possible.
How to test solar panel amps using a multimeter the first thing to do is place your solar panel outside in direct sunlight.