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 voltage on a solar panel with multimeter.
There are two types of multimeters.
The next step is to ensure that you select a voltage range that can display the possible maximum voltage of your solar panel in an open circuit.
Including the solar panels.
A multimeter is a device that you can use to test the voltage and current of any device.
Some exceptions may apply based on panel type.
For example if your solar panel is 20 volts rated your digital multimeter should be set to a reading of 200 volts for an accurate reading.
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.