To measure amperage or voltage of solar panel you need to set the function to dc amperage or dc voltage.
Does the amperage increase with each additional solar panel.
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.
Measure the solar panel controller output voltage try to get maximum voltage by angling the panels.
Measure the battery voltage.
If it is proceed.
5 hours x 290 watts an example wattage of a premium solar panel 1 450 watts hours or roughly 1 5 kilowatt hours kwh.
When stringing panels in parallel each additional panel increases the current amperage of the circuit however the voltage of the circuit remains the same equivalent to the voltage of each panel.
It may be that you can never get more than 12 13v.
In arranging solar panels you have two options for modifying the power output according the ohm s law.
For example if the solar panel is rated at 175 watts and the maximum power voltage vmp is given as 23 6 volts then calculate the current as 175 watts divided by 23 6 volts which is equal to 7.
Thus the output for each solar panel in your array would.
For the sake of example if you are getting 5 hours of direct sunlight per day in a sunny state like california you can calculate your solar panel output this way.
Testing solar panels for volts.
You can either wire multiple panels in series to increase voltage with current amps remaining the same as any one panel or wire the panels in parallel to increase current with the voltage output remaining the same as any one panel.