When manufacturers label photovoltaic panels as "18V," they're referring to the nominal voltage under standard test conditions (STC). But here's the twist: actual operating voltage often fluctuates du. What is the voltage of 18 volt solar panel? The voltage of an 18-volt solar panel, typically referred to as rated voltage, is primarily meant to provide sufficient output under standard test conditions (STC), which usually refers to an intensity of 1000 watts per square meter at 25 degrees Celsius. It's a bit confused about some of the stats on panels we have been looking at, for example, 100 watt 12 volts panel and 100 watt 18 volts panel. In the majority of cases there are no differences other than name. This sounds a bit weird, but it's really not. These conditions serve as the industry standard for evaluating solar panels, making it easier to compare panels accurately.