Measure output power of a solar panel
A maximum power point tracking (MPPT) chip will keep the solar cell working at it's peak power output. These chips sometimes have an output that will report the instantaneous power. 

You can more simply measure the power using two multimeter. One multimeter measures the potential across the cell, and the other measures the current. Like this:







simulate this circuit &ndash; Schematic created using CircuitLab

The instantaneous power is simply the product of voltage and current. Do note that the power delivered depends very much on the load. 



Power delivered by the solar cell (the blue line on the graph) peaks at a point between the maximum voltage and maximum current delivery. This is important to choose the right load in order to maximize the power delivered by the solar cell. Alternatively, you can use a MPPT, which automatically tracks the maximum power point.