Measuring power from my solarpanels

Today I finally had some spare hours to do the following; measure the DC voltage and current from my solar panels (2 pieces, total 380 watt peak) and convert this into power. That way I can finally see how much is actually generated by the solar panels.

I used an Arduino, adapted my Wireless Garden SensorStick and attached it to a rail-to-rail OpAmp from Microchip....