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....

My solar panels are currently connected to a Soladin converter from MasterVolt. It is already operating for more than a year now and working flawlessly. The Soladin converter only has some LED's to show it is operating. The faster the LED's on the Soladin flashes, the more power is converted back to the outlet. But it is just an indication... fast flashing LED's could be anywhere between 100w or 300w....

The floating voltage of the solar panels combined are around 85volts DC. When the panels are connected to the Soladin, the maximum power point tracker (MPPT) will keep the panels around 70 volts. Since these panels can deliver a top power of 380 watts, the current should never exceed 380w/70v= 5.5 amps. 

But due the fact of how I placed my panels (they are mounted on my balcony, not the roof) the highest power I ever measured with full sun was 270Watts. So for my specification of measuring the power, I had a maximum current of 5 amps with a maximum voltage of 100v. 

To measure the current, I used 0.02ohm resistors (actually, it where 5 pieces of 0.1ohm resistors connected in parallel). I used such a low resistor value to minimize the current measurement losses. By using this resistor, at 5 amps current there will be a loss of power of:  0.5W (5A*0.02R= 0.1V * 5A ).
But, using a low resistance shunt, it means also that there is a low change of voltage over the shunt. In this case, at 5 amps it will have a voltage of just  100mV (I*R=U). So I had to amplify this using a rail to rail OpAmp. I used an Microchip MCP6242 (dual opamp). 

Please see schematic drawing below. I've tested the circuit and it is very linear and working good! Even on the breadbord .

[todo]

The output is of the schematic above is forwarded to my previous SensorStick design. It incorporates several analog inputs, various sensors and a wireless transmitter from Hope RM12B 433mhz Electronics (cheap and VERY good!). 

Only minor modding was needed to the SensorStick. It is already now outputting data to my Cosm online data (oh, btw Cosm is re-branded to Xively!)

Well heck, the sun is already down now :(. The time I connected my prototype to the solarpanel, there was only 2 volts on the terminals... well got to wait for tomorrow then.. and see if everything is working as it should... do you watch with me? 

*Please keep in mind that in case there is a failure inside the Soladin, or the MPPT goes to a voltage lower than 70volts, the current can easily exceed double the current!!

Oh, by the way, please keep in mind that the voltage is dangerously! These electronics are placed in an area were no one else but me gets.