helo. im still a beginner using arduino and doing a simple pv datalogging to SD card, record the power output in Watts of solar panel by the voltage reading through an analog input. I am using 12V 83.3mA solar panel and i just want to measure power output not supplying any electricity. Do i need to set up a voltage divider to lower it around 5V into Arduino analog pin for voltage reading and a load resistor for the current calculations for power output? Thanks 
Yes to both questions.
If not supplying electricity then the power is ZERO. Power = Current times Voltage and if current = 0 then power = 0. So a loading resistor is needed.measure power output not supplying any electricity
If you are using a know value load (resistor) then you only need to measure the Voltage across the load to calculate power using Ohms Law and the Power equation.
If you want to measure the power to an unknown load (some real circuits or a battery) then you also need to measure the current drawn.
hi waltr, thanks for the reply what i meant was i will be not be using the power output from solar panel to charge up any device. i just want to record the power output data. pardon for my poor english 
calculate power using Ohms Law and the Power equation
→ u meant calculate current using ohms law? and then power formula P=IV?
does that give me the actual power output from solar panel? just curious. should i consider the voltage measurement from the other resistor on voltage divider setup
Again … if the output of the PV panel is not connected to anything, it will generate a voltage but not deliver any power. The voltage a PV panel will generate is dependent on how much light it is receiving and the load that it’s connected to. If you want the voltage measured, and any power reading derived from it, to have any real world meaning, you need to attach a load resistor to the PV panel. That load resistor should be representative of what you want to use the PV panel for.
If the PV outputs a voltage larger than the Arduino can safely handle, then you can split that load resistor into 2 resistors, making a voltage divider so that the voltage going into the Arduino is now small enough (5V or 3.3V Max, depending on which Arduino you have). Since you know the resistance and measure the voltage across one resistor, you can compute the current being supplied by the PV panel. Knowing that current and both resistances you can compute the total power.
If your panel claims to output 12V while supplying 83 mA, then perhaps you want verify if this is true. If you attach a load of 150 ohms to the panel, then at 12V the PV should also deliver 80 mA. That’s also (12V * 0.08A = ) 0.96W !!! Now split that 150 ohms into 2 resistors and measure the voltage across the smaller resistor. Be aware you can’t use standard 1/10 or 1/8 W resistors as they will burn up.
“Surprisingly” similar answers here: http://forum.arduino.cc/index.php?topic=189314.0