A good datalogger that I use is the EagleTree v3. It conencts inline between your battery and the ESC and will measure the Volts and Amps directly. I also have the Thermocouple for Battery, ESC or Motor temp (mostly use it for motor) and also the Hall Effect RPM sensor. I got it from Tower when they sent me a 20 off $50 coupon. It has been very handy. It has allowed me to do static tests (it will hook up live with your computer via USB) and measure thrust and motor efficiencies. I have plugged in the speed readings into CriveCalc and that will predict perfomance from different props.

I would strongly suggest this over a wattmeter, especially if you are really into Electrics. The price is in the ballpark, and the features for inflight recording of multiple pieces of data are outstanding.
Curtis