Is there any simple instrumentation to wirelessly monitor power consumption by an electric plane?

Ummm, not real sure what you mean by wirelessly. Use of something like an Astro meter or Watt meter, plugs inline between the pack and esc and reads out volts/amps/watts. Gives you real time readings.

And there are some units available that allow you to download the info to your computer after a flight, aka like a flight recorder.
That way you see how your system is performing over an extended period of time.

For downloading info after a flight, check out the specs on the Castle Creations ICE ESC's. I believe they allow it.

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