Using the Phantom 2's battery as reference (5100mah), how does one calculate the power drain IN MaH from a FPV transmitter? Mine is 600mw, so how do I figure this out? I'm probably wrong about this, but at 600mw, is that 600/60 = drain per minute = 10mw per minute. 10mw per minute x appx. 20 minute flight time = 200mah. Is this right?

Your vid Tx draws a very small amount of current, less than 1A. Impact on flight time on a Phantom battery is negligible. The best way to judge your real-world results is to fly a few test flights with the Tx powered on, a few with it powered off, and then a few without it mounted. You'll get more of a difference in flight time from the weight of the Tx than you do from it's power drain.

What about a 2000mw Tx? If 600mw is negligible, than wouldn't 2000mw be not too bad either? Also, how exactly can I calculate the power drain?

You cannot calculate current draw on output power - unless you know the efficiency of the TX. Watts = Volts x Amps and Amps = Watts / Volts, so 2W@10V should be 200mA, but a 2W TX will actually pull about 1.6-2A, which is not negligible. This will decrease flight time and increase current draw, so the batteries work harder. It knocks about 2 minutes off, compared to a 200mW TX in my experience.

Hi The immersion RC draws 3 watts of electrical power to generate 600mw of RF. 3 Watts of power at 11 volts is a current draw of 273 ma. A battery rated at 5100mah would run the transmitter (alone) for 18.7 hours. The effect on a typical P2 flight time would be small, if you were getting 20 minutes before, my maths (might be wrong) shows that reducing by 19 seconds. I hope this helps. Huw

Hi Using the same maths, an TX drawing 1.6 amps would reduce the flight time by 112 seconds, so ties in with observations. I am using: New Flight time = Old Flight time x (1+(TX current draw/15.5)) All times in seconds. Cheers Huw

I used the 2W TX as a worse case example, but if the Immersion manages much better efficiency, then as you say, this will have a small effect on flight time.