Using a Matt's PR as a reference I did a custom FW that calculates and log power (Watts) and used energy (Watts-hour). My goal was to see what will be the difference between mAh-based and wattage-based estimation approaches.
Today I did some real flights and collected logs. Here are my results:
As voltage decreases current is slightly increases. The power is excpectingly stays the same all the time (though it is noissy because it was windy).
CurrTot is the mAh used and EnrgTot is the Wh used. No much difference though.
Above is the log when hovering. Pwr is the copter power in watts, AvTime is the available (estimated remaining) time. It is calculated as [battery capacity in Wh] / [power in Watts]. This calculation provided pretty linear and accurate remaining fly time.
In same way we can calculate available (remaining) distance copter can fly.