TURNIGY 130A WATTMETER

This is a very accurate R/C amp/watt meter which can be used to predict model airplane flight time by knowing how many amps are being drawn from your battery with any given propeller.  This allows you to ensure peak currents are safe for motor, ESC, wiring and connectors.  
Check battery health and performance.
Evaluate R/C battery charging efficiency.  
Choose the best propeller or gearing and the most efficient motor  
Check for wiring and connector power losses  
Measure power and energy consumption of ANY device with a battery