Time is energy over power; range is speed times time
With usable energy E and a constant electric power P, endurance is E/P and still-air range is V × E/P. Twenty kilowatt-hours at 90% usable and a 40 kW cruise is 0.45 hours; at 100 knots that is 45 nautical miles. If you do not have a power number, level-flight power required is W V / (L/D), and electric power is that divided by a chain efficiency η (motor, controller, and propeller). Unlike the Breguet model, mass is treated as constant — the battery does not get lighter as you fly.
Climb, reserves, battery temperature, and a peukert-like rate effect are not in this budget. Use the airframe and battery manual for a dispatch range. For a hovering multirotor, the drone flight time page is the better model.
Sources
- Specific-energy endurance t = E / P.
- Level-flight power P = W V / (L/D) (Anderson, Introduction to Flight; FAA Pilot’s Handbook of Aeronautical Knowledge, power required).
- This site’s Breguet range and endurance calculator, for the contrasting fuel-mass-fraction model.
Frequently asked questions
How is this different from Breguet range?
Breguet integrates a changing fuel mass. A battery’s mass stays put, so endurance is just usable energy over power.
What should I use for η?
A combined motor, controller, and propeller efficiency. 0.80 is a round default, not a measurement of your airplane.
Does this include a reserve?
Only through the usable fraction. Lower it to leave energy in the pack at landing.