Estimating Endurance for an Electric UAV Without Fooling Yourself

The naive calculation and why it misleads

The obvious estimate divides capacity by current draw:

t = C / I

A 5,000 mAh pack drawing 25 A suggests 12 minutes. In practice you will see meaningfully less, for reasons that compound rather than add.

Reserve is not optional

Lithium polymer cells are damaged by deep discharge. A pack taken repeatedly below roughly 3.5 V per cell under load loses cycle life quickly, and a cell driven to full depletion may never recover its capacity.

Plan to land with 20–25% remaining. That is not conservatism, it is the definition of usable capacity. Immediately, twelve minutes becomes about nine.

Capacity falls with discharge rate

Rated capacity is measured at a low discharge rate, often 0.2C or 1C. At higher rates, internal resistance causes voltage sag, and the pack delivers less usable energy before reaching the cutoff voltage. This is the Peukert effect — less pronounced in lithium chemistry than in lead-acid, but present.

At 5C, expect a few percent below rated. At 10C and above the loss becomes significant, and transient loads may trigger a low-voltage cutoff well before the average draw would suggest.

Hover is not cruise

For a multirotor, hover power is governed by disc loading. From momentum theory, induced power varies with thrust to the power of 3/2 divided by the square root of disc area. The consequence is that a heavier aircraft on the same rotors costs disproportionately more power.

Payload penalties are therefore worse than linear: adding 20% to all-up mass costs noticeably more than 20% of hover power. This is why multirotor endurance degrades so sharply with payload, and why oversized rotors are one of the few genuinely free improvements available.

For fixed-wing aircraft, cruise near the best endurance speed is far more efficient than any hover, which is why an endurance requirement often decides configuration before anything else does.

Losses to account for

Each stage in the chain takes a share:

• ESC and motor efficiency — typically 80–90% combined at the design point, and worse away from it
• Propeller efficiency — 70–85% for a well-matched propeller, considerably less if mismatched to the motor or flight speed
• Avionics, payload, datalink, heaters — individually small, collectively not
• Temperature — cold reduces available capacity and raises internal resistance markedly, and a cold-soaked pack may deliver noticeably less than the same pack at 20 °C

A more honest estimate

t ≈ (C × DoD × k_rate) / I_avg

where DoD is usable depth of discharge (≈0.75–0.8), k_rate derates for discharge rate (≈0.9–0.97), and I_avg is the average current including avionics and payload, weighted across the mission profile rather than taken at cruise.

Then reduce further for climb segments, headwind, and a reserve for diversion or a second approach.

Measure, then trust

Every estimate should be validated by a controlled endurance test: fixed payload, known conditions, logged voltage and current, flown to your actual reserve threshold rather than to depletion.

Do this a handful of times and you will have a correction factor for your airframe that is worth more than any model. Fly the test before quoting an endurance figure to a client, because the figure you quote becomes the figure you are held to.