How the Formula Works

This calculator measures how efficiently a drone motor converts electrical power into mechanical thrust. It’s a key metric for selecting motors and propellers and optimizing flight time.

The formulas are:

Power Consumption (Watts) = Motor Current Draw (A) × Battery Voltage (V)
Efficiency (grams/watt) = Motor Thrust (grams) / Power Consumption (Watts)

Step-by-step breakdown:

  1. Power Consumption (Watts): Electrical power drawn by the motor is the product of current (amps) and voltage (volts). This is the fundamental electrical power equation P = I × V.
  2. Efficiency (grams/watt): The thrust produced per watt of power consumed. Thrust is measured in grams, so dividing by power gives grams per watt. Higher values indicate a more efficient motor-propeller combination.

Example

  • Motor Thrust: 1500g
  • Motor RPM: 10,000
  • Current Draw: 15A
  • Battery Voltage: 14.8V
Power Consumption = 15 × 14.8 = 222 Watts
Efficiency = 1500 / 222 = 6.76 grams/watt

A motor producing 6.76 grams of thrust per watt is considered efficient.

Notes on RPM

While RPM is collected as an input, it’s primarily used for reference and validation. The efficiency calculation fundamentally depends on thrust, current, and voltage. RPM variations help you understand the operating point of the motor (e.g., a motor spinning at high RPM with low thrust may be stalled or inefficient).

FAQs

1. What is a “good” grams/watt efficiency?
For modern brushless motors, typical values range from 5–10 grams/watt. Larger propellers generally produce better efficiency (more grams/watt) than smaller ones at the same power. Racing drones (small props, high RPM) often achieve 3–5 grams/watt, while larger camera drones with efficient propellers can reach 8–10+ grams/watt.

2. Why does my motor have better efficiency at lower RPM?
Generally, larger propellers spinning at lower RPM are more efficient because they move more air per revolution. This is why camera drones use large, slow-spinning propellers while racing drones use small, fast-spinning ones.

3. How do I increase my drone’s efficiency?

  • Use larger, more efficient propellers
  • Select motors matched to your propeller size
  • Reduce weight (lighter frame, battery)
  • Use higher voltage batteries (lower current for same power)
  • Ensure propellers are balanced and clean

4. Does this calculator account for ESC efficiency?
No. It assumes the motor is the only consumer. Real ESCs have 90–95% efficiency, so actual total power consumption is slightly higher. The grams/watt figure is a useful relative comparison, not an absolute measurement.

5. What’s the difference between thrust and efficiency?
Thrust is the force produced (in grams). Efficiency is the thrust produced per unit of power (grams/watt). A motor can produce high thrust but be inefficient (wasteful), while another can produce less thrust but be vastly more efficient.

6. Why is RPM important if it’s not in the main formula?
RPM indicates the operating point and helps you stay within the motor’s safe range. Motors have a maximum RPM (KV rating × voltage). Running way over recommended RPM can destroy the motor. RPM also correlates with thrust in a roughly quadratic relationship.

7. Can I use this to compare different motors?
Yes. If you test different motors and propellers at the same voltage and measure thrust and current, you can compare their grams/watt efficiency. The one with the higher grams/watt at your desired thrust level is generally more efficient for your application.

Reference Links