Enter the voltage and the start-up energy requirement of the motor into the calculator to determine the appropriate capacitor size.

Capacitor Size Formula

The following formula is used to calculate the capacitor size for an electric motor.

C = E/(.5*V^2) 
  • Where C is the capacitor size (farads)
  • E is the start-up energy requirement of the motor (Joules)
  • V is the voltage (volts)

To calculate a capacitor size, divide the start-up energy by one half of the voltage squared.

Capacitor Size Definition

A capacitor size is defined as the total capacitance required in a capacitor to handle a certain voltage in an electric motor with a given start-up energy.

How to calculate capacitor size?

Example Problem #1:

First, measure the voltage of the motor. For this example a voltmeter is used and the voltage is found to be 100 V.

Next, determine the energy startup requirement of the engine. This was published with the engine itself and found to be 400 Joules.

Finally, calculate the capacitor size using the formula above:

C = E / (.5*V^2)

= 400 / (.5*100^2)

= .08 farads.

Example Problem #2:

In this next example, the voltage is measured to be significantly higher at 200 volts.

Next, the start-up energy of the motor is found to be 600 joules.

Finally, using the same process, the capacitor size is calculated:

C = E/ (.5*V^2)

= 600 / (.5*(200^2))

= .03 farads.

capacitor size calculator