Motor Capacitor Size Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Electrical results can look precise while a unit, operating mode, or component assumption quietly changes the answer. Motor Capacitor Size Calculator keeps the calculation tied to the quantities on this page, so you can trace the displayed motor capacitor size back to the values you entered rather than treating it as a black-box number.

What this calculator does

The Motor Capacitor Size Calculator turns Voltage, Startup energy into the page’s Motor capacitor size using the circuit or component relationship below. Where the page shows supporting quantities, they come from the same idealized model so you can cross-check the headline rather than reading one isolated number.

How to use it

Start with the fields that actually drive this result: Voltage, Startup energy. Keep units consistent with the menus beside the fields and avoid mixing values measured under different conditions. After calculating, change one input at a time if you are comparing scenarios; that makes cause-and-effect much easier to see.

How the calculation works

The current page treats the capacitor as an energy-storage element and rearranges E = ½CV² to C = 2E/V². It therefore estimates capacitance from entered startup energy and voltage rather than from motor horsepower or a manufacturer start/run-capacitor table.

Example

Using the page’s default example (Voltage = 110 V; Startup energy = 9 J), the calculator reports Motor capacitor size of 1,487.603306 µF. Change one driving input at a time and confirm the result moves in the direction predicted by the equation; that is a quick way to catch a unit or mode mistake.

How to interpret the result

Interpret the Motor capacitor size as an idealized circuit/component quantity for the exact mode and units entered. Use it to compare designs or check hand calculations, but keep component tolerances, ratings, frequency dependence, and real operating conditions in view before turning the number into a hardware decision.

Limitations and notes

This page is an energy-equivalent capacitance calculation, not a motor-manufacturer start/run capacitor selection tool. It does not use motor horsepower, winding impedance, duty cycle, phase angle, or required starting torque.