How this calculator works
A load can use the difference between the capacitor energy at its initial and final voltages. Using only the initial voltage assumes discharge all the way to zero. Add practical allowance for efficiency, capacitance tolerance, series resistance and ageing before selecting a part.
Formula
C = 2E/V². Across a voltage window: C = 2E/(Vi² - Vf²).
Worked example
Storing 50 J at 10 V requires 1 F. Delivering 50 J while dropping from 10 V to 5 V requires 1.333333 F.
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Frequently asked questions
Can this select a capacitor from motor horsepower alone?
No. Motor starting depends on the auxiliary winding and required phase shift. Use motor-specific data.
Which voltage belongs in the equation?
Use capacitor terminal voltage, including any maximum charging voltage. The voltage-window mode also needs the lowest usable terminal voltage.
Should I choose the exact result?
Choose an appropriately rated component and check tolerance, pulse duty, temperature and service life. The calculated value is an ideal estimate.