Power Supply Hold-Up Capacitor Calculator (Bulk Capacitor Sizing)

What this tool does

When the input disappears, the bulk capacitor alone feeds the converter, and the converter keeps working while the capacitor voltage falls from V₁ to the lowest voltage V₂ it accepts. The energy released is ½C(V₁² − V₂²), and it has to equal the output power times the time, divided by the efficiency: C = 2Pt ÷ (η(V₁² − V₂²)). The capacitor is sized on the voltage swing, not on the voltage: letting a 400 V bulk capacitor fall only to 300 V uses about 44 % of its stored energy. One mains cycle is 20 ms at 50 Hz, a common target; remember capacitors lose capacity with age and cold.

Formula

C = 2 P t ÷ (η (V₁² − V₂²)) · E = P t ÷ η · t = η C (V₁² − V₂²) ÷ (2P)

Variables

Symbol	Meaning	Unit
`p`	Output power to be held	W
`eta`	Converter efficiency	%
`th`	Hold-up time wanted	ms
`v1`	Capacitor voltage when the input fails	V
`v2`	Lowest voltage the converter still works at	V
`cc`	Capacitor you plan to fit	µF
`CN`	Capacitance needed	µF
`EN`	Energy the capacitor must give	J
`TH`	Hold-up time your capacitor gives	ms

Worked example

Output power to be held150 W
Converter efficiency85 %
Hold-up time wanted20 ms
Capacitor voltage when the input fails380 V
Lowest voltage the converter still works at300 V
Capacitor you plan to fit470 µF

Capacitance needed129.8 µF
Energy the capacitor must give3.53 J
Hold-up time your capacitor gives72.4 ms

Limitations

Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.
The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.