Capacitor energy and hold-up time
Stored capacitor energy is useful for first-pass hold-up checks, but usable hold-up depends on the allowed voltage drop and load profile.
Can capacitor energy directly tell you hold-up time?
Only as a first pass. Energy is 1/2 * C * V^2, but hold-up depends on how much voltage drop the load can tolerate, the load current or power, effective capacitance, and discharge path.
Model summary
- Stored energy: E = 1/2 * C * V^2.
- Usable energy between two voltages: Eusable = 1/2 * C * (Vhigh^2 - Vlow^2).
- For approximate constant power, hold-up time is Eusable / Pload before efficiency and converter limits.
Worked example
A 1000 uF capacitor at 12 V stores 0.072 J total.
If the load may run down only to 10 V, usable energy is 0.5 * 1000 uF * (12^2 - 10^2) = 0.022 J.
At 1 W load, that ideal usable energy is only about 22 ms before losses and minimum voltage limits.
Design sequence
Estimate stored energy, subtract the unusable voltage range, then check discharge timing and real capacitor behaviour under voltage, temperature, and ageing.
Common mistakes
- Using total stored energy instead of usable energy over the allowed voltage range.
- Ignoring MLCC capacitance loss from DC bias.
- Forgetting leakage, ESR, ripple current, inrush, and discharge safety.
When the approximation breaks down
- The simple energy model does not include converter efficiency, pulsed load profiles, ESR heating, or control-loop dropout behaviour.
- Safety-related discharge timing needs a standards and hazard review, not just an energy calculation.
Related calculators
Capacitor energy calculator
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Capacitor discharge calculator
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RC time constant calculator
Use the general charge and discharge time-domain solver.
Engineering conversion calculator
Convert uF, mF, joules, and milliseconds.