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Capacitors - Stored Energy

Potential power and energy stored in capacitors.

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Capacitor - Energy Stored

The work done in establishing an electric field in a capacitor, and hence the amount of energy stored - can be expressed as

W = 1/2 C U 2 (1)


W = energy stored - or work done in establishing the electric field (joules, J)

C = capacitance (farad, F, µF )

U = potential difference (voltage, V)

Capacitor - Power Generated

Since power is energy dissipated in time - the potential power generated by a capacitor can be expressed as

P = dW / dt                    (2)


P = potential power (watts, W)

dt = dissipation time (s)

Example - Capacitor, energy stored and power generated

The energy stored in a 10 μF capacitor charged to 230 V can be calculated as

W = 1/2 (10 10 -6 F) (230 V) 2

= 0.26 J

in theory - if this energy is dissipated within 5 μs the potential power generated can be calculated as

P = (0.26 Joules) / (5 10 -6 s)

= 52000 W

= 52 kW

Be aware that in any real circuit, discharge starts at a peak value and declines. The energy dissipated is a very rough average power over the discharge pulse.

Capacitor - Time to Discharge at Constant Power Load

The time to discharge a capacitor at constant power load can be expressed as

dt = 1/2 C (U s 2 - U f 2 ) / P               (3)


dt = discharge time (s)

U s = start voltage (V)

U f = final voltage (V)

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