How to Calculate the Energy Stored in a Capacitor
A capacitor stores energy in the electric field between its plates. The amount of energy depends on two things: how much charge the capacitor can hold per volt (its capacitance, C) and how much voltage is applied across it (V). This calculator applies the standard electrostatic energy formula, E = ½CV², along with the related charge formula Q = CV, to convert your capacitance and voltage into stored energy, stored charge, and — if you provide a discharge time — the average power delivered during discharge.
Where the ½CV² formula comes from
As a capacitor charges from 0 volts up to its final voltage V, the voltage across it rises in proportion to the charge already on its plates: at any instant, v = q/C. The work needed to move the next small bit of charge dq onto the plates is dW = v·dq = (q/C)dq. Integrating from q = 0 to the final charge Q gives the total energy stored: W = Q²/(2C). Substituting Q = CV produces the three equivalent forms used throughout electronics: E = ½CV² = ½QV = Q²/(2C). The factor of ½ appears because the voltage — and therefore the "cost" of adding each additional bit of charge — increases linearly during charging rather than staying constant.
Reading the results: charge, energy units, and discharge power
The calculator reports energy in joules (auto-scaled to µJ, mJ, or kJ as needed) and also converts it to watt-hours so you can compare a capacitor's storage capacity to a battery's — capacitors typically store far less energy per unit volume than batteries, which is why they excel at fast charge/discharge rather than long-term storage. Charge (Q = CV) is reported in coulombs, auto-scaled to µC or mC for typical component values. If you enter a discharge time, the tool divides total energy by that time to estimate average power (P = E/t); this is an average over the whole discharge, not the instantaneous peak, which depends on the circuit's resistance and is highest at the moment discharge begins.
Practical notes and safety
- Energy scales with the square of voltage, so doubling the voltage across a capacitor quadruples the stored energy — voltage rating matters as much as capacitance for total energy storage.
- Large electrolytic capacitors and camera-flash capacitors can retain a dangerous charge even after power is removed; always discharge them safely through a bleeder resistor before handling.
- Supercapacitors (ultracapacitors) combine large capacitance (up to thousands of farads) with low voltage ratings (a few volts), so high-voltage applications require stacking them in series.
- Real capacitors have equivalent series resistance (ESR), which dissipates some energy as heat during fast charge/discharge and limits true peak discharge power below the idealized P = E/t estimate.