Stored capacitor energy and electric charge
Energy and charge describe different quantities: joules measure stored work, while coulombs measure charge. Display rounding can show a small watt-hour value as nearly zero even when the joule output remains meaningful.
For a separate steady resistive load calculation, the Ohm’s Law Calculator relates voltage, current, resistance, and power.
The formula
The ideal stored-energy relation integrates the work required to charge a capacitor. It scales linearly with capacitance and quadratically with voltage. The watt-hour output expresses the same energy using an electrical energy unit.
Worked example
A 1,000 µF capacitor charged to 12 V stores 0.5 × 0.001 × 144 = 0.072 J, equal to 72 mJ or 0.00002 Wh. Its stored charge is 0.012 C. Doubling the voltage to 24 V increases energy fourfold to 0.288 J.
How to use this calculator
- Enter capacitance, capacitor voltage using the units shown.
- Check both measurements against their labels before calculating.
- Calculate and review stored energy alongside the supporting quantities.
Use the actual voltage across the capacitor
Supply voltage and capacitor terminal voltage can differ in a circuit, especially during charging or when capacitors are arranged in series. Use the voltage across the component being evaluated. The capacitance should also correspond to its operating condition; some capacitor types change effective capacitance with bias, frequency, or temperature.
Delivered energy can be smaller
Equivalent series resistance, leakage, converter losses, and a minimum usable voltage can reduce energy delivered to a load. To find energy released between two voltages, subtract the stored energy at the lower voltage from the energy at the upper voltage. This page models an ideal component and does not determine discharge current, component ratings, or safe handling procedures.
Assumptions & limitations
What this calculation assumes
- Capacitance is constant across the entered voltage.
What to keep in mind
- Excludes equivalent series resistance, leakage, voltage derating, and load conversion losses.
Common questions
Why does doubling voltage quadruple energy?
Voltage is squared in the stored-energy formula, so a factor of two in voltage becomes a factor of four in energy.
Can I calculate energy available above a cutoff?
Calculate energy at the starting voltage and at the cutoff voltage, then subtract. The difference is ideal released energy.
Can stored watt-hours predict runtime directly?
Only with a suitable usable-voltage range and load model. Converter losses and a cutoff voltage reduce deliverable energy.
Sources & further reading
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