Capacitor ESR power loss calculator

Enter the RMS ripple current in amperes and ESR in ohms, and it evaluates P = I_rms² × ESR in watts. The result helps expose a loss that ideal-capacitor calculations omit but switching converters, inverter DC links, and pulsed supplies must manage. It also reports the RMS voltage across the ESR and the energy dissipated during one hour. The browser calculator is free, while an API request costs $0.002. Use ESR measured or specified at the operating frequency and temperature, and use RMS current rather than peak current for a meaningful thermal-loss estimate.

Why ESR turns ripple current into capacitor heat

A practical capacitor is not an ideal reactance. Its foils, electrolyte, terminations, dielectric losses, and construction contribute an equivalent series resistance, commonly shortened to ESR. Alternating ripple current passing through that resistance dissipates real power even when the capacitor's average DC current is zero. The heating relationship is P = I_rms² × ESR, the same resistive-loss rule used for conductors, with RMS current capturing the waveform's mean-square heating effect. Squaring current matters: doubling RMS ripple current raises ESR loss by a factor of four when ESR stays constant. This calculator returns power_loss in watts, esr_voltage in RMS volts from I_rms × ESR, and energy_per_hour in joules from power × 3600 seconds. Those outputs separate three useful views of the same series-resistance effect. Watts describe the continuous thermal load, ESR voltage shows the resistive portion of ripple voltage, and hourly joules help compare cumulative heat energy. The tool does not claim that all dissipated power produces a particular temperature rise. Temperature depends on thermal resistance, mounting, airflow, nearby heat sources, capacitor geometry, and how ESR itself changes as the part warms. Treat the computed watts as an input to a thermal assessment, not as a complete junction- or core-temperature model. Likewise, this result is distinct from energy stored in capacitance, which follows one-half C V squared and can be much larger or smaller without directly determining ESR heating.

Choosing RMS ripple current and a realistic ESR

Use the RMS value of the current that actually flows through the capacitor. Do not enter peak, amplitude, average absolute, or peak-to-peak current unless it has first been converted to RMS for the waveform involved. For a sinusoid, RMS is peak divided by the square root of two, but switching-supply ripple is often triangular, discontinuous, or rich in harmonics, so its conversion can differ. A current probe and waveform math, a converter simulation, or a documented capacitor-current calculation may provide the needed RMS value. ESR also needs context. Datasheets frequently specify impedance or ESR at selected frequencies and temperatures, while real operating ripple may span multiple harmonics. Choose a value representative of the frequencies carrying meaningful current and consider the worst credible temperature and tolerance. For a spectrum with materially different ESR at each harmonic, a single-number estimate can be insufficient; calculate each harmonic's I_rms² × ESR at that frequency and sum the powers when reliable data is available. Enter amperes and ohms to obtain watts. Milliamperes must be divided by 1000, and milliohms must also be divided by 1000 before entry. The calculator validates that both values are finite and non-negative, accepts zero as a legitimate no-loss boundary case, and rejects unsupported magnitudes rather than returning misleading infinities. Optional precision controls displayed decimal places only. It does not change the underlying multiplication, so reducing display precision cannot improve uncertain source data.

Using the loss result in a switching-supply design review

Start by comparing computed ESR power with the capacitor manufacturer's ripple-current conditions and any thermal guidance for the chosen package. A low numerical wattage is not automatically safe: a small component with poor heat transfer can experience a meaningful core-temperature rise, and elevated temperature can shorten life or increase ESR. Conversely, a physically large capacitor with effective cooling may tolerate more loss, but only its datasheet and validated thermal measurements can establish that. In a converter review, repeat the calculation at nominal load, overload, startup, and the operating point expected to produce maximum ripple. Include component tolerance, aging, low-temperature ESR increases, switching-frequency variation, and changes in control mode when they are relevant. Capacitors in parallel do not necessarily share ripple equally; differences in ESR, ESL, capacitance, placement, and trace impedance can concentrate current in one part. Calculate from the estimated current through each device rather than dividing total current blindly. The reported ESR voltage is useful for checking the resistive contribution to output ripple, although total ripple also contains capacitive and inductive components. The hourly energy number is a convenient scale comparison, not a claim that heat accumulates without leaving the package. For automated design sweeps, the API costs $0.002 for each successful request and applies the same deterministic formula as the browser tool. Final hardware should still be checked with appropriate bandwidth, a safe current-measurement method, and temperature measurements under representative enclosure, airflow, ambient, and load conditions.