Inductors store energy in a magnetic field; capacitors store it in an electric field. In a simple circuit model, voltage across an inductor controls how quickly its current changes, while current through a capacitor controls how quickly its voltage changes. Designers often use them together because these functions complement each other.
Choosing between them starts with the required circuit behavior. Equal size, similar price or a superficially similar energy figure does not make one a replacement for the other.
Compare the ideal models first
The following relations assume constant inductance L or capacitance C, use the passive sign convention, and exclude parasitics. Phase relationships apply to sinusoidal steady state.
| Property | Inductor | Capacitor |
|---|---|---|
| Stored field energy | E_L = ½LI² | E_C = ½CV² |
| Voltage-current relation | v_L = L × di/dt | i_C = C × dv/dt |
| Reactance magnitude at frequency f | X_L = 2πfL | X_C = 1/(2πfC) |
| Phase relationship | Current lags voltage by 90° | Current leads voltage by 90° |
| DC steady-state model | Zero ideal inductive voltage drop at constant current | Zero ideal current at constant voltage |
| Common circuit role | Manage current change or transfer energy | Support voltage or provide a frequency-dependent current path |
The DC row describes the ideal steady state after transients; it is not permission to ignore winding resistance, leakage or component ratings. Textbook foundations are OpenStax §15.2, magnetic energy and capacitor energy.
Worked comparison: same interval, different state variable
Illustrative calculations, not measurements: take an ideal 10 µH inductor and an ideal 10 µF capacitor. Assume the stated voltage or current is maintained over 10 µs and the component values remain constant.
Inductor: V_L = 1V
ΔI = V_L × Δt / L = 1 × 10µs / 10µH = 1A
Capacitor: I_C = 1A
ΔV = I_C × Δt / C = 1 × 10µs / 10µF = 1VThe inductor calculation gives a current change. The capacitor calculation gives a voltage change. In a real network the surrounding circuit determines whether those constant-voltage or constant-current assumptions hold.
For two separate operating points, the same ideal inductor at 2 A stores 20 µJ, while the capacitor at 5 V stores 125 µJ. Those numbers describe the selected values and operating points; they do not establish a general ranking of energy density, cost or efficiency.
At 100 kHz, the ideal reactance magnitudes are approximately 6.28 Ω and 0.159 Ω respectively. These are reactances, not loss resistances or predictions of the complete impedance of an unspecified real part.
In a converter, the question is usually how they work together
In a conventional buck output stage, the series inductor shapes current ripple and the output capacitor carries the difference between inductor current and load current. Output ripple and transient response depend on both parts, their losses and the controller.
Reducing L can increase current ripple; adding capacitance does not automatically preserve peak current, loop stability, startup or size. Begin with the controller's documented design procedure, then validate the assembled circuit. The buck inductor calculator can support an initial calculation; it does not qualify a replacement part. Coilcraft's DC-DC inductor selection note connects circuit requirements with datasheet limits.
Where the simple comparison stops working
For an inductor, examine L under DC bias, the manufacturer's Isat criterion, RMS-related heating, DCR and frequency-dependent losses. Isat and a heating-current rating are different specifications. A winding-loss calculation alone misses core and additional AC losses. See current and temperature ratings.
For a capacitor, examine operating voltage, effective capacitance, temperature, ESR, ESL and applicable ripple-current limits. High-permittivity ceramic types such as X5R and X7R can lose effective capacitance under DC bias; that behavior is not universal to every capacitor technology. Murata's DC-bias FAQ explains the distinction. Use the exact part's curves rather than assuming its printed nominal value remains available.
For both, inspect impedance over the relevant frequency range. In simple parasitic models, an inductor becomes capacitive above its first self-resonance, while a capacitor becomes inductive above its self-resonance. The actual curves and mounting matter. See Coilcraft's measurement guidance and Murata's impedance/ESR explanation. “Inductors block AC” and “capacitors pass AC” are inadequate design rules without frequency and topology.
Turn the comparison into a selection record
- Specify the current waveform, voltage range, signal or switching frequency and permitted ripple.
- Compare the exact parts under the same bias and temperature conditions, including maximum values and tolerances.
- Estimate losses, then check the board's thermal behavior and relevant transients.
- Check layout-dependent current paths and coupling; a universal placement distance cannot replace a board-level check.
- Keep the data sources, calculations and measured results separate. Decide on size, price and life using actual part data and requirements.
For more background, read what an inductor is or use the datasheet directory to find the original specifications.
Sources and revision note
The linked sources distinguish ideal physics from manufacturer-specific behavior. No component lifetime, cost advantage or percentage improvement is inferred from these examples.
Revised 2026-10-24: removed unsupported career and project claims, fixed heading structure and replaced blanket component rankings with conditional comparisons. Prepared with AI assistance; no independent engineering or native-speaker review is claimed. The original publication date and URL are retained in the editorial record.
