Capacitor Energy Calculator

Capacitors store electrical energy in an electric field between conductive plates separated by a dielectric insulator. Unlike batteries that use electrochemical reactions, capacitors store energy purely electrostatically, enabling extremely rapid charge and discharge cycles essential for power supplies, pulse power applications, and energy buffering. The fundamental energy relationship E = ½CV² shows energy increases with the square of voltage—doubling voltage quadruples stored energy, making voltage the dominant factor in energy storage applications. Capacitance ranges from picofarads in RF circuits to thousands of farads in supercapacitors for regenerative braking and backup power.

Capacitor Types and Applications: Ceramic capacitors provide low capacitance (pF to µF) with excellent high-frequency performance and kilovolt voltage ratings for RF and power electronics. Aluminum electrolytic capacitors offer high capacitance (µF to mF range) at moderate voltages (6V to 450V) for power supply filtering and bulk energy storage. Film capacitors (polypropylene, polyester) excel in AC applications, motor run capacitors, and inverters with self-healing properties and kilovolt ratings. Supercapacitors achieve farads to kilofarads using activated carbon electrodes, bridging conventional capacitors and batteries with 10-100× higher energy density than electrolytics and 10-100× higher power density than batteries.

Charge and Discharge Characteristics: Capacitor charging through a resistor follows exponential curves governed by RC time constant τ = RC. Voltage reaches 63.2% of final value in one time constant, 95% in three time constants, and 99.3% (effectively full charge) in five time constants. Discharge follows inverse exponential decay—63.2% energy released in one time constant. Constant current charging produces linear voltage changes per I = C(dV/dt), enabling precise timing circuits and achieving near 100% efficiency versus 50% efficiency for resistive charging which wastes half the energy as heat.

Equivalent Series Resistance (ESR): Real capacitors exhibit ESR from electrode and dielectric losses, dissipating energy as heat during charge-discharge cycles. Aluminum electrolytics show 0.01Ω to several ohms ESR; low-ESR types achieve <0.050Ω for switching power supplies. Power dissipated equals I²R where I is RMS ripple current. Excessive ripple current raises temperature, reducing lifetime per Arrhenius equation (lifetime halves for every 10°C temperature increase). Multiple parallel capacitors reduce equivalent ESR and distribute ripple current—four 100µF in parallel present one-quarter ESR of single 400µF capacitor.

Voltage Rating and Derating: Voltage rating specifies maximum continuous DC plus peak AC voltage before dielectric breakdown. Exceeding ratings risks catastrophic failure with fire or explosion in electrolytic capacitors. Conservative design derates to 50-80% of maximum voltage rating for reliability. AC applications require DC ratings exceeding √2 × VRMS (120V AC = 170V peak). Temperature derates voltage capability—a 350V capacitor may be limited to 280V at 105°C. Transient overvoltages demand additional derating or suppression devices.

Safety and Efficiency: Capacitors discharge instantaneously creating shock hazards—a 10,000µF capacitor at 400V stores 800 joules (lethal energy). Bleeder resistors discharge capacitors to <50V within 1-60 seconds, but can fail open leaving dangerous charge. Always verify zero voltage before handling. Polarity-sensitive electrolytics and tantalums explode violently if reversed or overvoltage. Supercapacitors demonstrate 95-98% round-trip efficiency versus batteries (70-90%), making them attractive for frequent cycling applications. Leakage current drains charge over hours (electrolytics) to years (ceramics).

Standards Reference: IEC 60384 (Fixed Capacitors for Electronic Equipment), IEEE 18 (Shunt Power Capacitors), ANSI C57.12.00 (Power Transformers).