Selection Guide
Supercapacitor Energy Calculation: Formulas, Usable Energy and Backup Time
Calculate supercapacitor stored and usable energy with E = ½C(V1² − V2²), estimate ideal backup time, and understand real-world losses.
Updated 2026-10-28 · 9 min read
The challenge
Stored energy at full voltage is not the energy your load can use. Converters stop working at a minimum voltage, so only the energy between V1 and V2 is available.
Stored and usable energy
E_usable = ½ · C · (V1² − V2²)
E in joules, C in farads, V in volts. Note: discharging from V1 to V1/2 releases 75% of the stored energy.
Ideal backup time (constant current)
t = C · (V1 − V2) / I
Why real results differ
- ESR drop reduces the effective starting voltage
- Converter efficiency and quiescent current
- Leakage current during long discharges
- Capacitance tolerance, temperature, and aging
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