Selection Guide

How to Calculate Capacitance for Backup Power Applications

Size supercapacitor capacitance for hold-up and backup: constant-current and constant-power formulas, tolerance, aging margin, and a worked example.

Updated 2026-10-28 · 9 min read

The challenge

Undersized capacitance causes brown-outs at end of life; oversized capacitance costs volume, money, and charge time. The goal is the smallest capacitance that still meets the requirement at the worst corner: cold, aged, and at minimum tolerance.

Constant-current loads

C = I · t / (V1 − V2)

Use this when the load draws a roughly fixed current directly from the capacitor (e.g. a linear regulator or a resistive load).

Constant-power loads

C = 2 · P · t / (η · (V1² − V2²))

Behind a DC/DC converter, current rises as voltage falls. Size by energy and include converter efficiency η.

Add margins

  • Capacitance tolerance (check datasheet; e.g. −10%)
  • End-of-life loss (often specified as −20% to −30% of initial)
  • Low-temperature reduction
  • ESR drop, which reduces the usable window

Worked example

A gateway needs 3 W for 8 s. The buck-boost converter works down to 1.8 V. Supply is 5.0 V (two cells in series). Efficiency is 88%.

C = 2·3·8 / (0.88·(25 − 3.24)) ≈ 2.5 F. With 30% end-of-life and 10% tolerance margin: 2.5 / (0.7·0.9) ≈ 4.0 F at 5 V, i.e. two ~8 F cells in series.

Try the calculator

计算器

Need help applying this?

Send your load profile and our engineers will suggest suitable parts.

Get selection help

Recommended supercapacitor types

Winding-Type Supercapacitors

Cylindrical cells built from wound electrode foils — the workhorse format from a few farads to thousands.

Explore

Radial-Lead Supercapacitors

Through-hole cylindrical cells for PCB mounting, typically from about 0.5 F to around 100 F.

Explore

Combined Supercapacitors

Two or more cells assembled in series as a single component to reach higher rated voltages.

Explore