Supercapacitor backup time calculator

Enter the effective capacitance, starting voltage, cutoff voltage, load power, and optional converter efficiency. The result includes runtime in seconds, minutes, and hours, along with both the energy available from the capacitor and the energy delivered after conversion losses. It is useful for early sizing, design comparisons, and checking whether a proposed bank offers enough ride-through time.

Model the voltage window that the load can actually use

A supercapacitor does not deliver the same usable energy merely because its nameplate capacitance stays constant. Stored energy depends on the square of voltage, so the relevant quantity is the difference between the energy at the initial voltage and the energy remaining at cutoff. This calculator applies one half times capacitance times the difference of the squared voltages. The cutoff is not necessarily zero: it should represent the lowest input voltage at which the downstream converter, controller, relay, or other protected load remains operational. Set the initial voltage to the bank voltage at the moment backup begins, after accounting for any normal charging limit or voltage derating. Set capacitance to the effective bank capacitance, not the sum of cell ratings unless that sum is correct for the actual series-parallel arrangement. Cells in series reduce effective capacitance, and tolerances, aging, temperature, balancing hardware, and leakage can reduce practical performance. Using conservative values for initial voltage and capacitance and a realistic cutoff produces a more defensible estimate than assuming every stored joule is available.

Convert usable energy into constant-power runtime

The runtime calculation assumes that the protected equipment consumes constant output power throughout discharge. After finding the capacitor's usable energy in joules, the calculator multiplies it by converter efficiency to estimate energy delivered to the load, then divides by load power in watts. Because one watt equals one joule per second, the quotient is backup time in seconds. The same value is also reported in minutes and hours for convenience. Efficiency is entered as a decimal: for example, 0.90 represents ninety percent. Use 1 only for an idealized calculation with no conversion loss. For a regulated electronic load, constant power is often a better first approximation than constant current because input current rises as the capacitor voltage falls. If your load is a simple resistor or genuinely draws constant current, its discharge curve follows a different equation and this result should not be treated as exact. Use the load's worst credible continuous power, or include startup and transient energy separately when those events materially affect the required hold-up interval.

Interpret the estimate and add engineering margin

The output is an ideal analytical estimate, not a guarantee for a physical bank. Equivalent series resistance causes an immediate voltage drop when the load is applied, which may make the system reach cutoff sooner than an energy-only calculation predicts. Wiring, protection devices, balancing resistors, self-discharge, converter quiescent current, and changes in converter efficiency also consume energy. Capacitance tolerance and degradation can be significant, especially across temperature and service life. For design work, calculate more than one scenario: a nominal case for comparison, a low-capacitance and low-efficiency case for expected operation, and a worst-case voltage-drop case that checks whether the load remains above cutoff at connection. Select a bank with deliberate margin above the required duration, then validate the completed assembly under representative load and environmental conditions. The returned usable energy is the capacitor-side energy between the two entered voltages; delivered energy includes the stated efficiency. This separation makes assumptions visible and helps reviewers reproduce the sizing decision instead of relying on a runtime number without context.