Battery calculators · FREE TOOL

Battery Backup Calculator

Build a small plug-in appliance load list and estimate energy, battery capacity, running watts and known startup demand.

1

Your inputs

Edit the example
Ordinary plug-in appliances

Use measured or manufacturer watts. Startup total is the total during starting, not extra watts. Blank means unknown.

1
2

Planning assumptions

%

Editable planning assumption. Follow the battery maker’s limits.

%

An assumption, not a measured equipment specification.

%

Adds this percentage to the energy estimate; not a state-of-charge cutoff.

W

Added once for the longest entered usage duration. Leave 0 if unmeasured; actual runtime may be lower.

Example estimate · editable inputs

Estimated nominal capacity needed

466.67 Wh

Appliance energy
280 Wh
Simultaneous running
70 W
Startup total
Unknown
Known startup lower bound
70 W

2 appliance startup value(s) unknown. Duty cycle reduces energy, not required running watts. Extra reserve is added to sizing; existing-station runtime is before that extra reserve.

BEHIND THE RESULT

How the calculation works

Load Wh = sum of running W × usage hours × duty fraction. Nominal Wh = (load Wh ÷ efficiency + idle W × longest usage h) × (1 + reserve) ÷ usable fraction. Running W is the sum of running watts; known startup totals assume simultaneous starts.

Duty cycle changes energy, not the inverter’s running-power requirement. Leave startup watts blank when unknown. A missing value never means zero surge.

A worked example

A 60 W laptop for 4 hours and a 10 W lamp for 4 hours use 280 Wh. At 80% usable, 90% efficiency, 20% extra reserve and zero idle draw, estimated nominal capacity is 466.67 Wh. Running demand is 70 W.

What this estimate can’t tell you

For ordinary plug-in devices only. Excludes life-support equipment, critical infrastructure, whole-home integration and transfer-switch systems. Appliance watts are illustrative until you measure or verify them.

Sources & methodology

Formulas are derived from the units shown. Read our methodology or report a correction. Sources reviewed 2026-11-01.