Backup Hours Local calculation · optional analytics

Battery runtime planner

How long will your backup power last?

Build your essential load, enter your battery, and get a practical runtime range—not a perfect-lab promise.

01

Battery setup

02

Essential load

150 W

What the number means

Useful planning, with the uncertainty left visible

The central estimate uses your rated battery energy, usable-capacity limit, inverter efficiency, and connected watts. The range is deliberately wider because batteries rarely match their label under every load and temperature.

The calculation

runtime = rated Wh × usable % × efficiency % ÷ load W

If the battery is listed in amp-hours, rated watt-hours are Ah × volts. For batteries in series, use the bank voltage and the Ah rating of one series string. Add Ah only for identical parallel strings.

Before you rely on it

  • Use measured running watts where possible, not only an appliance label.
  • Check inverter continuous and surge limits separately.
  • Allow more margin for old lead-acid batteries, heat, cold, or high discharge rates.
  • Use the manufacturer runtime chart for safety-critical or warranty decisions.

Common questions

Battery runtime basics

Why is my real runtime shorter?

Common causes are battery age, inverter idle consumption, startup surges, cold or hot conditions, and a load that varies during use. Small lead-acid batteries also lose effective capacity when discharged quickly.

Should I use watts, VA, or amp-hours?

Use watts for the connected load. Use watt-hours for stored energy. If a battery gives only amp-hours, multiply Ah by the battery-bank voltage. VA is an inverter or UPS power limit and does not directly state stored energy.

What usable-capacity percentage should I enter?

Use the value from the battery or system maker. If none is available, the presets are planning assumptions only: 50% is conservative for lead-acid and 80–90% is common for lithium systems.

Does this site upload my appliance list?

No. The calculation runs in your browser and your appliance list is not sent to a server. Optional Google Analytics starts only after you allow it.