Battery Runtime Calculator — Backup Load Planner

Plan how long a battery can power your outage loads. Switch appliances on or off, enter measured watts and duty cycles, and check the battery, goal and inverter together.

Plan an outage

Switch appliances on or off to see how much time each choice changes.

Illustrative night outage36 min short of the 8-hour goal

Estimated backup time7 h 24 minAt the average duty cycles you entered

Usable from this charge819 Wh1,024 Wh rated · 100% to 20%

Energy runwayGoal 8 h

Power onBattery reaches 20% at about 7 h 24 min

Average device load93 W

All selected loads on together210 W

Energy for your goal886 Wh usableAbout 1,108 Wh rated at the same charge range

Switching off Router or Lights (one at a time) would raise the estimate to 9 h 15 min. Choose based on what you need to keep powered.

An inverter continuous rating is not entered. Check it against the 210 W all-on load, and check startup surge and battery output limits separately.

Start with a complete example

01 / Battery and goal

Enter the energy you can actually use

Use your battery or power station's rated Wh. For a battery specified in amp-hours, multiply nominal volts × amp-hours to estimate Wh. The charge range is your own planning choice, not a chemistry recommendation.

Rated capacityBackup goalWh on the label or datasheetHours you need the loads powered

My battery lists Ah, not Wh

Multiply the battery bank's nominal volts by its amp-hours. Check how series and parallel batteries change those ratings before entering a bank.

Bank capacity (Ah)Nominal bank voltage (V)Ah for the entire bankV for the entire bank

Starting chargeStop at charge% when the outage begins% reserve or system cutoff

Inverter efficiencyInverter idle draw% from battery DC to appliance AC; use 100 for direct DCBattery-side W while the inverter stays on; use 0 for direct DC

Inverter continuous output, optionalW from its label; this checks all selected running loads together, not startup surge

02 / Outage loads

Choose what stays on

Duty cycle is the fraction of time an appliance runs at its entered watts. Use a measured average or manufacturer data when possible; a refrigerator's duty varies with room temperature and door openings.

Keep on during outage

ApplianceRunning wattsYour own device labelW while switched on

Duty cycle (%)100% means continuously on

Switching this off adds about 1 h 51 min

Keep on during outage

ApplianceRunning wattsYour own device labelW while compressor runs

Duty cycle (%)Illustrative fraction; replace with a measurement

Switching this off adds about 8 h 14 min

Keep on during outage

ApplianceRunning wattsYour own device labelTotal W when on

Duty cycle (%)Half the outage on average

Switching this off adds about 1 h 51 min

The example values are editable illustrations. Check actual watts, average run fraction, available battery energy and the inverter's limits before relying on a plan.

What the estimate means

Usable battery energy = rated Wh × (starting charge − stop-at charge) / 100. Average device watts = sum of each selected appliance's running W × duty cycle. Battery draw = average device watts ÷ inverter efficiency + battery-side idle W. Backup hours = usable Wh ÷ battery draw. The goal's required rated capacity uses the same charge range and load assumptions.

This is an average-load planning model. It does not model motor startup, inverter overload duration, battery voltage sag, cold, aging or the high-rate capacity loss of lead-acid and many UPS batteries. Manufacturer runtime curves are the better source for a specific UPS. A battery monitor or measured power draw makes an estimate more useful. Victron's battery-monitor manual explains discharge floor and the rate effect; CyberPower's UPS tool explains why model-specific runtimes vary.

For a device with a compressor or motor, check its startup requirement against the inverter's separate surge rating. If it will power critical medical or safety equipment, use the equipment manufacturer's backup guidance and verify the actual system under load.

Enable JavaScript to edit the plan. The illustrative 1,024 Wh battery has 819 Wh between 100% and 20% charge. Router 20 W, refrigerator 150 W at 35% duty and lights 40 W at 50% duty draw 92.5 W on average; with a 90% efficient inverter and 8 W idle draw, the estimate is 7 h 24 min.