Battery Backup Runtime Calculator

How it works

Delivered energy (Wh) = rated capacity × usable-capacity percentage × conversion efficiency. Runtime (hours) = delivered energy ÷ steady load watts. This energy calculation does not prove that the battery or inverter can supply the load’s continuous or startup power.

Worked example: 1,000 Wh battery and 300 W load

Apply the 90% usable-capacity limit: 1,000 Wh × 0.90 = 900 Wh. Apply 90% inverter efficiency: 900 Wh × 0.90 = 810 Wh delivered to the AC side. Divide 810 Wh by 300 W to get 2.70 hours.

This is an energy estimate at a steady load. A refrigerator, pump or power tool may cycle or surge, so its nameplate or measured average watts alone does not prove the inverter can start it.

Energy capacity and power capacity are different

DOE distinguishes energy capacity, normally expressed in kWh or MWh, from power capacity, expressed in kW or MW. Wh tells you how much energy can be stored; W tells you how fast the system can deliver it.

A large Wh number cannot compensate for an inverter whose continuous output is below the load or whose surge rating cannot start a motor or compressor. Compare both ratings with the exact equipment requirements.

Use demonstrated or usable energy when available

The advertised battery capacity may not equal the energy available at the output. Battery-management reserve, state-of-charge limits and conversion losses all reduce delivery. DOE’s battery evaluation method treats demonstrated capacity and efficiency as measured quantities and notes that efficiency depends on operating conditions such as charge/discharge rate and temperature.

If a manufacturer publishes usable Wh at AC output for a comparable load, use that figure and set the remaining percentage assumptions so you do not count the same loss twice.

Battery safety and outage planning

Use a complete product with the intended battery-management system, charger and enclosure; follow its manual for temperature, ventilation, placement, charging and output limits. CPSC says high-energy batteries need protection for charging, short circuits and cell balancing and should be evaluated as a complete system with the charger and end product.

For essential medical, safety or communications loads, do not rely on one calculator estimate. Include reserve, test the actual setup safely and maintain an alternative outage plan.

Sources used for assumptions: U.S. Department of Energy — energy capacity versus power capacity and storage losses; U.S. Department of Energy — Battery Energy Storage System Evaluation Method; U.S. Consumer Product Safety Commission — battery system safety and protection guidance; Jackery — Explorer 1000 v2 official specifications.

Frequently asked questions