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The total energy storage capacity of the battery bank.
For a battery rated in amp-hours (Ah) at a given voltage, multiply Ah x voltage to get Wh (e.g. a 100Ah 12V battery is 1,200 Wh).
The total wattage of everything running off the battery.
Add up the wattage of all devices you want to keep running.
Inverter losses AND the battery usable depth of discharge, multiplied together.
This one figure carries two things, so multiply them: a lithium bank at 95% usable depth behind a 90% inverter is about 85%, which is the default here. A lead-acid bank is the case people get wrong — it should only be taken to about 50% depth, so behind the same inverter it is nearer 45%, and using 85% for it overstates the runtime by almost double.
Estimated runtime
5.67 hours
Real-world runtime varies with battery age, temperature, and how deeply it's safely discharged (especially for lead-acid batteries, which shouldn't be fully depleted) — check your specific battery manufacturer's guidance.
- Usable energy
- 1,700 Wh
They open the calculator with your figures already in it
Battery Backup Runtime Calculator: 5.67 hours — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Runtime (hours) = (battery capacity in watt-hours x system efficiency) / load in watts
Inputs used
- Battery Capacity (Wh)
- 2000
- Connected Load
- 300
- System Efficiency (%)
- 85
Intermediate steps
- Usable energy
- 1,700 Wh
Confidence note: Real-world runtime varies with battery age, temperature, and how deeply it's safely discharged (especially for lead-acid batteries, which shouldn't be fully depleted) — check your specific battery manufacturer's guidance.
What this calculation does not cover
- Assumes the load is constant for the whole discharge. Anything that cycles - a fridge compressor, a well pump, a heat pump - draws its full watts only part of the time, so a runtime worked out from summed nameplates comes out far shorter than the bank really delivers, while one worked out from an average hides the peaks. The inverter's own standby draw also continues whether or not anything is switched on, and it is not in the figure unless you add it to the load.
- Runtime is an energy answer and says nothing about whether the system can carry the load at all. Whether the inverter and the battery can supply the continuous watts, and the starting surge of a motor or compressor, is a separate power question this calculation does not touch.
- Takes the bank as fully charged with nothing recharging it. No solar, generator or grid contribution is in the figure, and a bank already part-discharged when the outage began gives proportionally less than the runtime shown.
- The efficiency figure is fixed for the whole run. Capacity lost to cold, capacity faded over the bank's life, and the extra loss a lead-acid bank takes when it is discharged fast all have to be built into that one number yourself - no correction for temperature, age or discharge rate is applied.
- This is not a standby-battery compliance calculation. Fire alarm, emergency lighting and similar life-safety batteries are sized by a prescribed multi-stage calculation with its own derating and recharge requirements, set by the governing standard and the panel manufacturer's own sheet, and a general runtime figure does not substitute for it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Runtime (hours) = (battery capacity in watt-hours x system efficiency) / load in watts
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