Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Total runtime at this load
15.2
Days of backup (at chosen daily hours)
0.6
Energy needed per day of backup
19.2
Can battery start your surge load? (1=yes)
1
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How to use this
- 1Enter usable battery capacity (kWh).
- 2Enter round-trip efficiency (%).
- 3Enter total continuous load (W).
- 4Enter peak surge load (startup) (W).
- 5Enter battery continuous power rating (kW).
- 6Enter hours of backup needed per day (hrs).
- 7Read your total runtime at this load on the right — it updates as you type.
- 8Hit Share to keep the scenario or send it to someone.
About this calculator
Home battery specs quote usable kWh, but runtime depends entirely on what you're actually running. This calculator sums the wattage of the loads you select for outage backup, applies inverter efficiency, and divides your battery's usable capacity by that draw to get hours of runtime. It also flags whether your battery's continuous power rating (kW, not kWh) can even start your loads — a common gap where a battery has plenty of energy but not enough instantaneous power to run a well pump or AC compressor simultaneously with other loads. Depth-of-discharge and efficiency losses are built in so the number reflects real-world performance, not the marketing spec sheet.
Worked example
Using the values the calculator loads with:
Inputs
- Usable battery capacity: 13.5 kWh
- Round-trip efficiency: 90 %
- Total continuous load: 800 W
- Peak surge load (startup): 2500 W
- Battery continuous power rating: 5 kW
- Hours of backup needed per day: 24 hrs
Results
- Total runtime at this load: 15.2
- Days of backup (at chosen daily hours): 0.6
- Energy needed per day of backup: 19.2
- Can battery start your surge load? (1=yes): 1
What each field means
Inputs
- Usable battery capacity (kWh)
- The usable battery capacity used in the calculation, measured in kWh. Starts at 13.5 kWh so you have a working example on load.
- Round-trip efficiency (%)
- The round-trip efficiency used in the calculation, measured in %. Starts at 90 % so you have a working example on load. Accepted range: 60–100 %.
- Total continuous load (W)
- The total continuous load used in the calculation, measured in W. Starts at 800 W so you have a working example on load.
- Peak surge load (startup) (W)
- The peak surge load (startup) used in the calculation, measured in W. Starts at 2500 W so you have a working example on load.
- Battery continuous power rating (kW)
- The battery continuous power rating used in the calculation, measured in kW. Starts at 5 kW so you have a working example on load.
- Hours of backup needed per day (hrs)
- The hours of backup needed per day used in the calculation, measured in hrs. Starts at 24 hrs so you have a working example on load. Accepted range: 1–24 hrs.
Results
- Total runtime at this load
- Returned as a length of time and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Days of backup (at chosen daily hours)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Energy needed per day of backup
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Can battery start your surge load? (1=yes)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
What loads should I actually back up?
Prioritize the refrigerator, well pump or sump pump, a few lights and outlets, internet/router, and one AC or heat zone if you have the capacity. Trying to back up an entire house's central AC and electric range usually needs more power than a single battery module provides — that's a whole-home generator's job, not a battery's.
Why does surge load matter separately from continuous load?
Motors — well pumps, refrigerator compressors, AC units — draw 2-3x their running wattage for roughly a second at startup. If your battery's inverter can't cover that surge, the load will trip out even though the battery has plenty of stored energy. This is the most common battery-sizing mistake.
How much does depth of discharge affect usable capacity?
Manufacturers already report 'usable kWh' net of DoD limits in most modern lithium batteries (Tesla Powerwall, Enphase, etc.), typically 95-100% usable. Older lead-acid systems only allow 50% DoD, so double the nameplate rating to find true usable energy.
Can I stack multiple battery units?
Yes — most residential systems (Powerwall, Enphase IQ Battery, FranklinWH) support 2-4 unit stacks. Add usable kWh together and take the sum of continuous power ratings; this calculator's numbers scale linearly if you enter combined totals.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Battery Backup Runtime Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/battery-backup-runtime
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/battery-backup-runtime" target="_blank" rel="noopener">Battery Backup Runtime Calculator — RevenueLab</a> (2026).</p>
Source: [Battery Backup Runtime Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/battery-backup-runtime) (2026).