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Battery Backup Runtime Calculator

Estimate how long a battery backup system will power a load before depleting.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax

Estimated runtime

5.67 hours

Check your inputs

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
1700 Wh

For the dimensions entered, expect a estimated runtime of 5.67 hours. Note that battery capacity (wh) and connected load sit at the edge of the range this calculator was checked against, so treat the output as indicative rather than settled. Moderate confidence — sound arithmetic, but allow for the variation any real site introduces. Set for United States against IRC 2024. The market selector changes both the units and the code cited.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Runtime (hours) = (battery capacity in watt-hours x system efficiency) / load in watts

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Frequently asked questions

Why can't I use the full rated battery capacity?
Inverters lose some energy converting stored DC power to usable AC power, and many battery chemistries (especially lead-acid) shouldn't be fully discharged without shortening their lifespan — the efficiency factor accounts for both.
Does lithium vs lead-acid change this calculation?
The formula is the same, but lithium batteries typically tolerate a much deeper discharge safely (often 90-100% vs. 50% for lead-acid), meaning more of their rated capacity is genuinely usable — factor that into your efficiency/usable-capacity assumption.
How does this relate to solar panel sizing?
This estimates how long stored energy lasts without recharging; the Solar Panel Array Sizing Calculator estimates how much daily energy your panels can generate to recharge the battery — a complete off-grid or backup system design uses both together.