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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 in United States units and terminology, and each formula’s basis 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. Moderate confidence — sound arithmetic, but allow for the variation any real site introduces. Set for United States. The market selector changes the units and the trade terminology; any standard behind the formula is cited under sources.

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 — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.

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.

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.