How many days your battery bank lasts
Estimate how many days a battery bank can power your load with no charging, based on usable capacity for the chosen chemistry.
Days of autonomy is how long a battery bank can carry the full daily load with zero solar input — the standard cushion against a stretch of cloudy weather. It is a size-vs-cost tradeoff: each extra day means meaningfully more battery capacity for a cost used only a few days a year, which is why most systems target 1-3 days rather than a week or more.
Related calculators
Looking for a definition or a quick answer? See the solar & battery glossary for terms used on this page, or the FAQ for common sizing questions. For the full explanation, read the days-of-autonomy guide.
Sources & standards
- IEEE 1013 — Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic Systems — Widely used engineering practice for days-of-autonomy and depth-of-discharge sizing methods, referenced here for the general sizing approach.
Estimates for planning only. Verify with a meter and follow NEC / local code; have a licensed installer confirm any system before purchase or wiring. See the methodology for formulas and sources.