Battery Autonomy Calculator
How long does your battery system last during an outage or off-grid stretch?
About
Autonomy = (battery capacity × usable depth of discharge) ÷ daily load. The result tells you how many days a fully charged battery covers your loads with zero solar input — useful for outage planning and off-grid sizing.
How it works
- Enter total nameplate capacity in watt-hours (Wh).
- Enter your honest daily consumption in Wh — the most common error is underestimating standby loads.
- Adjust DoD to match chemistry. LFP can cycle to 100% but lasts longer at 80%; lead-acid should never go below 50%.
Frequently asked questions
What depth of discharge should I use for LiFePO4?+
LFP cells tolerate 100% DoD but cycle life drops 20–30% compared to limiting them to 80%. For daily-cycled storage, 80% gives the best calendar lifetime; for occasional outage backup, 100% is fine.
Why doesn't my battery last as long as the calculation says?+
Three reasons: inverter self-consumption (10–30 W idle), DoD that's lower than nameplate to protect cycle life, and capacity loss with age. Real-world autonomy is typically 80–90% of theoretical for the first 3 years.
Should I size my battery for 1 day or 3 days of autonomy?+
1 day is typical for grid-tied solar with battery backup (where outages are rare). 3 days is the off-grid standard, providing buffer for cloudy stretches. Beyond 3 days, generator backup is far cheaper than more battery.
Does a higher daily load proportionally reduce autonomy?+
Yes — autonomy is inversely proportional to daily load. Doubling the load halves the days. Reducing parasitic loads (always-on devices, inverter idle) is the cheapest way to extend autonomy.