Understanding your autonomy math
Why this number is an estimate, not a quote
This calculator assumes zero recharge for the entire autonomy period, which is the most conservative possible scenario. Real outages usually include at least some daylight hours where solar can recharge the system, which reduces the actual battery count needed. Use this number as a worst-case planning figure, not a guaranteed minimum. The solar recharge sizing tool models a more typical, less conservative scenario.
Why battery count jumps in whole steps
Batteries are manufactured and sold in fixed nameplate sizes, most commonly 10 to 13.5 kWh per unit. A calculated need that falls between two multiples of your unit size always rounds up to the next whole battery, since you cannot purchase a fractional unit. A calculated need of 60.4 kWh nameplate with 13.5 kWh units means buying 5 batteries for 67.5 kWh total capacity. Check the single vs multi battery cost tool to see how this rounding affects your total spend.
How realistic multi-day outage planning actually is
Most utility outages resolve within 24 to 72 hours according to typical restoration timelines tracked by the U.S. Energy Information Administration, so 1 to 3 days of autonomy covers the majority of real scenarios for most regions. Planning for a full week without any recharge usually only makes sense in wildfire public safety power shutoff zones or areas with a track record of slow utility restoration after major storms. The wildfire PSPS planning tool and 1 week backup tool both address these longer scenarios directly.
Why depth of discharge matters more as battery count grows
A conservative depth of discharge setting compounds across every added battery, since each unit contributes the same percentage of unusable reserve, a factor NREL includes in its Annual Technology Baseline assumptions for residential battery performance. Choosing 80% depth of discharge instead of 95% on a 5-battery system means giving up over 10 kWh of nameplate capacity that never gets used. The depth of discharge calculator isolates this factor on its own, and the depth of discharge vs cycle life calculator in our Battery Chemistry & Storage category shows how this setting also affects long-term battery lifespan.
What to do with this number next
Once you know your battery count, check the battery inverter pairing tool to confirm your inverter can handle that many units, or the cost per kWh comparison tool to compare what different brands charge for the total capacity you need. The inverter side of a multi-battery setup is covered in the inverter count for large array calculator in our Inverters & Equipment category.