Understanding zero-recharge battery sizing
Why this is the most conservative planning scenario
Sizing without any solar or grid recharge assumes the battery has to store every kWh needed for the entire outage duration upfront, with nothing added back in until grid power returns. This produces the largest, most expensive battery figure of any sizing approach on this site, since it makes no assumptions about favorable weather, working equipment, or a connected solar array. Use it when those assumptions genuinely don't apply to your situation, not as a default starting point if you do have working solar backup.
When zero-recharge sizing is the realistic choice, not just the cautious one
Homes without any solar installation, solar systems that aren't wired through a backup gateway to stay connected during an outage, a distinction the Department of Energy's inverter basics guide explains as a safety requirement for grid-tied systems, and locations with historically unreliable sun exposure during typical outage season, tracked in NREL's solar resource data, are all situations where zero-recharge sizing reflects genuine reality rather than excessive caution. The with solar recharge tool is the better starting point if none of these apply to you.
The middle ground most installers actually recommend
Few real installations size purely to one extreme or the other. Many installers instead size somewhere between the two, assuming partial rather than full daily solar recharge to account for less-than-ideal weather during an actual outage, producing a battery size larger than the optimistic full-recharge figure but smaller than this fully conservative zero-recharge figure, a middle ground the inverter itself also needs to support, as covered in the off-grid inverter sizing calculator in our Inverters & Equipment category. Comparing both extremes side by side, using this tool and the battery storage size calculator together, helps calibrate where a reasonable middle ground sits for your specific situation.
What to do with this number next
If this zero-recharge figure feels too large to be practical, the critical loads calculator can help trim your load list down, or the charge time from grid tool shows what a connected solar system would change about this math. The quick financing comparison tool in our Financing category helps weigh whether this larger, more conservative figure still fits your budget.