Emergency backup minimum sizing tool

Understanding minimum emergency backup

The direct answer for a typical minimum setup

For a fridge, a few lights, device charging, and a well pump running 15 minutes a day, critical load totals about 1,788Wh a day. For a 2-day outage at 12V with 80% depth of discharge, that needs roughly 372Ah of battery capacity, a genuinely modest system compared to a full off-grid build sized for indefinite use. The FEMA's generator safety guidance lays out the same rules from the regulator's side.

Why the inverter needs more headroom than daily load suggests

A well pump motor draws a large surge of power for a moment when it starts, commonly 2 to 3 times its running wattage, to overcome the initial mechanical resistance of getting the motor spinning. The inverter must be sized for that starting surge, not just the running wattage, or the pump simply won't start when called on, even though its steady-state power draw is comparatively modest. Pair this with the battery lifespan estimator to see the full picture before you commit.

Whether solar panels are necessary for this kind of backup

For short outages of a few days, a battery alone, charged from the grid beforehand, often covers the need without any solar panels at all, since the goal is riding out a brief interruption rather than sustaining indefinite off-grid operation. Solar panels become more valuable if outages could extend beyond a few days, or if grid charging isn't reliably available before the outage starts, such as in advance of a forecasted major storm. Ready.gov's power outage preparedness guidance covers the broader planning picture beyond just the battery and inverter sizing here.

Deciding whether to include a well pump

If your home relies on a well for water, including it is worth the added cost, since losing water access during an extended outage is a genuine hardship beyond mere inconvenience. It's a relatively small addition to daily energy load but has an outsized effect on required inverter size due to motor starting surge, so factor that tradeoff into your minimum backup budget.

What to check once you have a minimum sizing figure

Compare against the generator backup hybrid sizing tool if a generator is part of your emergency plan alongside battery backup, and check the off grid inverter sizing calculator for a fuller inverter sizing breakdown beyond this minimum estimate. If a generator ends up part of the plan, FEMA's generator safety guidance is worth reading before running one for the first time. For just the essential loads rather than this fuller picture, the critical loads only sizing calculator in our battery storage tools trims things down further, and the battery lifespan estimator in our maintenance and lifespan tools is worth checking since an emergency-only battery ages differently than one cycled daily.