Understanding 72 hour backup sizing
Why 72 hours is a widely used emergency preparedness benchmark
Federal emergency preparedness guidance, including Ready.gov's own disaster preparedness recommendations, commonly suggests households be self-sufficient for at least 72 hours following a disaster, reflecting how long it can realistically take emergency services to reach every affected household after a major event. Many people apply this same 72 hour figure to backup power planning specifically, treating it as a reasonable worst-case target without going to extreme lengths.
Why this is often the point to reconsider battery-only sizing
At 72 hours with zero recharge assumed, battery-only sizing starts to require a meaningfully large and expensive system, often 3 to 5 batteries even for a modest critical loads list. This is frequently the point where adding solar recharge to the plan, assuming reasonable sun exposure across the outage, becomes more cost-effective than continuing to scale battery count alone. The solar recharge sizing tool models this combined approach, and the quick financing comparison tool in our Financing category helps weigh whether the added solar equipment cost still comes out ahead.
Trimming the load list versus adding batteries
At longer durations, every watt of load compounds significantly, since the same load multiplied by 72 hours adds up fast, a reason ENERGY STAR's appliance efficiency database is worth checking before finalizing a load list for extended backup. Narrowing the load list down to genuine essentials, rather than scaling battery count linearly to cover a broader list, is often the more cost-effective path to reaching a 72 hour target. The critical loads calculator helps identify what can reasonably be trimmed.
What to do with this number next
If 72 hours still feels short for your region, the 1 week backup tool extends this same math further. Otherwise, the battery reserve percentage tool is worth checking before committing to a purely battery-scaled system at this duration. If chemistry-specific cold or hot weather capacity loss is a factor over 72 hours, the cold weather battery capacity loss calculator in our Battery Chemistry & Storage category models that directly.