Battery & storage
Enter your load, a typical outage duration, and a rare worst-case duration. Compare both battery sizes side by side before deciding where to land.
How it works
Step 1
Utility reliability data or your own outage history over the past few years both give a realistic typical figure.
Step 2
The storm season planning tool can help estimate this for storm-prone regions.
Step 3
Many households pick a middle ground rather than either extreme, similar to the tradeoff explored in the battery reserve percentage tool.
Most battery sizing tools ask for one outage duration and produce one battery size, which hides an important tradeoff: sizing for a rare 24 hour worst case costs roughly 4 times more battery capacity than sizing for a typical 6 hour outage, even though the typical outage happens far more often. Comparing both figures side by side makes this tradeoff explicit, similar to how the battery reserve percentage tool makes the backup-versus-savings tradeoff explicit too, and how the battery capacity fade projection tool in our Battery Chemistry & Storage category makes long-term degradation margin explicit in a similar way.
Many utilities publish reliability statistics, often reported as SAIDI (average outage duration per customer per year) and SAIFI (average number of outages per customer per year) figures, which give a data-backed sense of typical outage patterns in a service territory, tracked nationally by the U.S. Energy Information Administration. Your own household's outage history, tracked informally over the past few years, is also a genuinely useful data point, worth comparing against the storm season backup planning tool for your region.
Many households land somewhere between the typical and worst-case figures rather than picking either extreme, accepting that a rare severe outage might require reduced usage or a generator as a backstop while still getting solid coverage for the outages that happen most often, a layered approach Ready.gov's preparedness guidance also recommends rather than relying on a single backup source. This balanced approach avoids both the risk of frequent undersized coverage and the cost of a system sized entirely around a rare event.
Once you've picked a target duration between the two extremes, the battery storage size calculator can work out the exact battery size and count for that chosen figure. If financing affects how large a system you can justify for a rare worst-case outage, the down payment impact on payback calculator in our Financing category is worth reviewing.
Questions
A 600 watt load needs only about 4.4 kWh for a typical 6 hour outage but about 17.4 kWh for a rare 24 hour worst case. Many households size closer to the typical figure and accept that the rare severe outage may require conservation or a generator backstop.
Many utilities publish reliability statistics, sometimes called SAIDI and SAIFI figures, showing average outage duration and frequency for their service territory. Your own household's outage history over the past few years is also a useful, more personal data point.
Not necessarily wasteful, but it does mean paying for capacity that sits unused during the vast majority of shorter, more common outages. Whether that tradeoff is worth it depends on how disruptive the rare severe outage would be and your tolerance for that risk.
Yes, many households pick a middle duration, covering most outages fully while accepting that the rare longest outage might require reduced usage or a generator as a backstop. This balances upfront cost against backup coverage more evenly than either extreme.