Battery & storage
Enter your actual calculated need, what you're considering buying, and your load. See the coverage gap in kWh, percentage, and hours short.
How it works
Step 1
The battery storage size calculator gives you that baseline first, so you can see exactly what a smaller pack gives up. The battery capacity fade projection tool works out what this means for your battery chemistry choice.
Step 2
A budget-driven quote may fall short of the calculated figure without that gap being obvious at a glance; the cost per kWh comparison tool helps evaluate real quotes.
Step 3
A kWh gap is abstract; translating it into hours short makes the real-world impact concrete.
A 20 kWh calculated need met with only a 13.5 kWh battery leaves a 6.5 kWh shortfall, about 67.5% coverage of actual need. On its own, a 6.5 kWh gap is somewhat abstract, but translated into hours at a steady 1 kW load, it becomes a concrete 6.5 hours short of the originally intended backup duration, a gap that matters most against the typical outage durations tracked by the U.S. Energy Information Administration, a much clearer picture of the real-world impact an undersized system would have during an actual outage.
Undersizing often results from working within a tight budget and accepting a smaller system than the calculated need actually suggests, underestimating real household loads during the initial planning process, or simply skipping a careful sizing calculation before making a purchase decision. Running an actual load calculation with a tool like the battery storage size calculator before evaluating any specific quote establishes a clear target, making it much easier to spot a proposal that falls meaningfully short.
A battery smaller than calculated need isn't automatically a mistake if the household understands and consciously accepts the resulting shortfall, for example planning to reduce usage during a longer outage or intentionally treating the battery as partial rather than full backup coverage to manage cost, an approach Ready.gov's power outage preparedness guidance frames as one legitimate way to plan around a real budget. The real risk lies in undersizing unknowingly, discovering the gap only during an actual outage, rather than as a deliberate tradeoff, similar to the mirror-image risk covered in the battery oversizing risk tool, and to the capacity loss covered in the battery capacity fade projection tool in our Battery Chemistry & Storage category, where an accurate starting estimate matters just as much.
If this shortfall feels larger than you're comfortable accepting, the cost per kWh comparison tool can help evaluate whether a larger battery from a different quote closes the gap at a reasonable additional cost. If undersizing was driven by budget constraints, the monthly budget fit calculator in our Financing category can confirm whether a larger system still fits.
Questions
A 20 kWh calculated need met with only a 13.5 kWh battery leaves a 6.5 kWh shortfall, about 67.5% coverage of the actual need, which at a steady 1 kW load translates to roughly 6.5 hours short of the originally intended backup duration.
Undersizing often happens from working with a tight budget and accepting a smaller system than calculated need suggests, underestimating actual household loads during the planning process, or simply not running the sizing math carefully before making a purchase decision.
It can be a deliberate, informed tradeoff if the household understands and accepts the shortfall, for example planning to reduce usage during a longer outage or treating the battery as a partial backup rather than full coverage. The risk lies in undersizing unknowingly rather than as a conscious choice.
Running an actual load calculation with a tool like the battery storage size calculator before evaluating any specific quote establishes a clear target to compare against, making it easy to spot a proposal that falls meaningfully short of real household need before committing to a purchase.