Battery & storage
Enter your battery size and a reserve percentage. See exactly how much stays held back for outages versus how much is free for daily use.
How it works
Step 1
This starting figure accounts for depth of discharge before the reserve split is applied, the same base figure used in the usable vs nameplate capacity tool.
Step 2
Backup-focused households typically set a higher reserve, savings-focused households a lower one.
Step 3
The storm season planning tool can help decide when a higher reserve makes sense.
A reserve percentage setting tells the battery system to always keep a certain portion of usable capacity untouched during normal daily operation, held back specifically for an unexpected outage. A 20% reserve on a 13.5 kWh battery at 90% depth of discharge keeps roughly 2.4 kWh permanently in reserve, leaving about 9.7 kWh free for daily solar self-consumption or time of use arbitrage cycling, a depth of discharge figure the depth of discharge vs cycle life calculator in our Battery Chemistry & Storage category explores further. This setting is typically adjustable through the battery manufacturer's app.
Every kWh held in reserve is a kWh not available for daily use, which means a higher reserve percentage directly reduces the capacity available for capturing daily solar savings or shifting time of use costs. There's no universally correct reserve percentage, since the right setting depends entirely on how a household weighs the value of guaranteed backup capacity against the value of maximizing daily energy savings, a tradeoff Ready.gov's power outage preparedness guidance frames as a personal risk decision. A household in a hurricane-prone region might reasonably prioritize reserve differently than one focused purely on cutting electricity bills, the tradeoff explored in the self consumption battery size tool.
Because most battery systems allow changing the reserve percentage through an app at any time, some households raise their reserve heading into storm season or hurricane season when outage risk is higher, a seasonal adjustment consistent with FEMA's seasonal home preparedness guidance, then lower it during calmer months to capture more daily savings when the risk of an extended outage is lower. This seasonal approach captures more of both benefits across the year rather than locking in one fixed tradeoff permanently.
If you're deciding how high to set your reserve heading into a specific season, the storm season backup planning tool and outage frequency profile tool both help think through how much backup coverage your specific situation warrants. If your reserve strategy is built around a financed system, the monthly budget fit calculator in our Financing category helps confirm the numbers still work.
Questions
A 20% reserve setting on a 13.5 kWh battery with 90% depth of discharge keeps about 2.4 kWh permanently held back for outages, leaving roughly 9.7 kWh available for daily solar self-consumption or time of use savings.
Households prioritizing outage protection often set reserve to 50% or higher, especially heading into storm season, sacrificing some daily savings potential in exchange for a larger always-available cushion if the grid goes down unexpectedly.
Households prioritizing daily solar self-consumption or time of use arbitrage savings often set reserve closer to 0 to 10%, maximizing the capacity available for daily cycling while accepting a smaller cushion if an unexpected outage occurs.
Yes, many battery systems allow adjusting the reserve setting through an app, and some households raise it heading into storm season or hurricane season, then lower it during calmer months to maximize daily savings potential when outage risk is lower.
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