Quick definition
Every battery gradually loses some of its original capacity over years of use, a natural part of how batteries age through repeated charge cycles. State of health tracks this: it's the battery's current maximum capacity expressed as a percentage of what it could hold when brand new. A battery at 90% SOH can still fully charge, but that full charge now holds 10% less energy than it did on day one.
Unlike state of charge, which resets and cycles daily, SOH moves slowly and steadily in one direction over the battery's entire multi-year lifespan, functioning as the long-term aging indicator rather than a moment-to-moment reading.
At a glance: most home battery warranties guarantee a minimum SOH threshold, commonly around 70%, by a specific number of years or cycles, giving homeowners a concrete benchmark for what healthy long-term decline should look like.
Why it matters
SOH is essentially a battery's aging trajectory made visible and trackable, rather than something that only becomes apparent when a battery suddenly can't hold as much charge as it used to. Watching SOH decline gradually over the years, and comparing that trajectory against the manufacturer's warranty guarantee curve, gives homeowners a genuinely useful way to confirm their battery is aging normally rather than degrading unusually fast.
This is exactly why SOH sits at the center of most home battery warranty structures. Rather than guaranteeing a battery will last a specific number of years with no capacity loss at all, which wouldn't reflect how batteries actually work, warranties instead guarantee that SOH won't fall below a specific threshold, commonly around 70%, before a specific number of years or cycles have passed. Tesla's US Powerwall warranty is a well-known real example: it guarantees the battery will retain 70% of its rated 13.5 kWh capacity ten years after installation, with no cycle limit when used for solar self-consumption. This gives a concrete, measurable standard both the manufacturer and homeowner can reference. For the numbers behind State of health (SOH), the panel cleaning frequency calculator in our Maintenance & Lifespan section walks through the calculation step by step.
Understanding SOH also helps set realistic expectations for a battery's usable life over a system's full multi-decade lifetime. A battery doesn't suddenly stop working when it reaches its warranty threshold; it simply holds somewhat less energy per full charge than it originally did, a gradual, expected pattern rather than a sudden failure to plan around. How fast that decline happens genuinely varies: NREL research that simulated residential storage across several US climate regions found predicted battery lifetimes ranging from about 7 years to more than 20 years depending on the cell's chemistry, design, and operating conditions.
Where you'll see this
The battery capacity fade projection tool projects your specific battery's expected SOH trajectory over its full lifespan.
Since SOC and SOH are closely related but distinct concepts, the battery state of health calculator works through how a current SOH reading gets estimated for your specific battery.
This term comes up in battery warranty documentation, in long-term battery monitoring dashboards, and any conversation about how much usable capacity a battery is expected to retain years into its service life. The safety standard for stationary batteries, UL 1973, is what a home battery is tested against before it is sold for residential use.
Related terms
Frequently asked
This varies by chemistry and specific product, but a gradual, roughly linear decline over many years, generally tracking or beating the manufacturer's warranty guarantee curve, is the expected healthy pattern. A sudden, sharp drop rather than gradual decline would be more concerning and worth investigating.
Not directly in terms of daily operation, since the battery management system still charges and discharges based on current state of charge. SOH mainly affects the total usable energy available at 100% charge, which gradually shrinks as SOH declines over the years.
Most home battery products come with a warranty that guarantees a minimum SOH threshold, commonly around 70%, by a specific number of years or cycles. If actual SOH falls below that guaranteed threshold within the warranty period, the manufacturer typically owes a remedy under the warranty terms.
Operating within moderate temperature ranges, avoiding extended time at very high or very low charge levels where a manufacturer recommends against it, and following manufacturer guidance generally helps support a healthier long-term SOH trajectory, though some gradual decline over the battery's lifetime is normal and expected regardless.