Quick definition
Just like a car's fuel gauge tells you how much gas is in the tank right now, state of charge tells you how full a battery is at this exact moment, expressed as a simple percentage. A battery at 100% SOC is fully charged; one at 20% has mostly discharged and has relatively little usable energy left before hitting its depth of discharge limit.
SOC changes constantly throughout the day as a battery charges from solar production and discharges to power a home, making it one of the most frequently referenced numbers on any battery monitoring app.
At a glance: SOC is estimated rather than measured with perfect precision, calculated by the battery management system using voltage, current, and other data rather than a direct physical measurement of stored energy.
Why it matters
SOC is the everyday, moment-to-moment number that matters most for actually using a home battery day to day. It's what tells a homeowner whether their battery has enough charge to get through an evening, whether it's ready to provide backup power if the grid goes down, or whether it still has capacity available to soak up more solar production before hitting full.
Understanding SOC's relationship to depth of discharge also matters for interpreting what a given percentage actually means in practice. A battery showing 20% SOC on a system with an 80% depth of discharge limit is already close to its practical usable minimum, even though 20% might sound like there's plenty left. The raw SOC number alone doesn't tell the whole story without also knowing the battery's specific DoD limits.
SOC is genuinely a different concept from state of health, even though the two numbers sometimes get confused. SOC changes constantly and resets to 100% every time the battery charges fully; state of health changes slowly over years and reflects the battery's overall aging and capacity loss, a completely separate, longer-term measure of the battery's condition. The safety standard for stationary batteries, UL 1973, is what a home battery and its management system are tested against. To see State of charge (SOC) in a real example, the backup runtime estimator in our Battery & Storage section applies it to a working system.
Where you'll see this
The battery state of charge estimator works through how SOC estimation actually happens for a specific battery chemistry and system.
Since SOC and long-term battery condition are related but distinct, the battery state of health calculator covers the complementary long-term health metric.
Since SOC determines whether a battery is ready to cover an outage right now, the battery size for outage frequency profile tool helps figure out how much reserve capacity to plan around in the first place.
This term comes up constantly in battery monitoring apps, in home energy management systems, and any conversation about whether a battery has enough charge available for a specific need right now. The Department of Energy's bidirectional charging page explains how a battery's charge level decides how much backup power it can offer a home.
Related terms
Frequently asked
State of charge is a real-time, constantly changing number showing how full the battery is right now, like a fuel gauge. State of health is a slower-moving measure of the battery's overall condition and remaining capacity relative to when it was new. Both matter, but they answer completely different questions.
It's generally estimated rather than measured with perfect precision, since there's no way to directly count the exact energy stored in a battery. The battery management system estimates SOC using voltage, current flow, and other data, refined through algorithms designed to keep the estimate as accurate as reasonably possible.
For many lithium battery chemistries, spending extended time at a full 100% charge can contribute to somewhat faster long-term degradation compared to staying within a more moderate charge range, though the effect and its significance vary by chemistry and specific product. Checking manufacturer guidance for a specific battery is the reliable way to know what's recommended.
SOC reflects the percentage of total rated capacity, not necessarily the percentage you can actually access, since depth of discharge limits often reserve some portion of the battery as a protective buffer. A battery showing 20% SOC might already be at or near its practical minimum usable charge level, depending on the specific DoD limit set for that battery.