Battery State of Health (SOH) Calculator
State of Health (SOH) is the standard way to express how much a battery has degraded at a specific point in time — the percentage of its original rated (nameplate) capacity that a real measurement shows is still usable. This calculator works backward from one measured capacity reading, combining it with the system's age in service and an end-of-life threshold to report SOH, capacity fade, the implied average annual degradation rate behind that reading, and remaining useful life to end-of-life. For projecting future capacity instead of diagnosing a current reading, see the Battery Degradation Calculator. Pair the result with our BESS Round-Trip Efficiency Calculator to see how fade erodes the energy actually delivered each cycle, and our Capacity Market Revenue Calculator to see how shrinking usable capacity reduces accredited capacity payments over time.
The original rated (nameplate) energy capacity of the battery system in MWh.
The currently measured usable capacity of the battery system in MWh, from a capacity test or performance report.
How long the battery system has been operating in service, in years.
Most warranties and project models define end-of-life as 80% of original rated capacity.
Healthy — within normal degradation range
12.5% of original rated capacity (100% − 87.5% SOH)
Within the typical 2-3%/year range for utility-scale LFP systems
(87.5% SOH − 80% EOL threshold) ÷ 2.5%/year
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →The rate behind the number tells the real story. A battery at 87.5% SOH after 5 years is degrading at an average of 2.5%/year — squarely within the typical range for utility-scale LFP systems. The same 87.5% reading after only 2 years would imply 6.25%/year — more than double the norm, and worth investigating well before the system nears its 80% end-of-life threshold. A single SOH reading in isolation can't tell you that; the implied annual rate can.
How battery state of health is calculated
State of Health (SOH) is a percentage comparing a battery's currently measured usable capacity to its original rated (nameplate) capacity — the standard way to express how much a battery has degraded at a specific point in time, based on an actual measurement rather than a projection. Four quantities tie the calculation together.
State of Health (%) = (measured current capacity (MWh) ÷ original rated capacity (MWh)) × 100. At 400 MWh rated and 350 MWh measured, SOH is 87.5%. Capacity Fade (%) = 100% − State of Health (%), and Capacity Fade (MWh) = original rated capacity − measured current capacity — 12.5% and 50 MWh at the defaults.
Implied Average Annual Degradation Rate (%/year) = Capacity Fade (%) ÷ age in service (years). This works backward from the one measured reading to estimate the average annual rate of loss that would produce it. At 12.5% fade over 5 years, the implied rate is 2.5%/year — squarely within the typical 2-3%/year range for utility-scale LFP systems. The same 12.5% fade over only 2 years would imply 6.25%/year, more than double the norm.
Remaining Useful Life to End-of-Life Threshold (years) = (State of Health (%) − end-of-life capacity threshold (%)) ÷ Implied Average Annual Degradation Rate (%). At 87.5% SOH, an 80% threshold, and 2.5%/year, that is (87.5 − 80) ÷ 2.5 = 3.0 years. If SOH is already at or below the end-of-life threshold, the result is 0 years rather than a negative number. The primary flag grades the SOH reading itself (excellent above 90%, healthy 80-90%, below the 80% end-of-life threshold under 80%); the secondary flag grades the implied annual rate against the 2-3%/year utility-scale LFP benchmark. Data sources: industry-standard 80% end-of-life threshold (warranties and project financial models); typical utility-scale LFP degradation benchmark (2-3%/year for well-designed, moderately used systems).