Systems, methods, and devices are provided for estimating a state-of-health (SOH) in an energy storage system, where the estimate accounts for both power fade and capacity fade. The SOH estimation system comprises a SOH estimation module configured to receive, and to determine the estimate of the SOH based on: a nominal capacity input; a voltage input; a current input; a nominal open circuit voltage curve; an operational resistance model; a nominal resistance model; an operational dynamic model; and a rated discharge current. The SOH estimation module may further comprise a SOH aggregation module configured to receive a power rating estimate, PRn, and a normalized capacity estimate, NCn, and may be configured to determine the estimate of the SOH based on the power rating estimate, PRn, and the normalized capacity estimate, NCn.