What is battery capacity?
Battery capacity describes how much charge or energy a battery can store under stated conditions. For portable power, watt-hours or kilowatt-hours are the clearest starting point because they include both voltage and charge capacity.
Nominal capacity and usable energy
Manufacturers may state capacity in watt-hours (Wh), kilowatt-hours (kWh) or amp-hours (Ah). Amp-hours need the nominal voltage before they can be compared as energy. A first-pass nominal energy calculation is volts multiplied by amp-hours, but use the manufacturer's stated energy figure and test conditions where available.
Nominal capacity is not the same as energy delivered to an appliance. Battery management limits reserve part of the battery, and energy is lost through the inverter, cabling and other conversion stages. Temperature, discharge rate, battery age and operating mode can also change the result. That is why a capacity label should not be presented as guaranteed runtime.
Capacity does not replace power ratings
Capacity answers how much energy is available. Output power answers what equipment the system can run at one time. A large battery can still have an inverter or DC output that is too small for a high-demand load. Check continuous power, peak demand, connector ratings and any per-output limits separately.
Size from the job
Complete a load audit for the full operating period. Record running demand, starting demand, hours of use and which items operate together. Estimate energy for each load, total it, and keep practical headroom for uncertainty and field conditions. Then check whether the system can recharge before the next required use.
For critical work, validate the selected system with the actual equipment and a representative duty cycle. Record the starting state of charge, loads, ambient conditions and recharge method. A repeatable field test is stronger evidence than a runtime estimate based only on the front-panel capacity figure.