Nameplate capacity is not delivered AC energy
A battery may advertise a nominal or usable kilowatt-hour figure, but the energy that reaches loads also depends on state-of-charge limits, inverter losses and operating conditions. Confirm whether the published capacity is nominal, usable or warranted usable energy and whether the specification changes with system configuration.
Reserve settings trade runtime for resilience
Backup reserve keeps part of the battery available for outages instead of using it for everyday bill savings. A 20 percent reserve means the entire advertised capacity is not normally available for tariff shifting. Some systems change reserve dynamically for storms or grid events, so planning should distinguish everyday dispatch from emergency energy.
Temperature and power can reduce accessible energy
Cold or hot conditions can restrict charge/discharge power and, in some systems, temporarily reduce accessible capacity. High-power loads also expose inverter and battery-current limits. Runtime calculations should therefore use both kWh and kW rather than assuming every stored kilowatt-hour can be delivered at any requested power.
Degradation changes the answer over years
Lithium batteries gradually lose capacity with age and cycling. Warranty retained-capacity thresholds and throughput limits provide a better long-term boundary than assuming year-one energy forever. For a critical backup design, consider whether the system will still meet the target outage duration near the later years of the warranty.
Make assumptions visible
Use the usable vs nominal capacity guide, battery warranty guide and battery runtime calculator. Record reserve, efficiency and degradation assumptions so a quote can be compared on the same basis.