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UPS runtime is usable battery energy divided by load, after inverter conversion losses. Runtime falls faster than linearly with load because of the Peukert effect on lead-acid batteries, so a UPS rated for thirty minutes at half load may deliver under ten at full load. The critical check is that runtime comfortably exceeds generator start and transfer time.
UPS runtime
usable energy = battery capacity x inverter efficiency; runtime = usable energy / load; margin = runtime - generator start time
Ratings are quoted at a specific load and with new batteries. Capacity declines with age and temperature, and the Peukert effect reduces effective capacity at high discharge rates.
At least annually with a load test. Impedance monitoring catches degradation earlier, and lead-acid strings typically need replacement every three to five years.