Free tool
UPS Runtime Calculator
Estimate how long a UPS will carry your load from the battery capacity, voltage, load in watts and the efficiency of the inverter.
Why the real runtime is always shorter
The calculation takes the energy in the bank, removes what cannot safely be drawn, then divides by the load after inverter losses. Batteries are rated at a slow twenty hour discharge, so pulling hard for a few minutes returns noticeably less than the label suggests. That effect is already built into the estimate.
Depth of discharge matters more than anything else. Running a lead acid bank flat every day will destroy it within months, which is why 50 percent is the sensible default and 80 percent is a deliberate trade against battery life.
Batteries also age. A three year old bank can hold well under three quarters of its original capacity, so size for what you will need at the end of the battery life, not the beginning.