How UPS runtime is estimated
Backup time is battery energy divided by load power. Battery energy in watt hours is bank voltage times total amp hours times depth of discharge, and multiplying by inverter efficiency gives the energy that actually reaches the load. Convert the UPS rating from kVA to kW with the load power factor before dividing, since a 10 kVA UPS at 0.9 PF is carrying 9 kW.
The plain division is optimistic. Battery capacity is published at the 8 or 20 hour rate, while a UPS empties its bank in 15 to 60 minutes. At that rate a VRLA string delivers considerably less than its nameplate, which is why this tool applies a high rate factor of roughly 0.6 to 0.9. VRLA banks are normally sized to 80% depth of discharge with an end voltage of 1.75 V per cell.
Final sizing should follow the battery maker constant power discharge tables, stated in watts per cell, together with IEEE 485 for vented lead acid or IEEE 1184 for UPS applications. Those procedures add an aging factor of 1.25, a temperature correction and a design margin, none of which are included in this quick estimate. For data center, healthcare or other critical loads, have a licensed electrical engineer or the UPS vendor confirm the battery sizing.
Frequently Asked Questions
Slightly more than double, in fact. Adding a parallel string halves the discharge rate on each battery, which recovers part of the high rate capacity loss. The tradeoff is a larger charger, more floor space and extra cooling load.
VRLA batteries are considered end of life at 80% of rated capacity, so just before replacement the runtime is about 80% of the calculated figure. Adding the standard 1.25 aging factor at design time covers that.