Validate watts and VA
A load can fit the watt rating and still exceed VA when power factor is low.
UPS backup planning
UPS Battery Runtime Calculator
Estimate how long a UPS will power your equipment while checking watt capacity, VA demand, power factor, battery chemistry, aging and high-discharge losses.
Watts + VA validationChemistry-aware DoDBattery age deratingPeukert-aware estimate
A UPS estimate with real constraints
The model checks whether the UPS can carry the load before estimating battery energy after chemistry, aging, reserve and conversion losses.
Derate the battery bank
Nameplate energy is reduced by depth of discharge, health, reserve and high-current losses.
Estimate practical runtime
Usable energy is divided by battery-side demand including inefficiency and UPS idle draw.
Runtime = effective usable battery Wh ÷ [(load watts ÷ efficiency) + UPS idle watts]
Why VA and watts both matter
Watts measure real power. Volt-amperes include current that does not become useful power. This calculator estimates VA as load watts divided by power factor and checks both UPS limits. Read UPS VA vs watts for the full sizing method and worked examples.
For a battery-only estimate, use the Battery Backup Time Calculator.
Peukert adjustment
Lead-acid capacity falls at high discharge rates. The calculator compares estimated current with the common 20-hour capacity-test current and applies the selected exponent only when discharge is faster. Manufacturer curves remain the best source.
Series and parallel banks
Enter total Ah at the complete bank voltage. Series batteries raise voltage without increasing Ah; parallel strings increase Ah.
For series and parallel module counts, use the Solar Battery Bank Size Calculator.
Measure the equipment
A plug-in power meter is better than adding label maximums. Include monitors, routers, switches and storage. Compare a device-by-device result with the Home Office Backup Calculator.
UPS runtime questions
How long will a 1500 VA UPS run?
VA alone cannot determine runtime. Battery voltage, Ah, load watts, power factor, efficiency and battery condition are also required. See the transparent 1500 VA UPS runtime table for load-based examples.
Why ask for VA and watts?
They are different limits. A low-power-factor load needs more VA for the same watts.
Should lead-acid use 50% DoD?
Fifty percent is a conservative planning value, but the UPS cutoff and manufacturer specifications take priority.
Does battery age reduce runtime?
Yes. Heat, age and cycling reduce capacity. Use the battery-health input to model remaining capacity.