UPS Runtime Calculator
Configure Your UPS
Battery Energy Allocation
The calculator progressively adjusts nominal battery energy for battery characteristics, condition and conversion losses.
UPS Sizing Indicators
These values provide additional headroom guidance for selecting a UPS rather than operating directly at the calculated load.
Battery Watt-Hours Do Not Guarantee UPS Runtime
UPS batteries do not behave like an ideal fixed-energy source. Available battery capacity changes with discharge rate, battery chemistry, age, temperature, cutoff voltage and battery-management behavior. UPS inverter and conversion losses also reduce the energy delivered to connected equipment.
Use this calculator for preliminary planning. For a specific UPS model, validate runtime using the manufacturer's published runtime table, runtime curve or configuration tool.
UPS Runtime & Sizing Formulas
| Calculation | Formula | Purpose |
|---|---|---|
| Nominal Battery Energy | Wh = Battery Voltage × Ah | Theoretical stored battery energy |
| Usable Energy | Nominal Wh × Battery Factor × Condition × Efficiency | Planning estimate of energy delivered to the load |
| Runtime | Hours ≈ Usable Wh ÷ Load Watts | Approximate battery operating time |
| Apparent Power | VA = Watts ÷ Power Factor | Checks the UPS apparent-power requirement |
| Watt Utilization | Load Watts ÷ UPS Watts × 100 | Real-power capacity utilization |
| VA Utilization | Required VA ÷ UPS VA × 100 | Apparent-power capacity utilization |
Check Both Watts and VA
UPS systems commonly have both a watt rating and a volt-ampere rating. Connected equipment must remain within both limits. Power factor describes the relationship between real power in watts and apparent power in VA.
A load can therefore remain below the UPS watt rating while still approaching or exceeding its VA rating. This calculator evaluates both limits independently.
Understanding Battery Voltage and Ah
Batteries connected in series increase voltage while the amp-hour rating remains unchanged. Batteries or equivalent battery strings connected in parallel increase amp-hour capacity.
For example, four identical 12-volt 9-Ah batteries connected in series form a nominal 48-volt 9-Ah bank, not a 48-volt 36-Ah bank.
UPS Runtime Is Model-Specific
Actual UPS runtime depends on the specific UPS, battery chemistry, battery count, discharge characteristics, inverter design, operating temperature, battery age, firmware, cutoff voltage, connected load and other manufacturer-specific behavior.
Manufacturer runtime curves and measured runtime data should take precedence over this generalized estimate when planning a production deployment.