UPS planning
UPS Runtime Calculator
Estimate how long a UPS can support your equipment from battery energy, load watts, usable energy, battery health and UPS efficiency. Use the manufacturer's rated watt capacity when you know it.
Estimate UPS runtime
How to calculate UPS runtime
The calculator estimates usable battery energy, accounts for UPS efficiency and divides that energy by the battery-side load. It is a vendor-neutral planning estimate, not a model-specific runtime curve.
runtime hours = usable battery Wh ÷ (load watts ÷ UPS efficiency)
UPS VA versus watts
VA describes apparent power, while watts describe real power. A rough relationship is watts = VA × power factor, but the manufacturer's rated watt limit should be preferred. VA multiplied by an assumed power factor is labeled as an estimate and never overrides rated watts.
How battery age and usable energy affect runtime
Usable fraction is an editable planning assumption for the share of nominal battery energy considered available. Battery health separately represents remaining capacity compared with a healthy battery. Lead-acid capacity can decrease at higher discharge rates, so manufacturer runtime curves are preferred for specific UPS models.
Worked UPS runtime example
A 216 Wh battery with a 50% planning usable fraction provides 108 Wh of usable energy. At 100 W load and 90% UPS efficiency, the battery-side load is about 111 W, giving an estimate of approximately 58 minutes.
Limitations and methodology
UPS manufacturers may use proprietary runtime curves. Battery age, temperature and discharge rate can change the actual result. Review the methodology and sources for the broader planning approach.
UPS Backup Runtime Reference Matrix
Typical uninterruptible power supply backup minutes across standard consumer and enterprise UPS capacity classes and connected IT loads:
| UPS Capacity Rating | Wi-Fi + Modem (25W) | Laptop + Monitor (100W) | Gaming PC / Workstation (350W) | Rack Server + NAS (600W) |
|---|---|---|---|---|
| 650 VA / 360 W (1× 12V 7Ah = 84 Wh) | ~85 min | ~21 min | ~5 min (Load Limit) | Overload |
| 1000 VA / 600 W (2× 12V 7Ah = 168 Wh) | ~170 min | ~43 min | ~12 min | Overload |
| 1500 VA / 900 W (2× 12V 9Ah = 216 Wh) | ~220 min | ~58 min | ~16 min | ~8 min |
| 2200 VA / 1980 W (4× 12V 9Ah = 432 Wh) | ~440 min (7.3 hrs) | ~116 min | ~33 min | ~18 min |
UPS Runtime & Apparent Power Formulas
Calculates standby operating minutes of an uninterruptible power supply based on internal DC battery energy and AC inverter conversion losses.
Runtime (min) = [(Battery_Wh × Usable_Fraction × Health × Efficiency) / Load_Watts] × 60Variable Definitions
Battery_WhInternal Battery Energy(Wh)- Nominal internal battery pack rating (e.g. 2 × 12V 9Ah = 216 Wh).
Usable_FractionUsable Capacity Share(fraction)- Safety cutoff fraction (typically 50% for standard SLA batteries).
HealthSOH Degradation(fraction)- Available capacity relative to new factory condition.
EfficiencyInverter DC-AC Efficiency(fraction)- UPS inverter conversion efficiency (typically 80%–90%).
Load_WattsConnected Real Load(W)- Active power demand (VA × Power Factor).
Engineering Notes & Standards
- Real Watts = Apparent VA × Power Factor (typically 0.6 to 0.9 for standard desktop UPS models).
- Lead-acid runtime decreases faster at discharge rates above 0.5C due to internal resistance and Peukert losses.