Runtime
288minutes
Estimated runtime after UPS efficiency.
UPS Runtime Calculator
Estimate UPS runtime for routers, computers, NAS devices, and other small electronics from battery energy and load watts.
Use watt-hours when your UPS specification lists stored battery energy, or use battery voltage and amp-hours when those are listed. Do not treat the UPS VA rating as watt-hour capacity; VA is not stored energy.
UPS runtime inputs
Use basic mode when the specs list Wh. Use advanced mode when they list battery V and Ah.
W
Total connected device load in watts.
Wh
Use Wh from the UPS specifications. VA rating is not watt-hour capacity.
%
Accounts for inverter and internal conversion losses.
VA rating is not stored energy. Use watt-hours, or use voltage and amp-hours from the battery specification when available.
Results
Runtime
4.8hours
Total load
50W
Capacity used
300Wh
UPS efficiency
80%
Calculation breakdown
| Item | Value | Notes |
|---|---|---|
| Mode | Basic | Capacity entered directly in watt-hours. |
| Battery capacity | 300 Wh | Stored energy used by the runtime formula. |
| After UPS efficiency | 240 Wh | 80% efficiency applied. |
| Runtime | 288 minutes | 240 Wh / 50 W load. |
| Battery aging warning | Review condition | Older UPS batteries can deliver less runtime than their rated capacity. |
Formula
runtimeHours = capacityWh x (efficiencyPercent / 100) / loadWattsIn advanced mode, capacityWh is calculated as batteryVoltage x batteryAh before applying efficiency. The calculator does not accept VA as battery capacity because VA and Wh measure different things.
- capacityWh
- Battery energy in watt-hours, entered directly or calculated as voltage x amp-hours.
- efficiencyPercent
- Estimated UPS conversion efficiency.
- loadWatts
- Total connected device load in watts.
Example
An older UPS battery may deliver less energy than its rating, so the real runtime can be shorter than the estimate.
- UPS battery capacity is 300 Wh.
- Connected equipment uses 50 W total.
- UPS efficiency is 80%, leaving about 240 Wh available to the load.
- 240 Wh divided by 50 W gives 4.8 hours, or about 288 minutes.
How to use this calculator
- Find battery energy in Wh, or find battery voltage and amp-hours in the UPS battery specification.
- Add the watts for every device connected to the UPS.
- Choose basic mode for Wh or advanced mode for voltage and amp-hours.
- Review the runtime estimate and battery aging warning before relying on the plan.
Input guide
- UPS battery capacity: watt-hours of stored energy. This is not the VA rating.
- Battery voltage and amp-hours: advanced inputs used to estimate Wh as V x Ah.
- Load: total watts for router, modem, computer, NAS, monitor, or other connected equipment.
- UPS efficiency: estimated internal conversion loss between the battery and connected load.
Common mistakes
- Treating a UPS VA rating as if it were watt-hours.
- Forgetting monitor, external drive, modem, or NAS load.
- Expecting an older battery to match its original rating.
- Assuming runtime is fixed even though UPS firmware, temperature, load level, and battery condition affect results.
Limitations
The estimate uses a simple energy divided by load formula. It does not model UPS discharge curves, low-load behavior, battery age, temperature, firmware reserve, overload behavior, power factor, or manufacturer runtime tables.
FAQ
Can I calculate runtime from VA rating alone?
Not accurately. VA describes apparent power capacity, not stored battery energy. Look for Wh or battery voltage and Ah.
Why does an older UPS run for less time?
Battery capacity declines with age, temperature, storage history, and discharge cycles.
Why is VA not enough for runtime?
VA describes apparent power capacity for supported load. Runtime depends on stored battery energy, usually listed as Wh or battery voltage and amp-hours.
Should I include monitor, modem, and NAS loads together?
Yes. Add the watts for all connected devices and enter the total load. Runtime drops as load increases.
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- CPAP Battery Backup CalculatorEstimate CPAP backup runtime and nights from battery capacity, humidifier use, reserve, and efficiency.
Methodology and disclaimer
UPS runtime is an estimate. VA rating is not the same as usable battery capacity; use watt-hours or battery voltage and amp-hours from the UPS specifications when available.
The calculator uses watt-hours directly in basic mode. In advanced mode it multiplies battery voltage by amp-hours to estimate watt-hours, applies UPS efficiency, then divides by connected load watts.