Estimate how long a UPS battery backup can run your devices using battery energy, load, and real-world loss settings.
Table of contents
How to use our UPS Runtime Calculator
- Choose how the battery label lists capacity under How is the battery capacity listed?: Battery energy in Wh, or Battery capacity in Ah plus Battery voltage in V.
- Enter Connected load in watts by adding the devices plugged into the battery-backed outlets, not the surge-only outlets.
- Open Advanced options if you want to change UPS efficiency, Battery energy you plan to use, Battery health, UPS max output, or Number display.
- Click Calculate and read Estimated runtime in minutes first, then use the hours value for longer backup times.
- Sanity-check the result: if Share of UPS max output used is near or above 100 percent, runtime math may still show a time, but the UPS may overload before that time is useful.
Definitions
UPS: A battery backup that keeps power going to connected equipment when normal AC input power fails [1].
Battery energy: Stored energy measured in watt-hours, or Wh. More Wh usually means more possible runtime at the same load.
Connected load: The power your plugged-in devices are using right now, measured in watts, or W.
Battery capacity: A battery rating in amp-hours, or Ah. The calculator multiplies Ah by Battery voltage to estimate Wh.
Battery voltage: The UPS battery string voltage, measured in volts, or V. This is not the same as the wall outlet voltage.
UPS efficiency: The percent of stored battery energy that reaches your devices after conversion losses.
Battery health: Your estimate of how much of the rated battery capacity remains. Older batteries often need a lower value.
UPS max output: The highest watt load the UPS can supply. It checks overload risk, but it does not tell battery runtime by itself.
Common mistakes and quick fixes
Mistake: Entering the wall outlet voltage in Battery voltage.
Fix: Use the UPS battery string voltage for Battery voltage, such as the voltage printed on the battery pack or manual.
Mistake: Using the UPS VA rating as Battery energy.
Fix: Enter stored energy in Battery energy, or use Battery capacity and Battery voltage if the label gives Ah and V.
Mistake: Leaving Connected load at a guess that is much lower than the real devices use.
Fix: Add the watts for every device on the battery-backed outlets, or measure Connected load with a wall watt meter.
Mistake: Setting UPS efficiency, Battery energy you plan to use, or Battery health blank because you are unsure.
Fix: Keep the default percent values, or enter a number from 0 to 100 for each required percent field.
Mistake: Ignoring UPS max output when the devices include a PC, monitor, or printer.
Fix: Enter UPS max output if you know it so Load safety check and Share of UPS max output used can flag overload risk.
Limitations & Key Assumptions / Boundary Conditions
- This is an estimate from stored battery energy and load. A manufacturer runtime chart or a real test is better for one exact UPS model.
- Runtime can differ because UPS electronics, battery age, temperature, recent charging, and battery chemistry affect usable energy.
- The calculator treats Connected load as steady. Startup spikes, gaming loads, laser printers, and motors can make the UPS alarm or shut down sooner.
- UPS max output is checked separately from battery energy. A large battery does not guarantee the UPS can safely supply a high watt load.
- Battery energy from Battery capacity times Battery voltage uses label ratings. Real usable energy can be lower, especially under heavy load.
- If Battery energy is 0 Wh or the usable percent settings make usable energy 0, the estimated runtime is 0 minutes even with a valid load.
Methodology
How the runtime is calculated
The calculator first finds Starting battery energy. If you choose Battery energy, it uses the Wh value you entered. If you choose Battery capacity and Battery voltage, it estimates watt-hours from amp-hours times volts.
battery_energy_wh = battery_capacity_ah * battery_voltage_v
Next, it reduces that starting energy by UPS efficiency, Battery energy you plan to use, and Battery health. Each percent is divided by 100 before multiplication.
usable_energy_wh = battery_energy_wh * (efficiency_pct / 100) * (usable_battery_pct / 100) * (battery_health_pct / 100)
Runtime in hours is Energy available to devices divided by Connected load. Runtime in minutes is the hour value times 60.
runtime_hours = usable_energy_wh / load_w
runtime_minutes = runtime_hours * 60
If UPS max output is entered, the calculator also checks how much of that output the Connected load uses.
load_percent = (load_w / ups_max_output_w) * 100
Worked mini-example
For a 216 Wh battery, 300 W Connected load, 85 percent UPS efficiency, 80 percent Battery energy you plan to use, and 100 percent Battery health, the Energy available to devices is 216 * 0.85 * 0.80 * 1.00 = 146.88 Wh. Runtime is 146.88 / 300 = 0.4896 hours, or 29.376 minutes.