Photo by Domaintechnik on Unsplash
To calculate UPS runtime, convert the battery bank to watt-hours, reduce it for depth of discharge, battery health, reserve and high-current losses, then divide the usable energy by the UPS input draw. The simple capacity ÷ watts shortcut is only a rough upper limit.
Runtime (hours) = usable battery energy (Wh) ÷ total DC draw (W).
For a realistic estimate, total DC draw is the connected load divided by inverter efficiency, plus the UPS idle draw.
If you only need an answer, use the free UPS runtime calculator. It checks watt rating, VA rating and surge capacity before showing a result. The guide below explains every number so you can verify the estimate instead of trusting a black box.
The UPS runtime formula
Start with the battery bank’s nominal energy:
Then estimate the energy that is actually available:
Finally account for conversion losses and UPS electronics:
Runtime (hours) = usable energy (Wh) ÷ total DC draw (W)
This is more honest than using V × Ah ÷ load. That shortcut assumes a new battery, 100% discharge, zero reserve, no inverter losses, no UPS self-consumption and no capacity loss at high discharge current.
Calculate UPS runtime step by step
- Add the running watts. Measure the equipment at the wall when possible. For a computer, include the monitor, router, modem, storage and any other devices connected to the UPS.
- Check watts and VA separately. Required VA equals watts divided by power factor. A 300 W load at a 0.70 power factor requires about 429 VA.
- Find the battery voltage and amp-hours. Read the UPS label, manual or battery pack specification. Batteries in series increase voltage; parallel strings increase amp-hours.
- Choose realistic derating values. Set depth of discharge, battery health, reserve and efficiency for the battery and UPS you actually have.
- Allow for high-current discharge. Lead-acid capacity falls as discharge current rises. A Peukert adjustment prevents an optimistic result at heavy loads.
- Verify startup and peak demand. The UPS must handle the running watts, required VA and any short startup surge. Runtime is irrelevant if the inverter trips first.
Worked example: 24 V UPS with an 18 Ah battery bank
Assume a small UPS has a 24 V, 18 Ah lead-acid bank and powers a 250 W load. Use 85% efficiency, 10 W idle draw, 50% depth of discharge, 90% battery health, a 10% reserve and a Peukert exponent of 1.15.
1. Nominal energy: 24 V × 18 Ah = 432 Wh
2. UPS input draw: 250 W ÷ 0.85 + 10 W = 304 W
3. Battery current: 304 W ÷ 24 V = 12.7 A
4. High-rate factor: approximately 0.67 for this example
5. Usable energy: 432 × 0.50 × 0.90 × 0.90 × 0.67 ≈ 117 Wh
6. Estimated runtime: 117 Wh ÷ 304 W ≈ 0.39 hours, or about 23 minutes
A naive calculation would give 432 Wh ÷ 250 W = 1.73 hours. That result is more than four times the realistic planning estimate because it ignores discharge limits, conversion losses, battery condition, reserve and high-current loss.
VA is not the same as watts
Watts measure real power. Volt-amperes measure apparent power. A UPS must satisfy both ratings:
For a 250 W load at a 0.80 power factor, required capacity is 312.5 VA. If the equipment has poor power factor, the VA limit can be reached before the watt limit. Always compare the result with both values printed on the UPS. The complete UPS VA vs watts guide explains how to size both limits with headroom.
| Input | What it changes | Common mistake |
|---|---|---|
| Running watts | Continuous energy demand | Using the power supply’s maximum label instead of measured draw |
| Power factor | Required VA | Assuming VA and watts are interchangeable |
| Efficiency | Battery-side draw | Assuming the inverter is 100% efficient |
| Depth of discharge | Usable battery energy | Planning to drain lead-acid batteries completely |
| Battery health | Available capacity | Calculating with the new-battery rating after years of use |
| Idle draw | Energy used by UPS electronics | Ignoring a loss that matters most at light loads |
| Startup surge | Whether the UPS can start the load | Checking runtime but not peak inverter capacity |
What values should you use?
Inverter efficiency
If the manufacturer publishes an efficiency curve, use the value near your expected load. Otherwise, 80% to 90% is a reasonable planning range for many small UPS units. Efficiency is often worse at very light or very heavy load.
Depth of discharge
For lead-acid batteries, a conservative planning value is often 50%. Lithium systems may permit deeper discharge, but use the battery management system and manufacturer specification rather than assuming 100% is available.
Battery health
Use 100% only for a verified new battery. Lower the value for age, heat exposure, repeated deep cycles or a failed load test. Runtime that has fallen suddenly is often a battery problem rather than a math problem.
Reserve
A 10% to 20% reserve protects against measurement error and unexpected load changes. Critical equipment should use a larger margin and should be tested under controlled conditions.
Why the label runtime may differ
Manufacturer charts are measured with a specific new battery pack, ambient temperature and controlled load. Your result can be lower because batteries age, warm rooms accelerate degradation, connected equipment changes load, and many UPS units shut down before the battery is completely empty.
The best method is to calculate first, then perform a supervised runtime test with non-critical equipment. Record the actual watts and shutdown time. That gives you a model-specific correction factor for future planning.
Common calculation mistakes
- Using VA as if it were watt-hours.
- Multiplying volts by amp-hours without checking whether batteries are wired in series or parallel.
- Ignoring UPS idle consumption.
- Using 100% efficiency and 100% depth of discharge.
- Ignoring battery age and temperature.
- Using only average watts when a device has a startup surge.
- Assuming a 1500 VA UPS always contains the same battery capacity as another 1500 VA model.
- Rounding every intermediate number too early.
Calculate your UPS runtime
Enter battery, load, efficiency, health, reserve, power factor and surge values for a checked estimate.
Frequently asked questions
How long will a 1500 VA UPS run?
VA alone cannot determine runtime. You also need the UPS watt rating, connected watts, battery voltage and amp-hours, efficiency, battery condition and allowed depth of discharge. Two 1500 VA models can have very different battery packs. Compare loads in the 1500 VA UPS runtime table.
Can I calculate runtime from Ah only?
No. Amp-hours must be multiplied by battery-bank voltage to estimate watt-hours. You must then reduce that energy for usable depth, health, reserve and discharge-rate losses.
Should I use the computer power supply rating?
Usually not. A 650 W power supply does not continuously consume 650 W. Measure actual wall power or estimate each device using the appliance wattage library, then add a sensible margin.
Why does runtime collapse at high load?
Higher load increases battery current, conversion losses and voltage sag. Lead-acid batteries also deliver less effective capacity at high discharge rates, which is why the calculator includes a Peukert adjustment.
For other backup systems, compare the full calculator library, use the battery backup time calculator for general battery banks, or estimate computer and network loads with the home office backup calculator.
Pingback: UPS VA vs Watts: Choose the Right UPS Size
Pingback: How Long Will a 1500VA UPS Last? Runtime Table