UPS Battery Runtime Calculator
Enter your UPS capacity and the watts your devices actually draw to see how many minutes of backup power you'll have, before the next power cut catches you off guard.
Calculate UPS runtime
Enter your UPS specs and connected load to estimate runtime.
~23 minutes
At 350W load on a 1500VA / 0.9 PF UPS with new batteries.
How to use this calculator
Enter your UPS capacity in VA and the power factor (found on your UPS spec sheet, default 0.9 for most modern units). Enter the total watts drawn by devices connected to the battery outlets. The load percentage shows how heavily you're loading the UPS -- heavy loads dramatically shorten runtime. The battery age adjustment accounts for VRLA battery degradation over time.
Tips for success
- Keep essentials on battery outlets only. Connect only essential devices (computer, monitor, router, NAS) to the battery outlets. Use the surge-only outlets for peripherals like printers and speakers.
- Lower load, longer runtime. A lower connected load dramatically extends runtime -- removing one monitor can add 5-10 minutes.
- Replace batteries every 3-5 years. Most UPS units have a battery self-test function; run it annually.
- Compare to UPS display. If your UPS has an LCD showing runtime, compare it to this calculator -- a large discrepancy may indicate a weak battery.
- Need 30+ minutes? For a server room or home office, consider a larger UPS or an external battery pack (EBM).
Practical note
This calculator uses a standard Peukert-approximation model for typical VRLA (sealed lead-acid) UPS batteries. Actual runtime will vary based on temperature, battery age, load type, and UPS efficiency. Always treat this as an estimate and test your UPS under realistic load to verify actual runtime before you need it in an emergency.
How to find your connected load wattage
- Use a plug-in watt meter. A Kill A Watt meter on each device gives the most accurate measurement.
- Check device spec labels. Actual draw is often lower than rated wattage under typical use.
- Typical loads: desktop PC 150-400W, LCD monitor 25-80W, router 10-25W, NAS 20-50W per drive bay.
- Add a 10-15% buffer to your measured load for safety margin.
FAQ
What's the difference between VA and watts on a UPS?
VA (volt-amps) is the apparent power rating; watts is the real power. The difference is the power factor. A 1500VA UPS with a 0.9 power factor can deliver 1,350 watts of real power. Older UPS units had power factors of 0.6-0.7; modern line-interactive and online UPS units typically have 0.9-1.0. Always check your spec sheet.
How long should a UPS run a computer during an outage?
A typical 1000VA / 900W UPS running a desktop PC (200W), monitor (50W), and router (15W) at 265W total load will run for roughly 20-30 minutes with a new battery. That's plenty of time to save work and safely shut down. If you need longer runtime for outage bridging, add an external battery module (EBM) or size up to a 2000VA+ unit.
How do I know if my UPS battery needs replacement?
Signs include: the UPS beeps when connected to power (not on battery), the runtime under load is significantly shorter than specs, the UPS display shows a battery fault, or the battery is 3-5 years old. Most UPS units have a front-panel self-test you can run. A UPS that only gives you 2-3 minutes under a 300W load when it should give 15+ minutes needs a new battery.
Can I use a UPS to power a chest freezer or refrigerator during a short outage?
Only with a very large UPS and careful sizing. A chest freezer may draw 150-500W at startup (surge) and 50-150W running. The surge can be 3x the running wattage. Most consumer UPS units are not rated for inductive motor loads. If you want to bridge a freezer for 10-15 minutes while starting a generator, use a pure sine wave UPS rated at 2x the surge wattage of the appliance.