UPS manufacturers quote runtime at specific loads, but your setup probably does not match their test conditions. This calculator uses the actual battery specs inside your UPS to estimate runtime at your real load — giving you a more accurate number than the marketing chart on the box.
Common UPS Models and Their Battery Specs
| UPS Model | VA Rating | Battery | Runtime at 200W | Runtime at 400W |
|---|---|---|---|---|
| APC Back-UPS 600 | 600VA | 12V 7Ah | ~10 min | ~4 min |
| APC Back-UPS Pro 1500 | 1500VA | 2x 12V 9Ah (24V) | ~28 min | ~12 min |
| CyberPower CP1500AVRLCD | 1500VA | 2x 12V 8.5Ah (24V) | ~26 min | ~11 min |
| APC Smart-UPS 2200 | 2200VA | 4x 12V 9Ah (48V) | ~45 min | ~18 min |
| CyberPower OR2200LCDRT2U | 2200VA | 4x 12V 9Ah (48V) | ~42 min | ~17 min |
Runtime drops sharply at higher loads. A UPS that lasts 30 minutes at 200W might only last 7-8 minutes at 600W. Always calculate at your actual load, not the UPS maximum capacity.
Example: Home Office UPS Sizing
You work from home and need UPS backup for your desktop PC, monitor, and internet equipment during brief power flickers and short outages (up to 15 minutes).
Equipment: desktop PC (250W), 27" monitor (45W), router + modem (20W), USB hub and desk lamp (15W). Total: 330W.
Your APC Back-UPS Pro 1500 has two 12V 9Ah batteries in series (24V, 9Ah). Usable energy at 80% DoD and 88% efficiency: 24 x 9 x 0.80 x 0.88 = 152Wh. At 330W load: 152 / 330 = about 27 minutes.
That is enough for short outages and graceful shutdowns. For longer backup, either reduce the load (laptop instead of desktop saves 150W) or add an extended battery module if your UPS supports one.
Frequently Asked Questions
Calculating your home office energy costs? The <a href="/electrical/kwh-calculator">kWh calculator</a> estimates monthly electricity usage and cost.
UPS runtime depends on battery age as much as battery size. A 3-year-old UPS battery may deliver only 60-70% of its original capacity. If your UPS runtime matters for your work, replace the batteries on schedule and verify with an annual discharge test — not during the next storm.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.