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Homelab · Tool · By Mohammed Almuhanna
UPS Runtime & Sizing Calculator
Two answers in one tool. In Runtime mode, enter your load and battery to estimate how long your UPS will keep your gear up. In Sizing mode, set a target runtime to see what size UPS and battery you need. Treat every result as an estimate. Real runtime drops as batteries age and under heavy load.
How the runtime math works
Runtime is stored energy divided by how fast you drain it. The calculator takes the battery's watt-hours, applies an inverter efficiency factor, and divides by your load in watts. A common consumer UPS carries two 12 V 9 Ah blocks, about 216 Wh; feeding a 50 W NAS through a 90% efficient inverter, that is nearly four hours on paper.
You will not get four hours. Inverter overhead eats a bigger share at light loads, batteries lose real capacity within two to three years, and runtime collapses near the unit's rated maximum. The paper number is a ceiling. Size for the shutdown window you actually need, then add margin, and treat the vendor's runtime chart as the best source for a specific model.
VA vs watts, and the power factor input
UPS boxes advertise VA because it is the bigger number. Your gear draws watts, and watts = VA times power factor. The calculator defaults to 0.9, which fits modern line-interactive units; cheap units run closer to 0.6, so a "1500 VA" UPS may only carry 900 W. Check the watt rating on the spec sheet, not the VA on the box.
I do not run a UPS myself. My grid is stable and UPS prices here are genuinely steep, so I took that risk deliberately, with a real offsite backup behind it. If your power blinks even a few times a year, buy one. The sizing guide walks through the watt math, and the NAS UPS guide covers which models are actually worth the money.
UPS calculator FAQ
How long will a UPS run my NAS?
Battery watt-hours times inverter efficiency, divided by the NAS draw in watts. A typical 1500 VA consumer unit keeps a 50 W NAS up for roughly an hour in practice, which is plenty for a clean automatic shutdown. That shutdown trigger over USB matters more than raw runtime.
What is the difference between VA and watts?
VA is apparent power, watts are real power, and watts = VA times power factor. A 1500 VA unit at 0.9 power factor delivers up to 1350 W; the same VA at 0.6 delivers only 900 W. Always size by the watt rating.
What size UPS do I need for a homelab?
Add up the watts of everything you want protected, then buy a unit whose watt rating gives you 30 to 40 percent headroom. Running a UPS near its maximum shortens runtime sharply and stresses the unit. The what size UPS guide has the full walkthrough.
Do I need a pure sine wave UPS?
For a NAS or a PC, yes. Modern power supplies with active PFC can reject or misbehave on the stepped "simulated sine" output of cheaper units, exactly at the moment the UPS switches to battery. Pure sine wave removes that failure mode.