How long will my UPS actually last during a blackout?
You bought a UPS, or you're shopping for one. The question is always the same: how many minutes will my gear actually run when the power dies? Manufacturers print "up to 15 minutes" on the box, but that's at 30% load. Your gear may sit at 70%. And then instead of 15 minutes you get 3. Maybe less.
Most online calculators get this wrong. They divide battery energy ÷ load and call it done. They ignore 3 things that change everything: the difference between VA and W, depth of discharge (DoD), and the Peukert effect: sealed lead-acid batteries perform worse under heavy load.
Enter: your UPS (pick a preset or type VA + W + battery), your load (preset or watts + power factor). You get: realistic runtime, warnings (overload, mismatch, pure sine wave needed), and a chart of how runtime changes with load.
How to use it
- Pick your UPS from the list (popular models) or type VA, W and battery (V × Ah) manually. The numbers are usually on a sticker on the back.
- Pick your load: typical router + NAS, gaming PC, office PC, server rack, fridge. Or type your own watts + power factor (use 0.85 if unsure).
- Look at the big number in the middle: that's your runtime in minutes. The color tells you: green = good, yellow = graceful shutdown only, red = useless.
- Read the warnings below: that's the difference between this calculator and the others. It catches overload, oversizing, pure sine wave issues, and lack of headroom.
- Advanced mode: change battery DoD, inverter efficiency, disable Peukert correction. Plus a chart showing how runtime varies with load.
When this is useful
Six typical situations where the calculator tells you whether your UPS is right-sized:
- Sizing a UPS for new hardware. You're buying a workstation with a beefy GPU. The shop says "1500VA, definitely enough". You enter: 800W, PF 0.95. Calculator says: 1500VA against 1425VA load, NEAR MAX, runtime 4 minutes. Just enough for a graceful shutdown. You bump to 2200VA and get 12 minutes.
- Checking if your current UPS makes sense. You have an APC 1500 from 2018, hooked up to a router, NAS and small server (300W). Calculator says: OVERSIZED + EXCELLENT (over an hour). OK, you could have bought smaller, but it works. Keep it until the battery dies.
- Setting up home office backup. You want to survive a 30-minute outage with a laptop, monitor and router. Total: 100W. Calculator says: 1000VA UPS = 50+ minutes runtime. Plenty.
- Building a home lab. You have 2 mini-PCs, a switch, a NAS. Total wall draw: 250W. Question: is a CyberPower 650 enough? Calculator: 650VA at 250W load = OPTIMAL, runtime 6 minutes. Good for short outages, not long ones. You consider a UPS with extended battery support.
- Picking a UPS for a retail POS terminal. Register + receipt printer + payment terminal = 80W. Calculator: smallest 600VA UPS gets you 40+ minutes. All you need is to ride out short outages without losing the receipt mid-print.
- Sizing a UPS just for the router so internet survives blackouts. Router + ONT + switch = 30W. Calculator shows that a small 650VA UPS gives 90+ minutes. Add an external battery and you get 4-6 hours. Best UPS use case for a home.
Related: build your own DIY pack in the battery pack calculator, spec a USB powerbank for travel in the powerbank calculator, and turn watts into a money figure with the electricity cost calculator.