UPS Runtime Calculator · Runs in your browser
UPS runtime calculator for home servers
By Tiny · how I pick
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How this estimate works
A UPS runs your gear from a lead-acid battery through an inverter. The calculator takes the battery's energy (volts × amp-hours), allows for inverter losses, and applies Peukert's law, the rule that a lead-acid battery delivers less total energy the faster you drain it. That's why doubling the load cuts runtime by more than half.
I calibrated it against a real spec sheet: CyberPower's CP1500PFCLCD (two 12 V 8.5 Ah batteries) is rated for 11 minutes at half load in CyberPower's own spec sheet (see the CP1500PFCLCD on Amazon), and this calculator gives about 11 minutes for the same setup. Near full load, real units cut off sooner than any formula predicts, so treat anything above 80% load as "a couple of minutes at most." For an exact figure on a specific model, check its manufacturer's runtime chart.
What a home-lab UPS is really for
Most home outages are either a few seconds or several hours. A UPS covers the blips completely, and for longer outages its real job is to buy time for a clean shutdown so your NAS and server don't corrupt data. Connect the UPS's USB cable to your server and set up automatic shutdown (NUT on Linux and Proxmox, or the built-in UPS setting in Synology, TrueNAS and Unraid). Then 10 to 15 minutes of runtime is plenty.
- Keep the network alive too: put the modem and router on battery so remote access keeps working through short outages.
- Don't put a laser printer or space heater on it. They pull huge bursts of power and can overload the UPS.
- Pure sine wave for PCs with modern power supplies. Many desktop power supplies (active PFC) prefer a UPS with pure sine wave output; cheaper "simulated sine wave" units can cause some of them to shut off when switching to battery.
UPS picks by size
Network gear and one mini PCCompact ~600 VA UPS
Enough for a modem, router and a low-power mini PC or Pi to ride out blips and shut down cleanly.
Check price on AmazonOther brands to consider: CyberPower CP600LCD
Mini PC plus a NAS~1000 VA pure sine wave UPS
The sweet spot for most home labs: a mini PC, a 2- or 4-bay NAS and network gear, with time to shut down properly.
Check price on AmazonOther brands to consider: CyberPower CP1000PFCLCD, APC Back-UPS Pro 1000
Desktop or GPU box~1500 VA pure sine wave UPS
For a full desktop or a GPU machine plus a monitor. Don't expect long runtime from a GPU under load; it's for a safe shutdown.
Check price on AmazonOther brands to consider: APC Back-UPS Pro 1500, Eaton 5S 1500
Not sure what your server actually draws? Measure it, then check what it costs to run with the power cost calculator. Building a storage box? Size it with the media storage calculator and the beginner NAS guide.
Quick answers
How long will a UPS run a home server?
It depends on the battery and the load. A low-power mini PC plus a router on a ~1000 VA UPS typically runs for well over half an hour, while a desktop under heavy load may get only a few minutes. Use the calculator above for your setup.
What size UPS do I need for a NAS?
For a 2- or 4-bay NAS plus network gear, a 1000 VA pure sine wave UPS is usually enough to ride out short outages and shut down cleanly.
Is VA the same as watts on a UPS?
No. VA is the apparent power rating; the watt rating is lower, often 55 to 65% of the VA figure. Size your UPS by its watt rating, with headroom.
How often should I replace a UPS battery?
Lead-acid UPS batteries typically last 3 to 5 years. Runtime drops as they age, and most UPS units warn you when the battery needs replacing.