The verdict first: measure your actual load with a plug-in power meter, buy a pure sine wave UPS rated for at least twice that in watts, and accept that your runtime will be measured in minutes. A UPS is for a clean shutdown and for riding out short cuts — it is not a generator.
VA versus watts, and why the box shows the wrong one first
Every UPS is advertised by its VA rating, and VA is the bigger, more impressive number. The one that constrains you is watts, and it is typically 60 to 70 percent of the VA figure.
A "1500VA" UPS is commonly a 900 to 1000 watt UPS. If your equipment draws 800 watts, the VA rating suggests enormous headroom and the watt rating tells you the truth. Buy on watts, every time.
Sine wave versus stepped approximation — the one that actually breaks things
This is the specification that matters most and gets explained least.
Mains power is a smooth sine wave. Cheaper UPS units approximate it with a stepped square waveform when running on battery. For a lot of equipment — network gear, LED lighting, most external drives — that is completely fine.
Active-PFC power supplies are the problem. Most modern PC and server power supplies use active power factor correction, and a meaningful number of them will not accept a stepped waveform at all. The symptom is not degraded performance. The symptom is the machine shutting off the instant the UPS transfers to battery, which is precisely the failure the UPS was bought to prevent.
The rule: if a computer, a server or a modern NAS is plugged into it, buy pure sine wave. If it is a modem, a router and an access point, stepped approximation is fine and much cheaper.
Sizing, in four steps
- Measure, do not estimate. A plug-in power meter on the rack's power strip for a day costs about twenty dollars and removes all the guesswork. Nameplate ratings on equipment are maximums, and real draw is usually far lower.
- Note the peak, not the average. Spinning disks starting together draw far more than they do at idle, and the UPS has to survive that moment.
- Double it. A UPS running near its watt rating has terrible runtime and shortens its own battery life. Fifty percent load is the comfortable target.
- Check the runtime chart for your actual wattage, not the headline. UPS runtime is wildly non-linear — a unit that runs ninety minutes at 100W may run eight minutes at 500W.
Runtime: what a UPS is actually for
Five to fifteen minutes at a realistic load is normal, and it is enough, because the job is not to keep working through an outage. The job is to:
- Ride out the sub-second dips and brief cuts that make up most power events.
- Give connected equipment time to shut down cleanly, which is what protects the data.
That second one requires the USB or network connection to be plugged in and the shutdown software configured. A UPS with the data cable in a drawer is a surge protector with a heavy battery in it, and this is the most common way a UPS fails to do its job.
The cheapest UPS win in the house
A small UPS on the modem, the router and the access point. Network gear draws so little that runtime is measured in hours rather than minutes, and the result is that your internet stays up through a cut — including for the phone you are using to find out how long the outage will last. Under a hundred dollars, and most people do it after the first outage rather than before.
Batteries are consumable
UPS batteries last roughly three to five years and they degrade quietly. A UPS with a dead battery reports itself as fine right up until the moment it is needed. Put a replacement date in a calendar, run a self-test periodically, and budget for the battery when you buy the unit — on some models the replacement pack is a substantial fraction of the original price.
Buying notes
Match the unit to the load, honestly
- Modem, router, access point (under 60W): a small 600VA unit. Hours of runtime, under a hundred dollars.
- NAS, switch, mini PC (100-250W): a 1000VA pure sine wave unit. This is the size most homelabs actually need.
- Adding a real server (300-600W): a 1500VA pure sine wave unit, and check the runtime chart at your wattage rather than the headline figure.
- Rack-mounted: a 1U unit if the floor space genuinely matters. Expect to pay significantly more per watt and to hear the fan.
Software support is a real specification
The point of the USB connection is graceful shutdown. Check that your NAS or hypervisor supports the brand before you buy — this is where APC's wider integration genuinely earns its premium over otherwise-equivalent hardware.
Do not put a laser printer on it
Laser printers draw enormous current when the fuser heats and will overload almost any home UPS. Manufacturers say this explicitly. The same applies to space heaters and anything else with a heating element.
Where the enterprise line stops making sense
Serviceable batteries, hot-swap and network management cards are worth real money in a building with a maintenance contract. In a house they are features you will not use at roughly twice the price per watt — which is why the Smart-UPS is the skip pick above.
