A UPS that's too small will shut down mid-outage right when you need it, and one that's oversized wastes money on capacity you'll never use. Sizing one correctly is a straightforward calculation once you know your equipment's actual power draw — here's how to work it out before you buy.

The two numbers on every UPS: VA and watts

UPS capacity is listed in both volt-amps (VA) and watts, and they aren't the same number — watts is the real power your equipment draws, while VA is a related but slightly larger figure due to how power factor works in electrical systems. Manufacturers typically list both; for sizing purposes, focus on the watts figure and compare it against your equipment's actual watt draw, not the VA number alone.

Step 1: Add up what you're actually plugging in

List every device you want protected — computer, monitor, router, modem, NAS — and find its power draw. The most reliable source is the wattage printed on the device's power supply or listed in its spec sheet; this is a maximum rating, so real-world draw is often somewhat lower. If you can't find a spec, a rough estimate for common gear: a desktop computer with a monitor draws roughly 150-350 watts depending on the hardware, a router or modem draws 10-20 watts, and a small NAS draws 20-40 watts.

Step 2: Decide what actually needs battery backup versus just surge protection

Not everything needs to ride out a full outage. A printer, for example, rarely needs to keep running during a power cut — plugging it into the UPS's surge-only outlets (most UPS units have a few) instead of its battery-backed outlets frees up capacity for what matters, like your computer and networking gear.

Step 3: Decide how much runtime you actually need

Runtime is what most people get wrong — a UPS is generally there to bridge a short outage or give you time to save work and shut down cleanly, not to run your setup for hours. Most consumer UPS units are sized to deliver somewhere in the range of 5 to 20 minutes at a meaningful load; getting significantly more runtime means either dropping your connected load well below the UPS's rated capacity or stepping up to a much larger and more expensive unit. If you specifically need extended runtime (like keeping a home network alive during long outages), that changes your sizing math significantly and is worth treating as a separate goal from basic surge/outage protection for a desktop.

Step 4: Leave headroom, don't size to the edge

A UPS run near its maximum rated load has shorter battery life over time and less margin if you add another device later. As a rule of thumb, size the UPS so your actual connected load is around 50-60% of its rated capacity rather than close to 100% — this also tends to give noticeably better runtime than the UPS's base rating suggests, since runtime scales non-linearly with load.

A worked example

Say you want to protect a desktop and monitor (250W), a router and modem (25W), and a small NAS (30W) — a combined load of about 305W. Aiming for that to be roughly half of the UPS's capacity points to a unit rated around 600-700W (roughly 1000-1200VA in typical consumer labeling). That leaves headroom for the load, a printer or accessory added later, and a longer runtime than running the UPS near its limit would give you.

If your networking gear (router, modem, and any mesh nodes) is the priority — for instance, to keep internet access alive for a cordless phone or security system during outages — that's a much lower combined wattage than a full desktop setup, and a smaller, cheaper UPS sized just for that gear is often the better buy than one large unit trying to cover everything.

Quick sizing checklist