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UPS Size Calculator

Size a UPS to keep a router, NAS, office desk or desktop PC running long enough.

Equipment to protect

Selected load3 equipment type(s)
ONT / fibre modem8 W - Typical draw
Router18 W - Typical draw
NAS45 W - Typical 2-4 bay NAS
0 to 5,000 W
0 to 20

Add every critical device. The listed watts are typical values: replace them with a measurement when you have one.

Runtime and headroom

5-720
0.70 is common for mixed IT loads
Applied to output power and battery
20% is prudent after a few years
Runtime, headroom and battery aging127 WhLoad with headroom / peak89 W / 120 WPower factor172 VA

Each setting below changes the VA, W or battery capacity recommendation.

Battery target for 45 min

127 Wh
UPS class600 VA
Output power target120 W
Minimum VA from power factor172 VA
Estimated startup peak100 W
Efficiency / battery retained82% / 80%

How to read the result

Headroom is applied to the whole sizing load, so it changes the battery (127 Wh), output power (120 W), and the 172 VA minimum. The 600 VA class is a commercial tier, so it can stay unchanged until the next tier is needed. Also check the maker's runtime chart at 71 W.

Frequently asked questions

Why are VA and watts different?

VA is apparent power. Watts are real output power. A UPS must satisfy both ratings, especially for PCs and NAS hardware.

How much margin should I keep?

20-30% is a good default. More margin helps with aging batteries, startup peaks and future devices.

Is a UPS enough for long outages?

Usually no. A UPS is mainly for clean shutdowns or short outages. For hours of autonomy, look at larger power stations or battery systems.

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