How to Size a UPS Battery Backup for Your Home Office (VA vs. Watts, Explained)
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How we wrote this: A sizing guide based on public guidance from ENERGY STAR, Eaton and CyberPower (linked below). We have not tested any UPS. Always follow the manufacturer's instructions for your model.
An uninterruptible power supply (UPS) is a battery box that keeps your computer, monitor and router running through a short outage. ENERGY STAR describes the small UPS under a desk as giving you enough time to save your work and shut down properly. Sizing one is easier than the spec sheets make it look.
Step 1: List what must stay on
Only plug in what you need during an outage:
- Computer (desktop tower or laptop dock)
- Main monitor
- Modem and router, so you stay online, especially if you use VoIP phones (the FCC notes VoIP may not work in a power outage)
Printers, especially laser printers, and space heaters generally don't belong on a UPS. Check your UPS manual for what it says not to connect.
Step 2: Add up the watts
Find each device's wattage on its label or power supply. Eaton's desktop UPS buying guide calls watts the key rating because it's the real power available.
Step 3: Check VA too
UPS units list two numbers, such as "1000VA/600W". CyberPower's explainer explains the difference:
- Watts = real power your devices use.
- VA (volt-amperes) = apparent power, which includes "reactive" power that does no useful work.
- Power factor = watts ÷ VA. A 1000VA/700W unit has a power factor of 0.7.
Eaton's VA vs. watts page gives the conversion VA = watts ÷ power factor and a worked example: 525 W at a 0.7 power factor is a 750 VA load. Your load has to fit within both of the UPS's ratings.
Step 4: Leave headroom
The manufacturers phrase it slightly differently:
- CyberPower recommends a UPS whose watt capacity is 20–30% higher than your total load.
- Eaton's desktop guide says never to load a UPS above 80% of capacity, which leaves a cushion for peaks and battery ageing.
Worked example: a 250 W desktop + 30 W monitor + 15 W modem and router = 295 W. With 25% headroom you'd look for at least ~370 W of output, and confirm the VA rating also covers your load.
Step 5: Read the runtime chart
Runtime isn't on the box in a useful way. Look up the model's runtime chart at your actual load. Eaton notes that a smaller load means longer battery life, and a larger load means shorter runtime. Decide what you need:
- Save and shut down: a few minutes is enough.
- Ride out short outages: you'll need more battery, or a model that accepts extra battery packs.
Other features worth knowing
- Topology: CyberPower's guide lists standby, line-interactive and online designs. These offer increasing levels of power conditioning, at increasing cost.
- Shutdown software: many units connect by USB and can shut your computer down automatically.
- Efficiency: ENERGY STAR certifies UPS models and has a product finder for comparing them.
What to look for: (Look for: ENERGY STAR certified line-interactive UPS battery backup)