Infinite Tech, UPS & Power Backup Guides

How to Calculate UPS Size for Your Computer

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October 29, 2026

On October 29, 2026

Short answer: Choose a UPS using your equipment’s real power consumption in watts and its apparent-power requirement in VA—not only the PC power supply’s printed maximum wattage.

Prepared October 2026. This article provides general shopping information, not an electrical installation certificate or a tested model-specific runtime guarantee.

Step 1: List every powered device

Write down the computer, monitor, router, ONT, external storage and any essential small device that will use battery power. Exclude appliances that do not need backup. Use a plug-in power meter for the most useful measurements at working load; a desktop drawing 120W at idle may draw much more while rendering, compiling or gaming.

Step 2: Measure or estimate watts carefully

Add the typical running watts, then examine demanding scenarios and transient peaks. If the PSU label reads 650W that is not necessarily the wall power draw, but an incorrectly low estimate can lead to overload. A small buffer may be insufficient for equipment with large spikes.

Step 3: Understand watts versus VA

The UPS manufacturer specifies both maximum real power in watts and apparent power in VA. A hypothetical 300W load with power factor 0.8 corresponds to roughly 375VA. This calculation is an illustration, not the actual power factor of every computer. If you can measure both W and VA, use those numbers; both must fit the UPS limits.

Step 4: Apply sensible headroom

Schneider Electric recommends sizing with meaningful headroom rather than continuously running near the device maximum; one guide illustrates staying near 80% of available power. For a hypothetical 400W peak-load workstation, a minimum target of about 500W rated output provides 20% reserve. A 500W rating is not the same as 500VA.

Step 5: Match transfer and output characteristics

A gaming system with active-PFC PSU, server or storage appliance may require an appropriate output waveform and transfer behaviour. Ask about simulated sine versus pure sine output, compatibility and topology on the exact unit; do not infer them from vendor naming alone. Sensitive loads may benefit from an online UPS.

Step 6: Calculate runtime as a separate task

Once output power is adequately sized, determine how many minutes you need for a safe shutdown. The Lightwave 1000VA UPS, Lightwave 1500VA UPS and Lightwave 2000VA UPS are candidates only after confirmed manufacturer watt ratings. You need battery energy and a verified runtime curve to predict duration; VA is insufficient.

Frequently asked questions

Should a 400W PC use a 400VA UPS?

Not necessarily: check the UPS’s actual watt output and allow headroom.

Is power factor always 0.8?

No; the actual power factor varies by load and operating conditions.

Can I simply buy the largest VA?

Oversizing costs more and does not guarantee longer backup time.

Compare available Lightwave UPS options

Sources and next steps

Check the Lightwave manufacturer catalogue for exact models and Schneider Electric’s UPS selection guidance for general sizing principles. Contact Infinite Tech Nairobi for a model-specific quote, full product specifications, delivery and warranty.