How to Size a UPS for Your Equipment
Measure real load in watts, read your battery rating, and use runtime math to pick a UPS that lasts through an outage.
Start with the real load in watts
The single most important number for runtime is the load in watts, and it is the one people most often guess wrong. A power supply rated for 650 watts rarely draws that much; a typical desktop and monitor together may pull 100 to 200 watts at idle. Read the wattage from the device label, the power supply sticker, or better still a plug-in energy meter that shows the actual draw while the equipment works. A router, modem, and ONT combined often total only 20 to 40 watts, which is why they can run for hours on a modest battery.
Translate battery ratings into watt-hours
UPS makers usually advertise a VA figure, which is apparent power and not a measure of stored energy. What the runtime calculation needs is watt-hours. If you know the internal battery, multiply its voltage by its amp-hours: a 12 volt 9 Ah cell holds 108 watt-hours. Larger battery banks stack cells in series and parallel, so a pair of 12 volt 100 Ah batteries wired for 24 volts gives 2400 watt-hours. Enter voltage and amp-hours in the tool and it computes the watt-hours before estimating runtime.
Apply the runtime formula and an efficiency margin
Runtime in hours is watt-hours times efficiency divided by load in watts. A 1200 watt-hour battery at 0.9 efficiency feeding a 300 watt load lasts (1200 x 0.9 / 300) = 3.6 hours, or 216 minutes. Efficiency accounts for the inverter converting DC to AC and typically sits near 0.9, dropping for older or overloaded units. Because the formula assumes a steady load and ignores the extra losses batteries show at high current, the result is best read as a ceiling rather than a guarantee.
Add headroom for aging batteries and safe discharge
A battery loses usable capacity as it ages, and draining lead-acid chemistry to empty shortens its life sharply, so most designs stop well before zero. To leave a safety margin, size for more runtime than you strictly need, perhaps assuming only 70 to 80 percent of the rated energy is truly available. If your goal is simply to ride out short blips or to shut a NAS down cleanly, a small UPS is plenty. For hours of coverage during long outages, plan for a larger battery bank or a generator handoff.