UPS Battery Backup Calculator: Size (VA and Watts) and Runtime

By Web Asset House Studio · October 29, 2026 · Runs in your browser; nothing you type is sent or stored.

List what has to stay on during an outage, then read off the minimum UPS rating in both watts and VA. If you know your UPS battery's voltage and amp-hours (on the battery label, in the manual or on the replacement battery's spec sheet), the second part gives a rough runtime.

Runtime here is an idealized estimate. Real runtime depends on the battery's age, temperature and charge, and lead-acid batteries deliver less of their rated energy at high loads. For a purchase decision, use your UPS maker's runtime chart for the exact model; CyberPower and other manufacturers publish them.

Equipment (watts from each device's label or power supply; example rows from our UPS sizing guide)

Device	Watts	Qty

Power factor of your equipment (Eaton: 0.9 or more for many newer IT devices, 0.60–0.75 for legacy PCs) Maximum load as a % of UPS capacity (Eaton: never above 80%)

Battery (for the runtime estimate) Battery voltage (V) Battery capacity (Ah) Number of batteries of this size Inverter efficiency on battery (%) — placeholder, use your UPS spec if listed Usable share of stored energy (%) — placeholder, lower for older batteries Runtime you want (minutes)

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Results

Total load	—
Load in VA (watts ÷ power factor)	—
Minimum UPS watt rating	—
Minimum UPS VA rating	—
Battery energy (V × Ah × count)	—
Estimated runtime at this load	—
Battery energy needed for your target runtime	—

Formulas used

Total load (W) = sum of each device's watts × quantity.
VA = watts ÷ power factor (Eaton). A 1000 VA/600 W UPS has to cover your load on both numbers.
Minimum UPS rating = load ÷ maximum load share. At Eaton's 80% ceiling that's load × 1.25, in line with CyberPower's advice to buy 20–30% more watt capacity than your load.
Battery energy (Wh) = volts × amp-hours × number of batteries.
Runtime (minutes) ≈ Wh × efficiency × usable share ÷ load watts × 60. Wh needed = load × minutes ÷ 60 ÷ (efficiency × usable share).

The efficiency and usable-energy figures are placeholders you should change, not measured values; no manufacturer we cite publishes a general figure for them. That is why the maker's runtime chart for your exact model beats any formula.

Worked examples

Sizing (from Eaton): equipment rated 525 W with a power factor of 0.7 is a 750 VA load. Sizing the UPS to run at 75% gives 750 ÷ 0.75 = a 1000 VA UPS. Our guide's desk setup: 250 W + 30 W + 15 W = 295 W; at the 80% ceiling you need at least 369 W of UPS output.

Runtime: two 12 V, 9 Ah batteries store 216 Wh. At 85% efficiency and 80% usable energy that's 146.9 Wh, which runs a 150 W load for about 59 minutes in theory. Expect less in practice, especially from an older battery.

Before you plug in

Eaton says never to plug a laser printer into a UPS's battery outlets; it can drain the battery. Space heaters don't belong on a UPS either.
Devices with active-PFC power supplies (common on ENERGY STAR and 80 PLUS computers) may need a UPS with sine-wave output, per Eaton.
ENERGY STAR certifies efficient UPS models and has a product finder.

Sources

Sources checked October 29, 2026.