UPS Runtime Calculator

This free UPS runtime calculator estimates how long an uninterruptible power supply will keep your computer, gaming PC, NAS, server, router or modem running during a power outage. Enter the load in watts and the UPS battery's voltage and amp-hours — printed on the battery or in the manual — to get the runtime, and see what size UPS in VA and watts you need. It accounts for inverter losses, battery age and the Peukert effect that shortens runtime at high loads.

How UPS runtime is calculated

Battery current (A) = load W ÷ (battery V × inverter efficiency)
Runtime (h) = 20 × (Ah × health ÷ (current × 20))^k
Recommended UPS = load × 1.25 W (≈ that ÷ 0.6 in VA)

The second line is Peukert's law for a battery rated at its 20-hour rate. The exponent k is about 1.2 for the sealed lead-acid batteries in most UPS units and close to 1.05 for lithium. It is why doubling the load more than halves the runtime of a lead-acid UPS.

Worked example

A 1500 VA UPS has two 12 V 9 Ah batteries in series (24 V). With a 150 W load and 85% efficiency, the batteries supply 150 ÷ (24 × 0.85) = 7.4 A. Runtime = 20 × (9 ÷ (7.4 × 20))^1.2 ≈ 42 min.

UPS runtime chart

Estimated runtime of typical UPS sizes with new sealed lead-acid batteries. Loads above what each size can supply are left blank.

UPS (battery)	20 W	50 W	100 W	200 W	300 W	500 W
600 VA (12 V 7 Ah)	2 h 32 min	51 min	22 min	10 min	6 min	—
1000 VA (12 V 9 Ah)	3 h 25 min	1 h 8 min	30 min	13 min	8 min	4 min
1500 VA (24 V, 2 × 9 Ah)	7 h 51 min	2 h 37 min	1 h 8 min	30 min	18 min	10 min
3000 VA (48 V, 4 × 9 Ah)	18 h 3 min	6 h 1 min	2 h 37 min	1 h 8 min	42 min	23 min

Battery layouts vary between models; check your UPS manual for the battery voltage and amp-hour rating. Manufacturer runtime charts are the best reference for a specific model.

Typical power use of devices on a UPS

Device	Typical watts
Wi-Fi router + modem	15–25 W
NAS (2–4 drives)	20–50 W
Laptop (charging)	30–90 W
Office PC + monitor	80–150 W
Gaming PC + monitor (under load)	300–600 W
Small home server	50–150 W

Related calculations

Need hours of backup rather than minutes? See how long a power station lasts with the battery runtime calculator.
For long outages, size a generator with the generator size calculator.
Find what an always-on PC or server costs per year with the electricity cost calculator.

Frequently asked questions

How long will a UPS last during a power outage?

It depends on the battery and the load. A typical 1500 VA UPS with a 24 V, 9 Ah battery runs a 100 W load for about 1 h 8 min and a 300 W gaming PC for about 18 min. Most home UPS units are designed to bridge short outages and give you time to save work and shut down, not to run for hours.

How long will a UPS run a Wi-Fi router and modem?

A router and modem together use about 15–25 W. A small 600 VA UPS (12 V, 7 Ah) runs 20 W for roughly 2 h 32 min. For longer internet backup, a small DC "mini UPS" or a portable power station is far more efficient, because it skips the conversion to AC and back.

What size UPS do I need for a gaming PC?

Measure or estimate the full load — PC, monitor and speakers — then choose a UPS whose watt rating (not VA) is at least 25% higher. A gaming PC drawing 400–500 W under load needs a UPS rated around 600–700 W or more, typically a 1500 VA model. Many modern PC power supplies with active PFC work best with a pure sine wave UPS.

What is the difference between VA and watts on a UPS?

VA (volt-amps) is the apparent power and watts is the real power the UPS can deliver. Watts = VA × power factor; many consumer UPS units have a power factor of 0.6, so a 1500 VA model supplies about 900 W. Always size by the watt rating.

Why does my UPS run for less time than expected?

Three reasons: lead-acid batteries deliver less capacity at the high currents a UPS draws (the Peukert effect), the inverter loses 10–20% as heat, and batteries age — most UPS batteries lose much of their capacity after 3–5 years. Lower the battery health setting to estimate an older battery.