How long will my UPS last?
An uninterruptible power supply (UPS) or a battery inverter runs your equipment from batteries during an outage. Its runtime depends on the stored energy, the load, the inverter efficiency, how deeply the batteries may be discharged, and how capacity shrinks at high currents.
The simple estimate
Runtime (h) = V × Ah × depth of discharge × efficiency ÷ load (W)
The Peukert effect
Lead-acid batteries are rated over 20 hours. Drawn faster, they deliver less than their rated capacity. Peukert’s law corrects for it:
t = H × (C ÷ (I × H))^k
where H is the rating time (20 h), C the rated capacity, I the battery current and k the Peukert exponent: about 1.1 to 1.3 for lead-acid, 1.05 for lithium.
Example
A small UPS with two 12 V 9 Ah batteries in series (24 V), feeding 150 W at 85% efficiency, 80% usable:
- battery current: 150 ÷ (24 × 0.85) ≈ 7.4 A, a discharge rate of 0.8C;
- simple estimate: 24 × 9 × 0.8 × 0.85 ÷ 150 ≈ 59 minutes;
- with Peukert (k = 1.2): about 34 minutes, much closer to manufacturer runtime charts.
What uses the power?
- A desktop PC with a monitor: 100 to 300 W.
- A router, modem and Wi-Fi access point: 15 to 40 W.
- A NAS with four drives: 40 to 80 W.
- A CPAP machine: 30 to 60 W, more with a heated humidifier.
- A gas furnace blower: 300 to 800 W.
Measure the real load with a plug-in power meter if you can; nameplate ratings are usually maximums.
Choosing a UPS
- VA and watts: a UPS has both ratings, such as 750 VA / 450 W. Keep the load under the watt rating, ideally under 80% of it.
- Runtime: small UPS units are meant to bridge short outages and allow a clean shutdown, usually 5 to 15 minutes at half load. For hours of backup, use larger external batteries or a generator.
- Battery life: lead-acid UPS batteries last about 3 to 5 years; heat shortens that. Test the runtime once a year.
- Lithium (LiFePO4) UPS batteries last longer and suffer less from Peukert losses, but cost more.
Tips
- Plug only critical equipment into the battery outlets; printers and space heaters can overload a UPS.
- Install the UPS software so the computer shuts down automatically before the battery runs out.
- Runtime does not scale linearly: halving the load usually more than doubles the runtime.