How to size an Eaton UPS
Updated: October 2026
You are adding a PC, a monitor, and a NAS on a scrap of paper, and the Eaton title in the other tab says 700 VA. How to size an Eaton UPS, for the models in our [Eaton UPS comparison](/hub/electrical-equipment/eaton-ups), starts with the watts on the labels and ends with a minute figure you refuse to treat as a promise. The prices and watt ratings are the amazon.co.uk snapshot of 19 October 2026. We did not clamp a meter on your PC. A qualifying purchase can earn a commission at no extra cost to you.
You are adding a PC, a monitor, and a NAS on a scrap of paper, and the Eaton title in the other tab says 700 VA.
You are adding a PC, a monitor, and a NAS on a scrap of paper, and the Eaton title in the other tab says 700 VA. How to size an Eaton UPS, for the models in our Eaton UPS comparison, starts with the watts on the labels and ends with a minute figure you refuse to treat as a promise. The prices and watt ratings are the amazon.co.uk snapshot of 19 October 2026. We did not clamp a meter on your PC. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Rated power | Runtime claim | USB shutdown | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Eaton Eaton 5E900UI 900 VA | 900 VA / 480 W | 24 minutes | USB and software | 9.2 | about £118.57 | View offer |
 Eaton Eaton 3S700B 700 VA | 700 VA / 420 W | 15 minutes | HID USB claimed | 9.2 | about £124.06 | View offer | |
 Eaton Eaton 5E2200UI 2200 VA | 2,200 VA / 1,200 W | 52 minutes | USB and software | 7.8 | about £376.32 | View offer |
On this page
Add the labels
Write down watts, not VA, from the label or the power-supply rating of each device that must stay on. Watts equal VA times the power factor. A 700 VA 3S is 420 W. The 5E900UI title is 900 VA and the en-GB datasheet pairs it with 480 W. Your sum has to fit under the watt rating, not under the VA number in the model name.
A 200 W PC plus a 30 W monitor is 230 W. That fits under 420 W on the 3S700B. A 500 W PC does not, even though 500 is less than 700 VA. Why the ranges differ is on the Eaton UPS buying guide.
Leave a margin
Eaton’s UK buying guide asks you to allow about 20% more VA than the load when you choose the size, and to keep the running load under about 80% of the UPS capacity. On a 420 W 3S, 80% is 336 W. A desk that adds up to 400 W is inside the headline and outside that ceiling. They do not add runtime.
A laser printer’s heater can demand far more than the PC you just added, and none of these listings publishes your printer’s inrush. Above 120% the 5E goes to fault quickly. Between 110% and 120% it allows five minutes on line. Do not put a kettle or a heater on the battery outlets.
Read runtime without fooling yourself
A minute figure on a listing is a point on a curve, for a load the writer had in mind. Eaton’s 5E datasheet gives typical one-PC times of about 24 minutes on the 900 VA USB column and about 52 minutes on the 2,200 VA column. The 3S700B listing says 15 minutes. None of those is your PC until your watts match the test load. Do not invent the curve from the single number.
The 3S Mini amazon fields say both 5 hours and 150 minutes. Prefer Eaton’s examples: about 80 minutes at 9 W and about 40 minutes at 15 W. A router-only load is on Eaton UPS for router. The 5E user guide gives VRLA float life as a minimum of 3–5 years at 25°C. That note is not a guarantee for the 3S Mini.
Use the USB port for shutdown
The minutes are for saving work and shutting down, not for finishing the evening. HID USB, where the listing claims it, lets the PC or the NAS see the UPS and start that shutdown. The 3S700B listing claims HID USB and includes a USB cord. The 5E titles name USB and shutdown software. The Mini does not claim a shutdown link, because it is not feeding a computer. The 3S700B also has two USB-A charge ports, which are not the same jack as the HID port.
One UK owner of the 3S700B said the software failed to load nine times out of ten. That is one review, and it is enough to make the first boot a test. Stop when the job is no longer a plug into the back of the UPS. A smell of burning, a swollen battery, or anything that keeps medical equipment alive is outside this page.
Common questions
Why is a 700 VA Eaton only 420 W?
Because watts are VA times the power factor, and Eaton states 420 W on the 700 VA 3S models, which is a power factor of 0.6. A PC rated 500 W is over that limit even though it looks smaller than 700. The 900 VA 5E is the same pattern at 480 W on the en-GB datasheet.
Will a laser printer’s inrush fit if the steady watts fit?
Not necessarily. The heater can draw a short spike well above the idle watts, and these listings do not publish that spike for your printer. If the PC and the monitor fit and the printer knocks the UPS into fault, the steady sum was incomplete. Leave the printer off the battery outlets unless you have a watt rating that covers it with room.
What is USB HID for, and what did UK owners of the 3S700B say about the software?
HID is the claim that the computer can see the UPS and shut down before the battery ends. The 3S700B listing claims it and includes a USB cord. One UK owner said the software failed to load nine times out of ten. Test the software on mains power before you rely on it in a cut. If it will not load, you do not get an automatic shutdown.
How long does a lead-acid battery last in years?
The 5E user guide gives a VRLA float life of at least 3–5 years at 25°C. That is a manual condition for that family, not a promise for a warm cupboard. Do not copy it onto the 3S Mini. UK reviews of the Mini describe failure around two years, including just after the warranty.
Where do you stop and call an electrician?
Stop when the work is not a plug into the UPS and the UPS into the wall. A burning smell, a swollen case, or any wiring into the consumer unit needs a competent person. Do not use these listings for medical or life-safety kit unless that kit’s supplier has agreed. Do not open a swollen battery. A plug-in UPS sized for a PC shutdown will not run a house for days.
Related pages
Sources
Public sources from the research digest, last checked on 19 October 2026. Shelf prices and star lines are the amazon.co.uk figures in the display file.
- Eaton 5E Gen2 datasheet (en-GB) (retrieved 19 October 2026)
- Eaton UK UPS buying guide (retrieved 19 October 2026)
- Eaton 3S700B product page (retrieved 19 October 2026)
- Eaton 5S550IBS product page (retrieved 19 October 2026)
- Eaton 3S Mini catalogue page (retrieved 19 October 2026)
- BSI: BS EN IEC 62040-3:2021 (retrieved 19 October 2026)
- Electrical Equipment (Safety) Regulations 2016, regulation 35 (retrieved 19 October 2026)
- GOV.UK: Electrical Equipment Safety Regulations 2016 (retrieved 19 October 2026)