I have divided this post into sections:
- Low Voltage Battery Back Up
- Computer UPSs.
- Alternative UPSs
1. Low Voltage Battery Back Up
My recommendation is to forget getting a 240V ac Uninterruptible Power Supply (UPS) and get a low voltage (Battery Back Up Unit (BBUU). These replace the route (and VoIP ATA power supply if needed) with a battery backed low voltage output. This is the same output voltage as a, for example, router power supply, except that when the power fails, the battery automatically takes over.
The Unit to choose depends on how long you wish to maintain the connection. Not a recommendation but an example of a BBUU: https://www.amazon.co.uk/Yuuki-Power-Router-Battery-10400mAh-Black/dp/B0FCV2Q8TB/ref=sr_1_8_sspa?cri...
Another example, : https://www.amazon.co.uk/APC-Back-UPS-Connect-Lithium-Ion-Controllers-White/dp/B0CJ5D89Z9/ref=pd_rhf...
2. Computer 240V UPSs
What is the difference to using a 240Vac UPS you may consider? Simplistically, the UPS is plugged into the mains, the mains is used to charge a low voltage battery. A built-in inverter then transforms the low voltage battery to 240V RMS 50Hz into which you connect your standard router power plug. So, during a power cut the UPS battery has to convert the low voltage dc battery to 240V ac in order for the router power supply to convert is back to 12Vdc. This all takes power, which has to be supplied by the UPS battery, hence shortening the potential run time.
In addition, the router and any other power supply these days will invariably use switch mode technology. These present a low power factor load on the inverter. Without getting into deeper geekish explanations, this results in significant additional load on the UPS battery than the simple router rated quoted wattage, with excess power having to be dissipated in the inverter.
A low voltage BBUU has the simple and very efficient task of converting the low voltage dc battery voltage to, for a Plusnet Hub, 12V dc. Hence much greater hold up time for a specific battery capacity.
Other Potential UPS Disadvantages.
A Google search for computer back up UPS will bring up many alternatives with impressive output claims such as 600kVA. The information that is missing is the battery capacity. Many of these are designed to simply give time to shut down a computer in an orderly manner, which can be less than a minute. As an example, if a 600VA UPS only has battery capacity to maintain 600VA for a minute, then all that is needed is a 10 Whr capacity battery. So, if this is used to power a 10W router, the maximum support time will be no more than an hour tops. There is another factor to also consider, some of these, especially the higher-powered ones, have, if you delve into the specifications, a minimum load requirement, needed to keep the Inverter under control, this might or might not be less than the load presented by a router.
3. An Alternative Option
Another option is to forget about a UPS designed to support a computer, but look to the leisure industry. There are some very interesting portably Inverters designed for outside living, such as camping. I have an Antek 521 https://www.amazon.co.uk/Anker-Portable-Station-Generator-Outdoor/dp/B09Q5DJV71?th=1. I have one, this will keep a 10W (unknown VA but considerably higher load as its feeding two switched mode power supplies working for just under 10 hours.
I have the option to use the 12V output for even greater hold up time. This is not continually in circuit so that in the event of a power cut I have to swap the mains plug over. On the other hand, I can also use it to keep a couple of lights on and forget my data connection for an evening,
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