No company operates its servers without a UPS, without an uninterruptible power supply. In the event of a power failure, the computers simply continue to run, backed up by batteries – long enough to survive a brief power failure. If the blackout lasts longer, the computers shut down to back up all data before the batteries are empty. This also makes sense in the private sphere – be it to be able to use the Internet connection, the WLAN and thus the VoIP telephone connection even in the event of a power failure. To protect expensive projector lamps in the home cinema from heat death caused by fan failure. So that the smart home alarm system can send a push message to the homeowner’s cell phone in the event of a break-in, so that important documents do not get broken while working in the home office – or so that the video surveillance of the garden and driveway continues. There are enough reasons; Especially since many UPSs also protect against voltage peaks – for example in the event of a lightning strike in the neighborhood.
The simplest emergency power supplies look like oversized power strips. A buffer battery and electronics are integrated, which ensure the uninterrupted power supply of the connected devices, filter voltage peaks and transmit feedback on the current battery status to a connected PC or Mac via USB. The battery capacities of these models are not particularly large. High-performance computers cannot be operated with it, but a Fritzbox, Power over Ethernet for cameras and a small smart home server can continue to run for fifteen minutes – and thus longer than the average power failure in Germany per year.
Larger models look like NAS or small PC cases and find their place under the desk. The functionality is the same, only the capacity of the batteries is higher – and the power that consumers can call up in the event of a power failure, but more on that later. Important when buying: Which connections are available? Because while the simple “socket outlet UPS” are equipped with well-known Schuko sockets, UPSs in this form factor often come with IEC sockets. You can easily connect servers and PCs, but not a Fritzbox or the power supply unit of a simple video surveillance system.
High-end models for professional use come in 19 “format for installation in the server cabinet, but this is not the case here.
There are technical differences in the structure of uninterruptible power supplies. While in the past one paid particular attention to online vs. offline, today three classes are in the foreground.
Class 1: Online UPS
The most expensive devices belong in this class. The mains voltage supplies the batteries, the connected loads are always fed from the UPS. This means there is no delay due to switching in the event of a power failure. However, the rectifiers and inverters inside are permanently in use and must be designed accordingly generously – and ultimately the components also need energy; the efficiency of these UPSs is not that great.
Klasse 2: Line Interactive
UPS of this type are based on a bidirectional inverter. Depending on requirements, this element either charges the integrated battery or supplies the loads from the battery. The switchover is faster than with offline UPSs, namely in 2 to 4 ms – that’s enough to prevent even sensitive devices from hiccups.
Class 3: Offline UPS
Offline UPSs are, so to speak, the opposite of online UPSs. In normal operation, the consumers are supplied with energy directly from the power grid. In addition, a rectifier charges the integrated battery. In the event of a power failure, the power supply is switched over to the battery and inverter. This process takes up to 10 ms – it can be too long for particularly sensitive devices; Such a micro power failure can cause problems. On the other hand, the efficiency is higher, since the energy consumed does not have to be converted twice, and the UPSs are cheaper. For comparison: ATX PC power supplies withstand a micro power failure of 17 ms. This is absolutely sufficient for a normal PC.
Since consumers behind an offline UPS are supplied directly from the input voltage, they also receive voltage peaks unfiltered. In comparison, line-interactive UPSs also protect against overvoltage and undervoltage, and online UPSs also protect against fluctuations in the mains frequency, against harmonics – and potentially also against lightning strikes, thanks to their complete independence from the power grid.
And which UPS should it be now? First of all, it is important to know what you want to protect with it – for example a Fritzbox, a PoE switch with connected IP cameras and a smart home server that runs on a NAS, an Intel NUC, a Raspberry or a Mac Mini. The data sheets of the device manufacturers reveal the maximum power consumption.
A current Fritzbox 7590 needs a maximum of 30 watts under load, a Mac Mini with an M1 processor just under 40 watts and an older model with an Intel processor up to 122 watts. The maximum consumption of all connected devices (plus some buffer) is the power that the UPS must achieve. Inexpensive models, such as the “socket outlet UPS” from APC, provide at least 300 watts – so easily sufficient for this purpose.
To be clear: it is actually much more complex. In addition to the real power discussed here, there is also an apparent power. When planning extensive emergency power supplies, for example for critical infrastructure such as company servers, one should delve deeper into the subject – but as a rule of thumb for use at home, this is sufficient. With one exception: some devices such as laser printers have comparatively high inrush currents that could overload a UPS. But as long as you “only” operate computers, monitors and network equipment, you don’t necessarily have to dig deeper into the subject. In any case, the question arises as to whether a printer still has to work in the event of a power failure.
How long the UPS can supply the devices before the lights go out depends on the capacity of the built-in batteries. Unfortunately, the manufacturers do not always give the capacity, but often give information such as “runtime at 100% load” or “runtime at half load”. Most manufacturers also offer corresponding configurators on their websites, which give tips for suitable models when specifying the connected consumers and calculate the probable running times. Caution, you should also plan for buffers here – because the batteries age.
Almost all UPSs have a USB connection, higher quality models additionally or alternatively have a network interface. A connected computer can use this to query the charge status of the battery. A behavior can be set using on-board tools or programs supplied – for example, that the computer should shut down when the charge level of the battery is only 30 or 50 percent.
Caution: While computers can be set so that they switch themselves on again after a power failure, a more complex solution is required here – because as long as there is still power in the UPS battery, this does not count as a power failure for the computer. He does not notice what is happening on the input side of the UPS. Expensive devices bring Wake on LAN with them to bring computers back to life. For inexpensive consumer models you have to look for another solution if continuous operation is desired. A Google search for “Wake on LAN after power failure” brings up some ideas and suggested solutions.
The batteries of consumer UPSs are usually designed for a service life of five years, in the professional segment it can be ten. Almost all UPSs draw attention to themselves with annoying beeps when the capacity of the built-in batteries has decreased too much. Replacing them is usually not a problem for the end user either; suitable batteries can easily be found for common UPSs, such as those from APC, and from third-party manufacturers that are significantly cheaper. After the original battery in an APC Smart UPS lasted almost exactly five years in practical use, a cheap replica battery has now been used for 1.5 years – we will update the article when the next replacement is due.
We summarize the huge range on the market here in a nutshell: The following price comparison shows the top 10 of the cheapest and the top 10 of the most popular UPSs.
UPSs in the multiple socket form factor are particularly practical for private use. We consider these products again here separately:
UPSs are useful. Be it to keep security functions such as alarm, telephone and video surveillance running, to prevent data loss in the home office or to implement the continuous operation of network storage devices, servers or smart home centers. For most purposes, cheap models are suitable, provided the performance is right – you don’t have to keep your PC running for hours at home. It is enough to bridge the five minutes required to save open documents and shut down.
In the private environment, emergency power supplies integrated in multiple sockets are particularly suitable, which ideally also protect against overvoltage. Suitable devices are available for under 50 euros, branded products – such as from APC – for under 70.