Choosing The Right Size UPS
ITS recommends all PCs use a battery backup device, or Uninterruptible Power Supply (UPS), to protect their hardware and data from all types of power problems. This is important because power loss can damage hardware, and unexpected shutdowns can corrupt file systems, potentially causing data loss or unbootable devices.
Levels of Protection
A UPS has a large battery that provides power whenever a power fluctuation occurs, such as a brownout (low voltage) or even high voltage. A standard surge protector, by contrast, only protects against lightning strikes or other power surges, and does not provide protection against brownouts, blackouts, or voltage fluctuations. Brownouts, in fact, are generally considered the most dangerous condition to electronics.
A UPS can sense any change in voltage and switch to its built-in battery. Many units always leave the battery in use, to avoid even a microsecond's delay while switching. The battery is then continuously charged from wall power. UPSs also provide surge protection, replacing the need for separate surge protectors.
The battery allows a UPS to protect against all the above conditions, at least until its battery runs out.
Battery Life
The reason so many different UPS models exist is differences in battery technology and capacity. A larger battery will provide more power, and/or allow connected devices to run longer. For example, UPS manufacturer APC has small, sub-$200 models which allow a standard PC to run for 5-15 minutes on battery power. This provides just enough time to gracefully hibernate, or for you to save your files and shut down. A UPS with more capacity would allow the same PC to run longer, or perhaps power two PCs for the same amount of time. Larger devices can power entire server racks including switches and routers, or entire data center rooms.
Newer models may use lithium batteries, which are more expensive. They do not require replacement every few years as with lead-acid batteries. Per APC, lithium-Ion batteries used in Smart-UPS can last 8 to 10 years or 3000 charge/discharge cycles, versus Valve Regulated Lead Acid (VRLA) batteries that typically last 3 to 5 years or 200 discharge cycles.
What Not to Connect
Never connect a laser printer to even a medium size UPS. When the fuser element heats up, a printer draws a large amount of current, more than a small or even medium UPS can provide. If this happens, it will trip the circuit breaker in the UPS. If you would like your laser printer to be protected, only a larger UPS model will be able to provide the power required.
Avoid connecting surge protectors or outlet strips to a UPS (or another surge protector). Against fire code in some jurisdictions, excessive power draw can cause overheating of in-wall wiring if too much power is pulled through one circuit.
Determine Your Requirements
The two most important considerations are the amount of power required, and the runtime, or battery life, you wish to have.
The power requirements depend on the number of devices connected to the UPS, and the power each requires. Even a small (450W) UPS can handle a standard PC and monitor, at least to immediately but safely shut it down. For multiple PCs, you may need a larger unit, 1000W or more, but note most UPSs will only shut down one device by default. Every device has a power requirement indicated on its power supply or case. This number is usually far higher than what the device typically draws, but if you add the wattage requirements of all your devices, that will give you some idea of a minimum UPS size.
If your home or office regularly has power fluctuations or outages, select a UPS that outlasts the majority of them. Your desired runtime may be anywhere from minutes to hours. For example, if your building has a generator which takes a few minutes to turn on, you only need a small UPS. For long outages, of course, the PCs and UPSs should be shut down to avoid discharging the battery unnecessarily, although huge UPSs are available that can last many hours, if business needs warrant.
Look at your needs holistically. If PCs will only be on for a few minutes, there is probably no need to keep servers, switches, and the Internet running for hours.
May 2026
(original version:
April 2000)
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