Picking a UPS (Battery Backup) for Your Mac or PC

A UPS (Uninterruptable Power Supply), also called a backup battery, is a wise investment for any Mac or PC user. Even if you use software to back up your files, a UPS can be a lifesaver if you unexpectedly lose power. Before you pick a UPS battery backup for your Mac or PC, determine the size necessary to run your equipment when the power goes out.

What Is a UPS for Computers?

There are different types of UPS systems. The type used with computers is a Standby UPS. It contains a battery that kicks in when the UPS senses a loss of power. The UPS sits between the power supply and the equipment it protects. It plugs into the wall outlet, and the devices plug into the UPS. A standby UPS for computer equipment provides two primary services:

• Conditions the AC voltage to reduce power surges that can disrupt or damage a computer system.
• Provides a computer with temporary power for a short while when the electrical service to a home or office goes out.

Most UPS devices are designed for electronics with small noninductive motors, including computers, stereos, and TVs. Devices with large inductive motors require specialized UPS devices.

UPS Size

For a UPS to do its job, it must be adequately sized to deliver sufficient power to all the connected devices. Size refers not to the physical mass of the device but rather to its capacity. Before purchasing a UPS, determine the amount of power used by all the devices you plan to connect to it and the amount of time you want the UPS to provide power in the event of a power outage.

Device Wattage

The amount of wattage the UPS system needs to deliver is a critical factor. A watt is a unit of power defined as one joule per second. When talking about electronics, wattage is measured as the voltage (V) multiplied by the current (I) in a circuit (W = V x I). In this case, the circuit comprises the equipment connected to the UPS, such as a computer, monitor, and peripherals.

Almost all electronic devices have the voltage, amperes, and wattage listed on the label. To find the total, add the wattage value listed for each device. If no wattage is listed, multiply the voltage by the amperage. This produces a value that is the maximum wattage the devices are likely to produce.

VA Rating

UPS manufacturers usually do not use wattage. Instead, they use a Volt-Ampere rating. The VA rating is a measure of the apparent power in an AC (alternating current) circuit. Since computers and peripherals use AC, the VA rating is the appropriate way to measure the actual power consumed.

There is an equation for converting wattage to VA. As an example, if a computer and the peripherals have a total wattage of 800, then the minimum VA rating you should be looking for in a UPS is 1,280 (800 watts multiplied by 1.6). Round this up to the next standard UPS VA rating available, most likely 1,500 VA.

UPS Runtime

Runtime is how long the UPS unit can power a computer system at the expected wattage level during a power outage. To calculate runtime, you need to know the minimum VA rating, the battery voltage, the amp-hour rating of the batteries, and the UPS's efficiency.

The needed values are rarely available from the manufacturer. However, these values sometimes appear inside the UPS manual or technical specifications. If you can ascertain the values, online UPS runtime calculators can do the math for you.

If you can't find all the parameters needed to perform the runtime calculation, visit the UPS manufacturer's site and look for a runtime/load graph. There are also online UPS selector tools that help you choose a UPS based on your needs.

UPS Runtime Example

As an example, a CyberPower CP1500AVRLCD uses a 12-volt battery rated at 9 amp-hours with 90 percent efficiency. It can provide backup power for 4.5 minutes to a computer system drawing 1,280 VA.

That may not sound like much, but 4.5 minutes is long enough for you to save your data, close files, and perform a shutdown in an emergency. If you want a longer runtime, pick a UPS with better efficiency, a longer-lasting battery, higher voltage batteries, or all of these. Choosing a UPS with a higher VA rating doesn't increase runtime. Still, most manufacturers include larger batteries in UPS models with larger VA ratings.

UPS Battery Replacement

On average, a UPS battery lasts three to five years before it needs to be replaced. Many UPS devices provide a warning when the battery needs to be replaced, and a few stop working. You may end up changing out the battery a few times during the life of the UPS, so knowing the cost and whether batteries are readily available is a good idea before selecting a UPS.