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

Protect your Windows or Mac computer from power surges and outages

CyberPower CP1500AVRLCD
Most UPS specifications contain enough information to calculate the runtime for your computer system.

 CyberPower Systems

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.

Information in this article applies broadly to a wide range of devices. Check the specifications of individual products before making a purchase.

What Is a UPS for Computers?

A UPS for computer equipment provides two primary services:

  • Conditioning the AC voltage to reduce power surges that can disrupt or damage your computer system.
  • Providing your computer with temporary power when the electrical service to your home or office goes out.

Most UPS devices are designed for electronics with small non-inductive motors, including computers, stereos, and TVs. Devices with large inductive motors require specialized UPS devices. If you're not sure if your device should be connected to a UPS, check with the UPS manufacturer.

UPS Size

In order for a UPS to do its job, it must be properly sized to deliver sufficient power to all connected devices. Size refers not the the physical mass of the device but rather its capacity. Before purchasing a UPS, you should know the amount of power used by all the devices connected, as well as the amount of time you would like the UPS to provide power in the event of a power outage.

Device Wattage

One of the important values to know is the amount of wattage the UPS system will need to deliver. A watt is a unit of power defined as one joule per second. When talking about electronics, wattage is measure as the voltage (V) multiplied by the current (I) in a circuit (W = V x I). The circuit, in this case, is what you're connecting to the UPS: your computer, monitor, and any peripherals.

Almost all electronic devices have the voltage, amperes, and/or wattage listed on the label. To find the total, you can simply add together the wattage value listed for each device. (If no wattage is listed, multiply the voltage by the amperage.) This will produce a value that should be the maximum wattage all of the devices are likely to produce.

You can use a portable wattmeter, such as the Kill a Watt meter, to directly measure the wattage used by all of your devices.

VA Rating

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

Fortunately, there is an equation for converting wattage to VA. As an example, if your computer plus the peripherals have a total wattage of 800, then the minimum VA rating you would be looking for in a UPS would be 1,280 (800 watts multiplied by 1.6). You would 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 will be able to power your 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 efficiency of the UPS.

Unfortunately, the needed values are rarely available from the manufacturer, though they will sometimes appear inside the UPS manual or technical specifications. If you can ascertain the values, there are online UPS runtime calculators that can do the math for you.

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

The hardest value to uncover is the UPS efficiency. If you can’t find this value, you can substitute .9 (90 percent) as a slightly conservative estimate.

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 any data and perform a graceful shutdown in an emergency. If you want a longer runtime, you'll need to pick a UPS with better efficiency, a longer-lasting battery, higher voltage batteries, or all of the above. Choosing a UPS with a higher VA rating doesn't increase runtime, but most manufacturers include larger batteries in UPS models with larger VA ratings.

UPS Battery Replacement

On average, a UPS battery lasts 3 to 5 years before it needs to be replaced. Many UPS devices will provide you with a warning when the battery needs to be replaced, but a few will simply stop working. You'll likely be 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.

Before buying, make sure the UPS provides a pass-through mode that lets the UPS continue to operate as a surge protector when the battery fails.