How Do I Determine What Size Battery Backup to Buy?


An uninterruptible power supply--also known as a "battery backup" or "UPS"--is used to provide power to electrical devices in the event of a power outage. An uninterruptible power supply converts the direct current electrical energy (which is supplied by an internal lead-acid battery) into alternating current electrical energy.

Since the internal battery in a UPS can only supply electrical energy for a finite period of time, it is important to choose a UPS that can provide enough power for that period of time.

Battery Backup Power Ratings

  • Each uninterruptible power supply has a power rating measured in Watts. This power rating is the maximum power in Watts the power supply can provide for a short period of time (typically less than five minutes).

    A battery backup which is supplying less than the maximum load will provide power for a longer period of time. For example, an APC BE350G 200 Watt UPS is capable of supplying 200 Watts of power for approximately one minute. The same UPS is capable of supplying 100 Watts for six minutes.

Determining Power Consumption

  • A battery backup will be able to supply electrical power for a longer period of time if the battery backup is not supplying maximum power. Therefore, to allow a battery backup to supply power for as long as possible, it is critical to determine how much power the battery backup is required to supply.

    Each electrical device plugged into a battery backup will draw electrical current. To determine how much electrical power the battery backup must supply, obtain the electrical current ratings of each electrical device the battery backup will supply power to. This rating can be found either on the device's power plug, or at the point where the electrical cable enters the device (such as the back of a refrigerator, microwave or radio).

    Add the electrical current rating values together, and multiply by 120 Volts. For example, if a radio with a current rating of 1 Amp and a cordless telephone with a rating of 2 Amps are to be plugged into the battery backup, the total power the battery backup must supply is 360 Watts (or, 120 Volts x 3 Amps).

Choosing a Battery Backup

  • To provide power for the longest period of time, choose a battery backup with a much larger (at least double) power rating than the electrical devices require. Never use a battery backup that provides less power than the electrical devices require; this may pose a fire hazard, and will damage the battery backup.

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