How Long Will A UPS Run?

"Run time” refers to the amount of time a UPS will provide power once the main utility power is interrupted. Standard battery run time for a common UPS is around 5 to 10 minutes under full load and about twice that under half load. (Run times are often quoted as minutes under full and half load.)

The battery run time during a power outage is a function of the current load placed on the battery and the size of the battery. The demand for current by the equipment being powered is expressed in "amps”. The amount of "juice” stored in a battery is expressed in terms of "amp-hours”. (One amp-hour is a current flow of one amp for one hour.) If you want to know how long a given set of equipment will operate off a UPS battery, add up the current in amps that the equipment will pull and divide the amp hours of the UPS by that number. (e.g. A ½ amp load on a 2 amp-hour battery will run for 4 hours.)

So, it makes sense that the smaller the load relative to the size of the battery the longer the run time. Over-sizing a UPS relative to the load is one way to get extended run time. Some UPS models allow special external battery packs to be connected to the UPS to extend run times. Check with your supplier or the product specifications.

A battery will not provide full voltage through its entire rated amp-hour life. Some reduction in voltage will occur at the end of the functional operation of the battery, just as a flashlight becomes dim but still works. Some UPSs will provide automatic shutdown or alarm when this voltage decline is imminent. So, if you have a specific required battery run time in case of a power outage, it’s best to have a bit of amp-hour head room on top of what you calculate.

"Middle market” and top of the line UPSs have replaceable batteries. Since even continually charged batteries have a limited service life of three to five years, being able to replace them is a nice feature. UPS batteries pose no danger with normal operation. But, they should be disposed of properly.

Recent advances in battery technology provide choices in battery designs for various applications and price points.  Lead acid  batteries are heavy and are the most common type used. A 600 watt UPS weighs around 30 pounds. A 2200 watt unit weighs around 80 pounds.  

Simple and sophisticated UPSs alike have warning indicators for loss of main utility power and for low battery power. A warning will also indicate when the battery is reaching the end of its useful life and needs to be replaced. In some UPS models the battery status can be remotely monitored over RS232 or Ethernet communication links.