UPS » Run Time

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“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 ˝ 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. For some brands a single battery size is used for two or more sizes of UPS. In this case the run time may be much longer for the smaller unit. These figures are almost always published by the manufacturer.

Be aware of the fact that 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 life, being able to replace them is a nice feature. UPS batteries are the sealed lead acid type. They pose no danger with normal operation. But, they should be disposed of properly. Although there have been great recent advances in battery technology, these components are still heavy. A 600 watt UPS weighs around 30 pounds. A 2200 watt unit weighs around 80 pounds. More sophisticated UPSs allow for additional battery packs to be connected to the UPS for additional run time. Typically, these just plug into the back of the UPS.

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.