UPS batteries are a crucial and sensitive part of a reliable UPS system.  The type of UPS chosen can have a dramatic effect on battery life and operating cost.

Short vs Long Critical Power Needs

To determine how much power is needed from the UPS, users should take into consideration what the critical power needs are.  For example, will UPS battery power be required:

• For a short time to simply shut down the protected load in a systematic and efficient manner (to eliminate scrap, protect business assets, etc.)
• For a longer time period - like bringing an emergency generator online? 

Full-Load vs half Load Run Times

UPS manufacturers publish full-load and half-load runtimes. 

• Full-load is the amount of time the UPS battery will run at its maximum capacity before the batteries are depleted. 
• Half-load is the amount of time the UPS battery can supply power at half of its maximum capacity. 

Typically under full-load conditions a UPS battery will last only 1/3 of the time it would under a half-load demand.

Incoming Line Voltage Stability

Incoming line voltage stability also plays a part in UPS battery life. 

Offline UPS — As we have seen, when deployed in a fluctuating voltage scenario, offline UPS (single-conversion / line interactive UPS) will go to battery power often.  This depletes the UPS battery power and shortens their service life (as well as speeds their replacement interval - thus driving up the cost of ownership). 

More critically, though, the depleted UPS batteries have less energy for an emergency situation like the power outage that they were initially purchased to protect against.  So they have limited availability when they are most needed.

On-Line UPS — Online double conversion UPS technology, when faced with a fluctuating voltage scenario, can supply the desired voltage without reverting to battery power.  That means that the batteries stay fully charged and ready for use during an emergency. 

Because the batteries stay charged, they provide full power during power outages.  They also tend to last for the stated service life - driving down operating costs while still providing a stable voltage output.

UPS Batteries - How to Size

Regardless of which type is selected, the UPS must be properly sized to accommodate the protected load.  Ideally, it should have at least 25 percent greater capacity than the total power capacity requirements of all the equipment that will draw from the system. 

To determine the power requirements, check the plates on the underside or back side of the equipment.  Most likely this will list required voltage.  Sometimes the power requirements will be listed in amps - if that is the case, simply multiply the amps by the line voltage (for example, 120V single-phase in the US).

Once you have the power requirements, multiply this figure by 1.25 to determine the capacity required (be sure to factor in anticipated growth which would add equipment to the system). 

Summary - Don't Buy UPS Batteries Solely on Price

There are countless industries and applications which may benefit from the protection that a UPS provides. 

While cost is surely a factor - the initial purchase price advantage enjoyed by line-interactive UPS technology may be offset with reduced equipment life and other downstream equipment costs which may not be readily apparent. 

In short, the lowest initial cost approach may not yield the greatest economy in the long term. 

There are many applications which can suffice very well with the less expensive offline, single-conversion UPS technology (line-interactive variety), but there are also applications which would benefit (operationally and economically) from the use of online, double-conversion UPS technology.

One key to making a good decision is gathering pertinent information and weighing appropriate options.  One size does not fit all.  But a sound decision can provide years of positive returns on the initial investment.