How to Know Which Uninterruptible Power Supply to Purchase

An uninterruptable power supply is a safety backup device that monitors incoming electric activates its own battery pack to supply power in case of an external power interruption. Computers, SCADA systems and critical medical or production equipment protected by the UPS will not detect interruption in power supply and will continue to run during a temporary power dip or brown-out.

Difficulty: Moderately Easy

Instructions

1. Determine the worst-case demand in power and time that you will need to have covered. Multiply the voltage rating of the equipment being protected times the equipment's usage at its heaviest current demand. This will give you the worst-case demand in volt-amps (VA). Most UPS systems will be rated by VA or KVA, thousands of volt-amps. The cost of a UPS system will be determined largely by the worst-case demand in volt-amps and time. A realistic estimate of how long the UPS will need to supply the KVA demand will determine the UPS capacity. For power dips and short brownouts, a 1/2-hour capacity should be more than adequate. Avoid optimism when setting your requirements. Having a UPS not up to demand is the same as having no UPS, as the system will shut down due to lack of power.

2. Contact a UPS vendor with your KVA and time requirements. For a PC that would need only protection from transient power dips, a UPS will cost $30 to $50. For a SCADA system or medical test unit that could face a 45-minute interruption, you will need to spend $20,000 or more on a UPS.

3. Choose a UPS from an accessible vendor who could service or upgrade your unit on short notice.

4. Choose a UPS in which you or your maintenance group are able to replace the batteries. Even the best batteries have a predictable service lifetime.

References

• UPS FAQs
• UPS overview
• Sample of a range of UPSs