Nowadays, UPS plays a very important role in keeping the critical power supply running continuously in the event of a power outage.However, it requires professional knowledge to select the right UPS size for your application, otherwise it will cause overload or waste.

Under-size UPS 

If you under-size uninterruptible power supply (UPS) systems, the data center will overload the available power.  Then the UPS will shut down and go into bypass mode. That is, it affords no protection from a power failure or brown-out.

Oversizing UPS 

Oversizing the UPS results in extra pay for unneeded power and cooling cost because a UPS at 40% load or less, the efficiency will drop quickly.

Then how to pick the right UPS for your applications? Because UPS systems are often sized at kVA (kilo-volt-amperes) rating. For example, a 1 kVA UPS means its circuitry can handle 1,000 VA(volt-amperes), you need to get the grand total VA rating of your devices which need to be protected?

Here are several steps to assist in sizing the right UPS:

1: Figure out how many equipments need to be protected;

2: List the amps and volts for each piece of equipment, multiply the voltage and the amperage to get the VA rating. (If your devices' power consumption is stated in Watts (W), please divide the watts by power factor. For servers, the power factor is often 0.9.);

3:add all the VA ratings together to get the total VA requirement;

4:Multiply the total by 1.2 for future growth and system expansion and you will get the gran total. That figure is the maximum size in VA or kVA that your UPS should be.  (Remember not to run a UPS for 100% load, it is not stable. )

Above is a commonly used method to calculate the size of the UPS system. But if your loads have an inrush of current during start-up or draw higher currents in normal operation (i.e. laser printers, blade servers, air conditioners, certain lighting systems, motors and compressors), you need to be careful and ask your ups supplier for help.