How to Size a UPS

So you need a UPS, but are unable to determine what size you need? We’re here to help you make an informed decision.

Before we started, let’s go over frequently used UPS terminology:

VA – Or volt-amperes is simply the amount of apparent power in an electrical circuit. Most UPSs are marketed by their VA rating or apparent power they have.

Watts – This is the amount of real power that a device draws. Besides a VA rating, a UPS will also have a wattage rating that states the amount of real power it delivers to your critical IT load.

Power Factor(.pf) – This is the ratio of apparent power to real power, expressed as a decimal. For instance, a UPS with a VA rating of 3000VA /2700W has a power factor of .90.

To get an accurate measurement of your IT load, we recommend that you use Watts in your calculations, since this represents the amount of real power that your device uses. See the video below for more on VA vs. Watts:

The first step in calculating your total power requirement is:

1. Add up the nameplate ratings of all the devices that you wish to protect with the UPS.

The nameplate rating of your IT device is usually found on the back panel.

Alternatively, you can check your IT manufacturer's manual for more information about the wattage rating.

2. Divide the total wattage by 1.7 to estimate real world load.

Manufacturers must ensure safety compliance and issue nameplate ratings that are usually well above the actual load. In real world applications, nameplate wattage ratings can be nearly twice the amount of actual wattage that your device uses. Using 1.7 is still an estimate, however. Determining the correct wattage is important in preventing over or under-sizing the UPS.

3. Determine runtime requirements.

Once the IT load has been decided, the next factor to determine is the runtime requirements. For backup power applications only requiring a few minutes of runtime, the internal batteries of a UPS are often sufficent. However, where increased runtime is required, additional battery modules must be added to the solution. These additional battery modules are manufacturer and model specific. Contact one of our representatives for assistance in determining your specific runtime requirements.

Future Growth Requirements

Even if you can precisely determine your current power needs, no one should size their UPS exactly to the capacity of the UPS. The power demands of servers, which usually make up most of the IT load, can fluctuate depending on the amount of processor load. Over-sizing the UPS by 15%-20% is a good rule of thumb to allow additional IT devices to be added in the future.

This is not an exhaustive list of elements to look for in a UPS. Other factors, such as the receptacle configuration, available power distribution, and voltage requirements are some additional important factors. For an individual assessment of your power or cooling requirements, please contact one of our representatives.