How to Size a UPS

Sizing Your UPS for Efficiency and Growth

Correctly sizing your UPS solution is the first step to developing an efficient system. The more closely the UPS capacity matches the load, the more efficiently the UPS will operate. However, a UPS with insufficient capacity may be unable to support your equipment during an outage or other power problem.

To find the proper balance, there are two major questions to consider: how much power is required to support connected equipment (capacity), and how long will the UPS need to support equipment during an outage (runtime)?



A UPS system’s capacity rating is the maximum power that the unit can output to support connected equipment. It is typically measured in Volt Amps (VA). To ensure sufficient protection and account for growth, we recommend using a UPS that has a VA capacity rating that is greater than the total power needed for the protected equipment, by at least 15%.



Runtime refers to the amount of time that the UPS will be able to support connected equipment from battery for a given load level. Runtime is important because it tells you how long your equipment will continue to run during a power outage before the UPS battery runs out. You can increase runtime by selecting a UPS with a larger battery, or by adding external battery packs to a UPS that accepts them.


How to Size Your UPS Solution

You’ll need a UPS with enough capacity to handle the full load, as well as a battery system (either internal or with additional external battery packs) that can support that load for long enough to restore power or safely shut down your systems. Expected growth and redundancy requirements should also be considered. We’ll go through the calculation below:

  1. Make a list of all the equipment that will be plugged into the UPS.
  2. Read the nameplate on each piece of equipment and write down the voltage and amperage. For each piece of equipment, multiply the voltage and the amperage to get the VA rating.
    Note: If your equipment is rated in watts, simply convert it to VA by dividing the wattage rating by the equipment’s power factor, which is typically 1.0 for blade servers and other networking equipment
  3. Add all the VA ratings together to get the total VA requirement for the equipment plugged into the UPS.
  4. To account for growth, you will need a UPS with greater VA capacity than the total VA requirement from step 3. SurgePro recommends assuming at least 15% growth over a five year period, though your actual plans for growth may require a larger allowance.
  5. Multiply the initial VA requirement by 1.15 (i.e., 1.0 to account for the initial VA requirement, and .15 to account for 15% growth).
    Note: Adjust this calculation for other rates of growth by changing the digits after the decimal (e.g., multiply by 1.1 for 10% growth, multiply by 1.25 for 25% growth).

Now That You Know Your Equipment, Find the Right UPS


Sizing for Redundancy

Mission-critical loads demand an extra level of protection in case of human error or other failure. Sizing your UPS solution for redundancy can provide that extra level of protection. Redundancy can be achieved by connecting multiple UPS systems in parallel (or a modular UPS system) to a common critical load bus (e.g., an ATS PDU).

The most common redundancy architecture is N+1, where N refers to the number of units (or modules) required to support the load. Thus N+1 configurations include one more unit (+1) than the load requires. This means that if any one unit fails, the remainder can continue to support the critical load without interruption.


Sizing for 3-Phase

Sizing a solution based on 3-phase utility power can be tricky. Contact one of our Surgologists for sizing help.

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