What Size Uninterruptible Power Supply Do I Need?

What Size Uninterruptible Power Supply Do I Need?

by Isaac Prakash March 25, 2023

Now that you’ve established you need an uninterruptible power supply, how do you figure out what size you need? With so much variation on the market, it can feel woefully overwhelming. Read on and we’ll help you cut through the noise.


Why choosing the right size UPS is important

The long and short of it is, you need to ensure that your UPS runs properly. If it isn’t large enough to support the equipment you have plugged into it, you’re going to have a bad time.


Factors to consider when determining UPS size

Here are the most important factors to consider when determining the appropriate size for your UPS…


Total power consumption of connected devices

Naturally, your UPS system must have a large enough capacity to easily accommodate all your connected devices and their respective power consumption.


Power factor and efficiency

You must also think carefully about how efficiently your UPS system will be able to run in the event of an emergency. It’s always worth allowing a buffer for optimal efficiency.


Overhead and future expansion

And indeed, you must allow yourself some room to breathe. Buying and installing a UPS system that accommodates your exact current requirements (rather than allowing room for growth) can stifle your operation. Factor in future expansion before settling on a UPS.


Calculating UPS size requirements

So, how do you go about calculating the size requirements for your UPS system?


Step-by-step guide to calculating power consumption of devices


  • Determine exactly how many electronic devices you will be connecting to your UPS.
  • Tally the voltage and amperage for each individual device.
  • Multiply each device’s voltage and amperage (volt-amp – aka VA), then add all of the VA ratings together.
  • Your UPS should exceed your VA totals by at least 10 to 25% to allow a buffer / account for future growth (e.g., multiplying your VA total by 1.15 gives a 15% buffer, etc.).
  • Finally, compare your VA total and your 10-25% buffer to your prospective UPS systems’ output/VA rating in order to find an appropriate fit.

    Example calculations for common devices (e.g. desktop computer, server, network equipment, home entertainment system)


    • A typical desktop computer uses between 30 and 70 watts of electricity at between 3 and 5-amps. Let’s say 45 watts by 4 amps if the PC is running all day. This will give it a rating of 180 (45 x 4). Following that, to calculate a suitable buffer (say 15%), the total would come to 207 (180 x 1.15).
    •  A Mac Mini might come in at 39 x 5 = 195 (224.25 accounting for 15% growth).


    Choosing the right UPS size

    Here’s how to choose the right UPS size for your requirements…


    Matching calculated power consumption with UPS power capacity

    Using the simple formula listed above, you can easily calculate how much power you need, and then compare it with some prospective UPS systems and their maximum power capacity.


    Considerations for runtime and battery backup

    Determining how much runtime and battery backup you have available is critical. Think carefully about how much time you will need to make the necessary arrangements in the event of an emergency. For example, do you only need a few minutes to save all your work and safely power your systems down? Or perhaps 10 to 15-minutes would be an ideal amount of time to get your backup generator up and running?


    Other factors to consider (e.g. outlet types, form factor, price)

    How much space are you able to sacrifice for your UPS system? The size and nature of your environment will determine what kind of UPS system you need (e.g., a tower, rack / wall-mounted or ruggedized model).

    In the same breath, you also need to be mindful of your budget. You don’t want to break the bank installing your UPS, however, you also need to ensure that you are investing enough money in a UPS system that can easily accommodate your needs. Find the right balance.



    Below are some of the more frequently asked questions. Can’t find the info you need? Simply contact us today and we’ll happily assist you.


    What happens if I choose a UPS with a lower power capacity than my devices require?

    Simply put, your UPS will be unable to provide the required protection and it will shut down immediately due to an overload.


    Can I connect more devices to a UPS than its power capacity allows?

    No. You must use less or equal to the UPS systems rated and recommended capacity.


    What if I need more power capacity than a single UPS can provide?

    You can have multiple UPS installed, depending on your requirements. It may be worth looking into having a backup generator installed that will provide enough power, and then looking for a UPS system that can provide enough juice until you can power up the backup generator accordingly.


    Do I need to consider power factors and efficiency when choosing a UPS size?

    Yes. Power factor and efficiency are critical factors that will determine the size and quality of your UPS system.

    Isaac Prakash
    Isaac Prakash


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