How to Size an Uninterruptible Power Supply

Figure out how much battery backup you need

What to Know

  • To size your needs: Total watts of your equipment x their total amperage and add 15% of that total to get your total need.
  • The difference in UPS capacity compared to its load can increase runtime if it’s significant enough.

This article explains how to determine the right uninterruptible power supply size to fit your needs.

How Big Should My Uninterruptible Power Supply Be?

The three major factors to consider when setting up a UPS are the intended load (i.e. the combined voltage and amperage of all connected electronics), the capacity (i.e. maximum power output), and the runtime (i.e. how long it can supply battery power for). A UPS is most efficient when the capacity closely matches the overall load without falling below the necessary levels. In other words, you’ll want to avoid using a UPS with a lower capacity than the intended load.

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The necessary runtime for your UPS depends on how long you want or need your connected devices to continue to operate during an outage. Larger batteries or external battery packs (if the UPS supports them) will increase runtime. More devices and more power draw decreases runtime.

  1. Figure out all of the electronic devices you’ll be connecting to the UPS.

  2. Determine both the voltage and the amperage for each device. This should be included in the device’s documentation, or listed on the device’s nameplate.

  3. Multiply the voltage by the amperage to calculate each device’s volt-amp (VA) rating, then add all of the VA ratings together to determine the total VA necessary fort your UPS.

  4. It is recommended to exceed your VA totals by around 10% to 15% (or more) to both provide a bit of a protective buffer and to account for growth over time.

  5. To account for this buffer/future growth, multiply your VA total by 1.15 for 15%, 1.20 for 20%, and so on.

  6. Compare your VA total (preferably with at least a 15% to 20% adjustment) to the UPS output/VA rating to find one that’s a good fit.

For example, if you plan to hook up a single M1 Mac mini (2020), which uses a maximum of 39 watts and around five amps if it’s used all day, the VA rating would be 195 (39 x 5). Accounting for 15% growth, the VA total would then become 224.25 (195 x 1.15).

How Do I Know My UPS Capacity?

The capacity of your UPS is its maximum power output (a.k.a VA rating). When shopping for a UPS, the VA rating should be listed along with the type (Standby, Line-Interactive, or On-Line), input voltage, etc on the box or product page. If you already have a UPS and want to check, the capacity should be in the documentation that came with the UPS and displayed somewhere on the UPS casing itself (likely on one side, the back, or the underside).

Misc UPS ratings

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How Long Will a 300W UPS Last?

The available runtime for a 300 watt UPS largely depends on the make and model itself, as well as how close to capacity the connected load is. Some UPS will last around three minutes with a full load, some up to five minutes, and so on. If the UPS is operating at a less than full load it will likely be able to provide backup power for a little longer than it would otherwise. Some UPS may also list their full load runtime, though there is a way to calculate it for yourself if necessary.

How Do You Calculate How Long a UPS Will Last?

The commonly-used formula for calculating the runtime of a UPS is fairly straightforward, but you have to know a few additional values first.

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  1. Find the capacity of the UPS battery in ampere hours (Ah). This information should be included with the UPS documentation, on the listing page, or on the battery itself.

  2. Find the input voltage of the UPS.

  3. Determine the total load for the UPS in watts by adding up the watts used by all of the connected devices.

  4. Multiply the battery capacity by the input voltage, then divide that number by the total load.

For example, a UPS with a battery capacity of 150Ah, 10V of input voltage, and a load of 700 watts should handle a runtime of around two minutes (150 x 10, divided by 700).

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