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I thought I was prepared for power outages. I had flashlights, a collection of topped-up external battery packs for my household devices, and the mental fortitude to eat all the ice cream in the freezer before it melted. Despite this, I was unprepared for a momentary dip in the voltage coursing through my home.

Last summer, during a heatwave, I was editing video without a care in the world when the time came to render and compress 45 minutes of flawless content. I hit the button to begin the process when the lights flickered and my screen went dark. Like Icarus, I had failed to take into account the punishing heat of the sun, and the energy stress of everyone in the area blasting their air conditioning to avoid heat stroke had been too much for the grid to handle. While the brownout’s effects on the rest of my home were limited to a few minutes of dimmer lights, all the work I’d done on my PC was shot. A process that would’ve been done that hour didn’t get finished until the next day. What I needed was an uninterruptible power supply. Chances are you need one too.

What’s that?

An uninterruptible power supply (UPS) is exactly what it sounds like. Essentially a surge protector combined with a battery and an inverter, it will save your electronics from the perils of abrupt power loss or fluctuation. When the power cuts out, the battery kicks in, and the direct current from the battery is inverted to alternating current your electronics can use. This gives you enough time to save your work and turn everything off properly, or gives low-power devices a few extra hours of life. The concept is simple enough, but figuring out the best UPS for your needs can be a challenge.

These power supplies can range in scale dramatically. On the low end, some provide just enough power to keep your modem and router running while you work on your laptop and wait for the power to return. On the high end, one can power the entire city of Fairbanks, Alaska for up to seven minutes. For the purposes of this article, we’ll assume you’re not working on a municipal scale.

There are three basic UPS types: standby, line interactive, and double-conversion. Standby models just switch to battery power as mentioned above, though their output is less consistent. In a standby UPS, power fluctuations above or below the desired range prompt a switch from mains current to the battery, which can take up to 25 milliseconds. These are fine for devices that can withstand a fraction of a second of a blip in their power without risk, such as modems, routers, and power supplies to devices with batteries. They can handle blackouts, brownouts, and power surges without incident.

A line interactive UPS is  best for most people’s needs, using automatic voltage regulation to counteract power fluctuations and provide stable current before switching to the battery. This means that during a brownout or surge, the internal transformer will continue providing power at the proper level before transitioning to the battery without any delay. The increased capability of handling power dips means this is what my computer would have needed to survive that heatwave last summer.

And finally, unless you are running a data center, you probably don’t need a double conversion UPS. These models used to be reserved for massive IT installations, but have since scaled down to the consumer level. Though they’ve become smaller, their prices remain high. That said, a double inversion UPS is the most stable of the three basic options because it never has to switch its power source. The AC goes straight to the battery, and the battery power is converted back to AC, at all times. As its output is always derived from the battery, a power outage will have no effect on connected devices until the UPS battery dies.

So, what do I need?

The average consumer should get a line-interactive UPS. If you’re powering a PC or medical equipment, look for those with pure sine-wave inversion, which provides current indistinguishable from properly functioning household electricity. Some cheaper models use simulated sine-wave output, which will be enough to run simpler devices without incident but will likely cause problems when running a PC or CPAP machine. 

[Related: 4 things to do long before you lose power]

The most important thing to look for in any UPS is its power capacity, as none of the other features matter if there isn’t enough power for the attached devices. A UPS that might keep a modem and router alive for several hours might only have enough juice to keep your PC and monitor on long enough to save and shut down. Check the volt-amps and wattage of the UPS against the power needs of the devices you’ll be plugging into it to ensure it can handle the workload. Volt-amps are not obvious on most devices, but can be estimated with a handy online calculator. Put the device wattage in the first box and 0.6 in the second box, as UPS manufacturer industry standards assume a 60 percent power factor for common personal computer loads. Once you’ve determined the volt-amps, err on the side of caution by getting a UPS that can handle at least 20 percent more. This extra capacity helps avoid potential overload, so as not to burn down your house.

Speaking of burning down the house, there are a few other safety notes to keep in mind: never plug a UPS into a surge protector or extension cord. Never plug a heater or anything with a strong motor into the UPS, as this may overload the device. Read reviews, and make sure the review you’re reading specifies the power capacity of the model in question. Check reviews of any UPS you’re looking at online for people complaining about them bursting into flame because they plugged too much stuff into it, and then do not repeat their mistake. When in doubt, read the manual.

Which uninterruptable power supply should I get?

We’ve run out of space for a full review, but our gear team has you covered for what they consider the best battery backups on the market. But we can provide a bit of a primer. If you don’t need a pure sine wave for sensitive devices, the CyberPower CP1500AVR is a top pick. If you need enough juice for your PC and monitors, the APC 1500 VA Sine Wave UPS is our choice for networks that need plenty of power and true sine-wave output. No matter what you end up getting, take careful notes and compare them against your needs to make sure you’ve got your specs covered. Then ensure you’re still looking at the corresponding model, because for all their wondrous utility, these are all black boxes with deeply unmemorable names.

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