5G, 5G, 5G! Carriers and smartphone makers can’t stop talking about how fast the fifth-generation mobile network is. Is the much-hyped technology worth a device upgrade? 5G is fast—when you can find it—but as my week of battery tests show, it’s a power hog, and there are no killer apps requiring the high data speeds just yet.

Carriers over the past year are offering aggressive trade-in deals—as much as $1,000 off—to get people to upgrade to the latest 5G models, after spending billions of dollars setting up the new mobile network. It’s working: According to Counterpoint Research, global sales of 5G smartphones surpassed 4G handsets for the first time in January.

Earlier this month, Apple added 5G to two more of its products, the entry-level iPhone SE and new iPad Air. The entire iPhone 12 and iPhone 13 lineup are 5G-ready, as are select models of the latest iPad Mini and iPad Pro. (Android phones, too, have long had 5G capability, though this review is focused on Apple.)

Your next phone or tablet will likely be 5G-capable, if your current one isn’t already. All unlimited data plans through AT&T, T-Mobile and Verizon include access to the network. 

As you can expect, there’s a trade-off between blazing speeds and conserving energy. When to use 5G will mostly likely depend on what you’re up to. Whether you’re new to 5G or just wondering what it’s doing to your phone, here’s what you need to know about how the mobile network will—and won’t—change your iPhone or iPad experience.

5G vs. 4G LTE

The carriers weren’t kidding about the speed: I recently downloaded a nearly 10-hour Spotify playlist in less than a minute over 5G. In one extreme example, my colleague Joanna Stern experienced a download speed of 1,300 megabits per second on Verizon’s 5G network in Jersey City, N.J. (Compare that to your own cellular and home Wi-Fi speeds using fast.com.)

To achieve that speed, Joanna was parked right underneath the cell tower that emits the fastest type of 5G, called millimeter wave, favored by Verizon. This superfast flavor requires you to be within a few hundred feet of the cell site to get top speeds, and trees, buildings and other obstacles can slow it down. Plus, Jersey City has the fastest median download and upload speed of any city in the U.S., according to broadband-metrics company Ookla.

Apple’s new third-generation iPhone SE supports 5G, but not the fastest flavor, called millimeter wave.

Photo: Nicole Nguyen/The Wall Street Journal

My city, San Francisco, isn’t even in the top 30. Walking around different neighborhoods and using Ookla’s Speedtest app to test Verizon, I saw 5G speeds ranging from 160 to 361 mbps for downloads. By comparison, my download speeds on the 4G network—generally referred to as LTE—ranged from 55 to 140 mbps. (For comparison, the national average internet speed in 2021 was 99.3 mbps, according to HighSpeedInternet.com.)

Scrolling through Instagram and Twitter, I didn’t notice a difference between 5G and 4G. I saw the most variation when downloading and uploading media. Over T-Mobile’s LTE network, a 30-second Cinematic mode video took 3.4 seconds to download from iCloud Drive. Over the carrier’s 5G, the download was so instantaneous, I didn’t have time to hit start/stop on my stopwatch.

(I didn’t have a chance to test AT&T’s network, but results would likely be consistent, as Apple uses the same 5G modems no matter the network.)

The Battery Test

While Apple offers a cellular battery-life rating for iPads, the company doesn’t share the same on its iPhones. An Apple spokesman said there are different tests across products since each device has a different function. 

So, I wanted to compare 5G and 4G battery performance for myself. I streamed a long YouTube video of relaxing ocean footage, with video quality set to “Auto,” on different Apple devices until they ran out of battery, first on 5G, then on LTE. It isn’t a perfect test, but it proved to be a consistent way to witness 5G’s added battery drain.

That’s why the most important takeaway from this experiment is the difference in battery performance over the two types of networks: T-Mobile 4G and 5G for the iPhone SE and 13 Pro; Verizon 4G and 5G for the iPhone 13 Mini and new iPad Air. Just note, the time a device lasts doesn’t say much on its own, since this isn’t real-life behavior (unless watching 10-hour oceanscapes while out on the town is your thing).

As expected, 5G was more of a drain on the battery than LTE in every case. (During my testing, I didn’t experience the extreme speeds associated with millimeter-wave 5G, which probably would have drained the battery even more.)

The new SE lasted nearly an hour longer on 4G than on 5G, while the new iPad Air and iPhone 13 Mini went for about 1.5 additional hours. And while the iPhone 13 Pro ran a remarkable 12 hours and 50 minutes on 5G, it still lasted about 2.5 hours longer on LTE.

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A YouTube spokeswoman said the “Auto” quality setting looks at many factors, including connection type and screen size, adding that the app might play higher resolution on 5G compared with 4G, but there are many things that can influence power consumption. She said the company is “always looking for ways to optimize battery consumption,” and made some improvements for streaming over 5G. 

Durga Malladi,

head of 5G at Qualcomm, which makes modems for Apple and other phone makers, explained that when you use an application on 5G, data can flow at a faster rate than what you’d see with a 4G connection. That’s a good thing, but it can drain your phone’s battery more quickly. This is also why millimeter wave is potentially even more of a battery hog than other frequencies. (Of Apple’s 5G phones in the U.S., only the new SE doesn’t have millimeter-wave support).

It’s hard to tell which exact flavor of 5G you’re connected to, though. The phone’s status bar could say 5G, which just means the carrier’s network is available. If you see 5G+, 5G UW or 5G UC, that can refer to both regular midband and superfast millimeter-wave networks. It’s like going to a liquor store where every bottle is unlabeled, and you have no idea what type of beverage you’re getting—or how potent. 

If you use your phone or tablet a lot away from home, a fast 5G signal means you don’t have to worry about connecting to a potentially risky public Wi-Fi network. It also enables you to do more data-intensive activities, such as playing a multiplayer online game like Minecraft, or live-streaming different angles of a game from a packed millimeter-wave-equipped football stadium. The downsides? Quicker battery drain, and potentially patchy coverage.

Enabling the 5G Auto setting activates Smart Data Mode, which switches the device’s connection to 4G for less data-intensive tasks to conserve energy.

Photo: Nicole Nguyen/The Wall Street Journal

Manage Your Battery 

If you want your iPhone battery to last longer, you can make it use 5G less. Go to Settings > Cellular > Voice & Data and set connectivity to 5G Auto. (It may also be under Cellular Data Options, depending on your carrier.) This activates Smart Data Mode, a setting that will dynamically switch your connection between 5G and LTE to preserve power.

The feature will rely on 5G to, for example, download a large movie, but when it’s doing background tasks, such as checking for email, the phone will drop back to LTE to save battery.

Your iPhone can also automatically switch to LTE once it hits a certain battery level. Open the Shortcuts app, and go to the Automation tab to create a Personal Automation. Select Battery Level and set the desired percentage. On the next page, tap Add Action and search for Set Voice & Data. Set Voice & Data mode to LTE.

The simplest option is to just turn off 5G until you need it. I set my cellular connection to LTE because I want to squeeze every extra minute out of my iPhone 13 Mini’s relatively small battery. But it’s there when, for instance, I’m panic-downloading a 10-hour oceanscape at the airport so I can relax on a long flight.

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Write to Nicole Nguyen at nicole.nguyen@wsj.com