Foremost is that efficiency largely depends on your energy source. By the time electricity from fossil-fuel-powered plants has reached your home, Dr. Murphy notes, it’s already lost about 60 percent of its energy (some estimates have that number higher). That scenario, he said, could drastically hinder the performance electric stovetops and microwaves.

Kettles suffer from the same electricity issue — but also come with additional drawbacks that Dr. Murphy says can cause their efficiency to vary widely. The auto-stop function, for instance, often runs much longer than is necessary, which wastes energy. And people tend to overfill kettles. “Habitually, you’re just heating a lot more water than you need to be,” he said. “In practice, you’re not likely getting any better than a microwave.”

To get the most out of your kettle, Dr. Murphy says you should only boil as much water as you need and monitor the boiling point, so you can manually shut it off.

Dr. Murphy also looked into starting with hot water from your tap as a way to improve results (the myth that cold water boils faster has been thoroughly debunked). He found that it can probably help, because hot water systems are relatively efficient. But because a certain amount of the hot water will inevitably get left behind in your pipes, unused, the benefits of starting hot are greater when you’re boiling large pots of water.

Dr. Murphy didn’t test methods such as induction cooktops, which others have since found to be quite efficient. Technology is always changing (scientists in Oregon, for example, have developed new surfaces that could improve boiling efficiency) but he doesn’t see any breakthroughs on the horizon that will make boiling water miraculously less energy intensive. The biggest thing individuals can do, he said, is be vigilant about our own habits.

“You just have to care about it,” he said. “You have to pay attention.”