Although the net energy required to heat a given amount of water to a certain temperature doesn’t change (regardless of how the water is heated), the quantity of energy actually consumed will depend on how much energy is lost in the process.

Keeping that in mind, the electric kettle will consume the least amount of energy, because the kettle’s heating element sits directly in the water. Electric kettles have efficiency ratings of about 90 percent, and use roughly 50 watts to boil 2 cups of water.

Gas stove-top burners, on the other hand, are the least energy-efficient option, because in addition to heating the water, energy goes into heating the teapot, other nearby objects, and the surrounding air. Given the apparent prevalence of escaping heat, energy-efficiency ratings for gas burners are estimated to be around 55 percent. Electric ranges with heating coils are slightly more energy-efficient (65 percent). However, due to higher costs and the emissions associated with electricity generation, electric ranges may actually be less sustainable than gas ones.

The actual amount of energy needed to heat a given quantity of water on a stove top will depend on the starting temperature of the water, the type of pot used, and the heat setting. That said, a stove-top teapot can use three times more energy—and take five times longer—than an electric kettle to brew the same two cups of tea.

A fourth option, although one never recommended by tea connoisseurs, is to heat your water in a microwave oven using the beverage setting or a temperature probe. (Heating water using the microwave's timer can, in rare circumstances, result in "superheating," which may cause your teacup to explode or result in severe burns to your skin.) Microwaves may use slightly less energy to heat water than electric kettles, but they aren’t necessarily an optimal choice and they certainly aren’t the safest for this particular purpose.

Regardless of your preferred method of tea brewing, here are a couple of tips to remember:

water you’re heating. You can save energy by heating only the amount of water you’ll need. teapot. A burner that’s too large will result in wasted energy, while a burner that’s too small will require a longer heating time.