Ask Pablo: Electric Kettle, Stove, or Microwave Oven?

Dear Pablo, For boiling water what is the most efficient, an electric kettle, a pot on a stove, or a microwave?

Whether you are making tea or cooking pasta, knowing which of these three options is the most efficient way to boil water can help you become a better tree hugger and might even save you a bit of money. Through some quick measurements and calculations I hope to provide a definitive answer to this question.

A large mug is around twelve ounces, or 350 ml, so I will use 350 ml of room-temperature water (17° C). I will be using an electric kettle made by Black & Decker, an electric stove made by General Electric with a Circulon two-quart saucepan, and a 900W microwave with a turntable. The electricity use of each will be measured using a Kill-a-watt meter until the water reaches the boiling point, or 100° C.

The Electric Kettle

Electric kettles are designed for their efficiency and many of them have names like Eco Kettle. In electric kettles the water is in direct contact with the heating element, there is no pot to heat and most kettles include an integrated lid. The electric kettle averaged around 1200 watts and took 125 seconds to boil the water, which translates to 0.04 kilowatt-hours (kWh) of electricity consumed. I cleared the cobwebs off of the thermodynamic part of my brain and calculated that the theoretical energy required to heat 350 ml of water by 83° C in 125 seconds is 972 watts. Dividing this by the actual wattage used gives us the overall efficiency of boiling water in an electric kettle, 81 percent.

The Stove

The problem with a stove is twofold; the heat needs to be transferred from the element to the pot, and then the pot needs to warm up before passing that energy to the water. Also, if you don't use a lid, there is a third source of inefficiency in heat loss due to convection. The 6-inch elements on my stove use 1250 watts and boiling 350 ml of water took 318 seconds and consumed 0.11 kWh, almost four times as much as the electric kettle. The theoretical energy required to heat 350 ml of water by 83° C in 318 seconds is 382 watts, giving us an overall efficiency of only 30.5 percent. Already it is quite clear that an electric kettle is far more efficient than the stove, more than twice as efficient. The next time you boil water for cooking pasta you might consider heating the water in the electric kettle first and then adding it to your pot.

The Microwave Oven

Since the water heated by the microwave will be contained by the mug we are not just heating the water, but also the mug to some degree. This will add to the time and energy required to bring the water to a boil but will also help to keep the water hotter longer when compared to boiling water poured into a room-temperature mug. Despite being a 900 watt microwave oven, the actual energy use was 1350 watts. The 900 watts most likely refers to the output of the microwave emitter itself, indicating a 67 percent efficiency just to generate the microwaves. Boiling the same amount of water took 191 seconds and used 0.07 kWh. Using the same calculations as before I determined that the actual efficiency of boiling water in the microwave oven is 47 percent, better than the stove, but still not as good as the electric kettle.

The Conclusion

The clear winner is the electric kettle, at 81 percent efficient, followed by the microwave, at 67 percent efficient, with the stove being the Hummer H2 of the bunch at 30.5 percent efficient. Assuming that you currently use the stove to boil water, switching to an electric kettle for your morning tea will reduce your daily electricity use from 0.11 kWh to 0.04 kWh. Over the course of a year this daily 0.07 kWh savings adds up to 25.5 kWh. Depending on where you live, you could potentially save between $2.50 and $5.00 per year. Of course most of us boil more water than just for making tea. If you extrapolate these savings out to every time you make soup, pasta, home-brew or boil a lobster it can add up.

Additional Considerations When Boiling Water

Regardless of your method for boiling water you can ensure maximum efficiency by boiling only what you need. Use your mug to measure the proper amount or get yourself an Eco Kettle. If you are in an office you might think that filling the electric kettle to the top is the most efficient but think again. Unless the water is going to be used right away a great deal of the energy is going to just end up in the air, where you HVAC system has to remove it. Besides, heating small batches of water is quicker than heating one big batch anyway.