The Best Way to Boil Water: Nitty-Gritty

August 31, 2011

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U.S Energy Information Administration, Annual Energy Review 2009 (August 2010)

Q: What is the best way to boil water in a modern home? Is it better to use the microwave, an electric kettle, a gas stove or an electric stove? Also, Is there any truth to the notion that once water has been boiled it shouldn't be boiled again—thereby wasting it?

Asked by Erin Craig,'84, Palo Alto, Calif.

Boiling basics

Boiling water may be just about the simplest of all kitchen tasks, and unless you're running a teahouse, it's not much more than an afterthought in your total energy use. Still, many strands of energy usage are tied up in the deceptively simple question of how best to boil water. The answer starts with where your energy comes from.

Electricity can be generated by burning coal or natural gas, from nuclear reactions or hydroelectric dams, or from emerging renewable energy sources such as wind and solar power. Coal is currently the cheapest and most widespread means of generating electricity in the United States, accounting for 45 percent of the country's total electricity production, according to the U.S. Department of Energy. It is also the most polluting, generating 93 percent of the sulfur dioxide and 80 percent of the nitrogen oxide emissions generated by the electric utility industry. Natural gas is next most common, accounting for a further 23 percent of the U.S. electricity supply. While considerably less polluting than coal, natural gas is still a major source of greenhouse gas emissions.

To reboil or not to reboil

The warning against reboiling water probably goes back to days when water sources were more suspect. Boiling water kills potentially dangerous microbes and parasites, but it can also concentrate inorganic poisons. If you're using clean water, you can reboil it as many times as you want—it won't affect the taste or your health. However, if your water comes from a well, all bets are off. Some ground water contains nitrates, found in fertilizer, and arsenic, which may occur naturally or as runoff from farms and industry. Boiling water increases the concentration of those contaminants. The more times you reboil the water, the higher the concentration and the higher the risk. Most city water does not have sufficient amounts of nitrates to cause health risks, and the City of Palo Alto passed all state and federal drinking water health standards.

But if you're thinking of reboiling water, this implies that like many people, including yours truly, you often boil more water than you need. As trivial as this "over-boiling" seems, the energy wasted can add up quickly: According to the U.K.'s Energy Saving Trust, over-boiling in the United Kingdom wastes enough energy to power about three quarters of the nation's streetlights.

English company EcoKettle resolves the problem, or laziness, of boiling unneeded water with a small technological fix. You fill the electric kettle, which looks like a Brita filter jug, to its maximum, then use a lever to choose how many cups of water you want boiled. The EcoKettle heats only that amount.

Ranking your boiling choices

Here's how different boiling methods rank in energy efficiency, according to Jonas Ketterle:

  1. An electric kettle or induction cooktop powered by renewable energy (solar panels on the roof)
  2. An electric stove powered by renewable energy (solar panels on the roof)
  3. A gas stove
  4. An electric kettle with grid electricity
  5. An electric stove with grid electricity
  6. A microwave

A gas stove instantly provides a hot flame, and the heat source vanishes as soon as the knob is turned off. When the kettle whistles and you turn off the electric stove, it is still unnecessarily heating up the air above the hot plate. An electric stove requires more energy and time to heat up, thus it takes longer to heat the water inside the kettle. However, an electric stove is more efficient in delivering energy to the kettle. A gas burner delivers only about 35 percent of its heat energy to the pot, whereas an electric stove conveys about 70 percent, according to the U.S. Department of Energy. But, the electric stove's efficiency in the home is offset by the inefficiency in electricity production at the plant.

Different types of gas and electric stoves consume varying amounts of energy. A gas stove either has an electric ignition or a standing pilot (the flame is always on). Not surprisingly, a standing pilot more than doubles energy consumption, according to the American Council for an Energy-Efficient Economy. An electric stove usually has either a solid disk or an exposed coil. The disk creates better surface contact with the pot or kettle, and thus heats it more effectively. It's also important to clean your kettle. Using boiling water and vinegar removes mineral deposits that reduce its energy efficiency.

The easiest thing you can do to limit your impact on the environment is to measure the exact amount of water you need to boil for your tea. This will also protect you from any adverse effects of reboiling water, should your water supply be unsafe. And that's so easy, you should have plenty of (personal) energy left over to tackle a few more energy tasks.

DAVID CARINI earned his master's in communication in 2010.

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