Everyday Efficiency: Boiling Water

A Watched Pot: What Is The Most Energy Efficient Way To Boil Water?

Inside Energy, February 23, 2016. Image credit: Unsplash

What is the most energy efficient way to boil 500 milliliters (about 2 cups) of water? And which method has the smallest carbon footprint?

For appliances – microwave, electric tea kettle, electric stovetop – the most efficient appliance is the one that can boil water while pulling in the least amount of electricity via its plug.

Inside Energy spoke with Tom Williams, a researcher at the National Renewable Energy Lab, to break down the rough efficiencies:

  • A microwave is about 50 percent efficient. Most of the energy is lost in the process of converting electricity to microwaves (which are part of the electromagnetic spectrum).
  • An electric stovetop is about 70 percent efficient, although that varies widely depending on the type of pot or kettle you use. Most of the energy is lost heating the air around the stove.
  • An electric teakettle is about 80 percent efficient, although again this varies from kettle to kettle. Electric kettles are generally very well insulated, and the heating coils sit directly in the water, so less heat is lost to the air.
  • An induction stove or hot plate is about 85 percent efficient. It creates an electromagnetic current directly in a pot to generate heat, losing very little to the air.

When it comes to electric appliances, the induction stove is the winner. (However, these are still a rarity in most homes, and they only work with magnetic cookware.) The electric tea kettle is a close second. Not all kitchens – or all kettles –  are the same, so keep in mind these estimates vary.

But what about a gas stove?

A gas stove burns natural gas directly, cutting power plants and electricity out of the picture. Much of that energy goes to heating the air around the flame, and doesn’t make it into the water. Tom Williams, of NREL, says that the efficiency gains you get from burning natural gas directly are offset by the inefficiency of the flame. Thus a gas stove rivals an electric stove in terms of its efficiency, at around 70 percent.

What about that carbon footprint?

To figure out the carbon footprint of our cup of tea, we need to consider the source of the electricity that is powering our appliances. According to the Energy Information Administration, roughly 29 percent of U.S. electricity comes from coal, 34 percent comes from natural gas, 20 percent comes from nuclear, and 16 percent comes from renewables (including hydro). This varies widely state-to-state. Compare Washington, where 69 percent of electricity generated comes from hydropower, to Wyoming, where 89 percent comes from coal. Coal-fired power plants generate between 2.07 and 2.17 pounds of carbon dioxide per kilowatt-hour of electricity produced, natural gas 1.21 pounds, and hydro and renewables, none.

Read More