I've been taking the Lantern's advice and eating at home more often than going out. Now I'm wondering how I should be preparing that home cooking. What's the greener choice for heating up my meals—the microwave or the oven?
Your foodie friends may cringe at the thought, but cash-strapped grad students and single urbanites have had it right all along: As a general rule, microwaves are more eco-friendly than conventional ovens. However, that doesn't mean you should consign yourself to a life of gummy casseroles and dry chicken just to save the planet.
First off, a little perspective. According to the Department of Energy, cooking accounts for a little less than 3 percent of an average home's energy use (PDF). So any changes you make aren't going to add up to much—you'll have a much bigger impact by looking to your light fixtures or your thermostat. On the other hand, it is fairly easy to cut energy consumption in the kitchen, so why wouldn't you?
As a general rule, any oven powered by electricity—whether or not it's a microwave—is going to cook food more efficiently than a gas-powered appliance. About 12 percent to 14 percent of the energy drawn by a standard electric oven goes toward cooking your food. With a gas-powered oven, that number drops to a little less than 6 percent. (Microwaves handily beat both, directing about 60 percent of their energy toward cooking.) Still, when compared with conventional electric ovens, gas-powered ovens are usually considered more efficient overall, since there are significant energy losses associated with generating electricity and then transmitting it to your home. So if you have a gas oven, use it with the Lantern's blessing. *
That's not an option for the majority of American households, though. About 60 percent of our conventional ovens are powered by electricity, not gas—and they're less efficient than standard microwaves. In the first place, conventional ovens operate at a higher wattage—about 3,000, compared with something between 600 and 1,650 for a microwave. They also cost us energy by cooking food more slowly. One University of Bristol study found that a chicken cooked in a convection microwave resulted in energy savings of 30 percent over a conventional electric oven.
You'll see the greatest energy savings when cooking up small portions. One Swedish study, for example, found that cooking a single portion of baked potatoes in an oven took 9.5 times as much energy as it did in a microwave. When that was increased to four portions, however, the oven used only 2.5 times as much energy per serving (PDF). When the meals get big enough—a pot roast plus vegetables, for example—you're better off skipping the microwave altogether.
Microwaves do have a few other environmental advantages. For one thing, they produce a lot less indoor air pollution than other cooking methods. Plus, they don't heat up your house the way an oven can, which means lower energy costs associated with both your A/C and your refrigerator. Heating up a meal on the plate you intend to eat off of also means fewer dishes to wash—although regular use of your microwave might encourage higher consumption of ready-to-eat convenience meals and all their extra packaging.
Whichever cooking method you choose, there are plenty of little things you can do to save energy. With a conventional oven, avoid preheating and turn the heat off a few minutes before your food is done. If you switch from metal pans to heat-retaining glass or ceramic ones, you'll be able to turn your oven down by 25 degrees. Microwave ovens can be made more efficient by keeping them clean—the appliance can't tell the difference between your dinner and old cheese splatters, so it'll expend energy heating up both. Also remember to keep the appliance unplugged when you're not using it. (Over its lifetime, the average microwave will use as much energy in standby mode as it will in actually cooking your food.)
In the end, though, the greenest way to cook your food is the one that produces the tastiest meal. As we've discussed here before, food waste is an important environmental consideration: Not only does it mean more garbage, it also means squandering all the resources that went into growing, storing, and preparing that food. Let's say you roasted your Sunday-night chicken dinners in the microwave rather than the oven for a whole year. Over 52 weeks you'd save 15.6 kilowatt-hours—or about 0.14 percent of your home's annual energy use. If saving that teeny bit of energy means you'd be more likely to toss your soggy leftovers, then by all means, fire up the big boy and cook that chicken right.
Is there an environmental quandary that's been keeping you up at night? Send it to firstname.lastname@example.org, and check this space every Tuesday.
Correction, March 27, 2009: The original version provided the wrong context for understanding why gas ovens might be greener than electric ovens. It failed to mention that the delivery of electricity to the home is relatively inefficient. (Return to the corrected paragraph.)