Regarding small, rechargeable appliances—such as cell phones, toothbrushes, cameras, etc: How much energy is consumed when the unit remains plugged into its charger after it has completed charging to 100-percent capacity? Also, does the charger (with no unit attached) consume energy if it remains plugged into the wall outlet?

Asked by Cecilia Lam, '86, Scottsdale, Ariz.


What if I told you there's an appliance in your house that, over the course of a year, could consume more energy while it isn't being used than when it is? The culprit is tucked away in your kitchen, and comes in handy when nuking a frozen dinner, heating up leftovers or illustrating the inherent wastefulness of standby power.

If you guessed your microwave, you're right on target. A quick back-of-the envelope calculation demonstrates this seemingly incongruous fact. Say your microwave uses 1,000 watts when heating up food, which is much more than the seven watts it uses while in standby mode. These seven watts of power run the microwave clock, and also ensure that the unit is always ready to respond to the push of a button. Since power, measured in watts, is a rate, we need to know the time elapsed in order to calculate how much energy is actually used.

Let's say you use your microwave six minutes out of each day—enough time to blast a frozen burrito, say. That means that it consumes 36.5 kWh (kilowatt hours) of electricity annually in its active mode (6 minutes * 1,000 watts * 365 days). The other 23 hours and 54 minutes of each day, your microwave is "off." In this off/standby mode, your microwave still consumes 61 kWh of electricity annually (23 hours, 54 minutes * 7 watts * 365 days). That's enough electricity to microwave more than 600 frozen burritos annually!

These values may seem small, but they really add up. You might recall from the Essential Answer that an estimated 10 percent of residential energy use can be traced back to vampire energy consumption from "off" or idle electronic devices and appliances. The actual utility of this vampire energy runs the gamut from sensible—powering a television to receive remote control signals—to absolutely gratuitous, such as leaving a wall wart charger plugged in all the time.

Unplugging your devices and supporting smarter charging technologies certainly helps reduce vampire energy loads, but it isn't the whole solution. Vampire energy arises not only from portable electronic devices left plugged in 24/7, but also a myriad of other sources that contribute to the tally, including inefficient internal circuit design.

Perhaps you'd like to be more proactive by personally tracking down the culprits most responsible for the vampire energy load in your house. If so, check out electricity usage monitors, like the Kill-A-Watt or Watts Up meters, which allow you to measure the energy used by individual appliances.

More power to those motivated enough to audit their own energy use. Most people, however—myself included—can't be bothered. And even when people do comb through their household appliances and find one that's an egregious energy hog, chances are that they won't just toss it and buy a more energy-efficient one: there may be significant replacement costs or hassle involved. Thus, the real battleground of energy efficiency is in the department stores and online retailers where people first purchase their appliances. If you were in the process of purchasing a fridge or microwave, you'd probably weigh your options, handle the products, ask friends and family and read reviews. Only then would buy what you deem best for yourself and your family. As it turns out, you should also factor in your energy budget: the decision you make can significantly impact your electricity bill years down the road.

This hypothetical scenario reveals a fundamental problem with marketplace incentives. A rational consumer should be concerned with not just the sticker price, but the total lifetime costs of a major purchase like a refrigerator, which includes both the upfront purchase price and the ongoing costs of energy and maintenance. A rational manufacturer, however, seeks to maximize its profit margins by reducing the cost of production. Since you're the one who will be paying the energy bill, the manufacturer has little incentive to spend more money on more energy-efficient components, and might even replace parts with inferior substitutes knowing full well that they'll cost the consumer more in the long run.

This is what makes programs like the U.S. Environmental Protection Agency and Department of Energy's Energy Star rating system a consumer's best friend. By doing the tedious testing work on our behalf and then presenting the results in a clear format—those ubiquitous yellow stickers—Energy Star makes it easy to pick out the most energy-efficient product in its class. These programs are effectively playing the role of the lone environmentalist with a Kill-A-Watt in hand, but on a much grander scale. Which is exactly what's needed in the war against vampire energy.

You probably didn't ask this question and expect to hear about mythical monsters in the reply, but hopefully you now have a better idea how to identify and vanquish all of the energy vampires around you. The good news is, common energy sense is a lot more effective than garlic or a silver stake.


Joshua Chan, '11, is a human biology major.