Open Mind

Like a lot of people, I like to make coffee in the morning. I generally brew about two cups, then drink ‘em slowly over the course of several hours. My coffee-maker keeps them nice and hot. But it occurred to me, that maybe it’s more energy-efficient to turn the coffeemaker off after it’s done brewing, then reheat my coffee in the microwave. Which uses more energy?

The calculation is pretty simple, actually. Electrical devices usually inform you of the amount of power they consume. But power is not the same as energy; power is the rate at which energy is consumed.

The SI (international system) unit of energy is the Joule. One Joule (1 J) is the energy required to take 2 kilograms (2 kg, about 4.4 pounds) of mass, and get it moving at a speed of 1 meter per second (1 m/s, about 2.2 mph). The SI unit of power is the Watt. One watt (1 W) is energy consumption at a rate of one Joule every second (1 W = 1 J/s).

If the power consumption is constant, then you can simply take the power level in watts, multiply it by the length of time in seconds, and you’ll get the energy consumption in Joules. To put it in terms of an equation (gasp!), let “P” be the power level, “T” be the length of time, and “E” be the energy. Then

E = P T

According to the fine print on the bottom of my coffeemaker, it runs at about 650 watts. If I leave it on for one hour (3600 seconds), the energy it consumes is 650 W x 3600 s = 2,340,000 J. My microwave runs at 1000 watts; reheating the coffee takes about 30 seconds. So, to reheat my coffee takes 1000 W x 30 s = 30,000 J. It’s usually about an hour between consuming the 1st cup of coffee and the 2nd. So, if I leave the coffeemaker on so my 2nd cup is hot, it costs 2,340,000 J, but if I simply re-heat it in the microwave it only costs 30,000 J. Therefore, leaving the coffeemaker on takes 78 times as much energy as reheating the 2nd cup in the microwave.

Watts and Joules aren’t the only units for power and energy. A common unit for power is the kilowatt (kW); that’s just 1000 watts. So, my microwave consumes power at a level of 1000 watts, or simply 1 kW — they’re the same power, just different units. A common unit for energy is the kilowatt-hour (kW-h); that’s the energy consumed by running at a power level of 1 kW for a period of 1 hour. Your electric power company probably sends you a bill in which the energy is measured in kW-h. Since 1 kW is 1000 W, and 1 hour is 3600 s, we can compute that 1 kW-h is 1000 W x 3600 s = 3,600,000 J. So, 1 kW-h = 3,600,000 J.

If you live in Great Britain, energy is often expressed in units of British Thermal Units (BTU). If you heat water in a laboratory experiment, energy may be expressed in units of calories. If you’re on a diet, the energy content of food is often expressed in calories, but this is not the same as the calorie used in lab experiments; the food calorie is the laboratory kilocalorie, which is 1000 calories. Here are some commonly used units of energy, and their equivalent in Joules:

• 1 kilowatt-hour (kW-h) = 3,600,000 J
• 1 calorie = 4.1868 J
• 1 food calorie = 4186.8 J
• 1 British Thermal Unit (BTU) = 1055.06 J.

Let’s compute the energy required for certain activities. Heating water requires about 1 calorie to heat each gram of water by 1^oC. For a water heater, the tank temperature should be no less than 130 degrees (Fahrenheit) to prevent bacterial growth. If your water heater raises the water from room temperature (about 68^oF, or 20^oC) to 131^oF (55^oC), then you’re increasing its temperature by 55 - 20 = 35^oC. That requires 35 calories for each gram of water. Since a gallon of water is about 3785.4 grams, to heat one gallon of water in your water heater requires about 132,489 calories. Since 1 calorie is 4.1868 J, heating the gallon of water takes just about 555,000 J of energy. If you can reduce your hot water consumption by one gallon, you’ll save 555,000 J of energy.

If you raise the temperature of your water heater by 10^oF (5.56^oC), it requires an additional 88,050 J of energy to heat each gallon of water. If you use 10 gallons of hot water during the day, that’s an additional 880,500 J of energy. And you’ll increase the risk of scalding yourself with the hotter water. If you reduce your water-heater thermostat from 141^oF to 131^oF, you’ll save 880,500 J of energy.

Suppose you leave a device plugged in on “standby,” and it only consumes power at a very low level, say, a mere 10 watts. If it’s left on all day, that’s a time period of 86,400 seconds. The total energy consumed is 10 W x 86,400 s = 864,000 J. If you unplug the device, you’ll save about 864,000 J of energy.

Suppose you replace a 100 W incandescent light bulb with a much more efficient 15 W compact flourescent light bulb. Running the incandescent bulb for 8 hours requires 100 W x 28,800 s = 2,880,000 J of energy. Running the compact flourescent for the same time period uses only 15 x 28,800 = 432,000 J. This saves 2,880,000 - 432,000 = 2,448,000 J of energy.

I started out with the coffeemaker. Suppose yours runs at 650 watts (like mine), but you like your coffee very hot, so to reheat it you’ll run it in the microwave at 1000 watts for 60 seconds. Leaving the coffeemaker on consumes 2,340,000 J of energy, while reheating it uses only 60,000. If you turn the coffeemaker off and reheat your coffee in the microwave, you’ll save 2,340,000 - 60,000 = 2,280,000 J of energy.

There are lots of other opportunities to conserve energy. Not only will this reduce the energy demand for the nation, and therefore reduce the amount of greenhouse gas created (electric power generation is one of the biggest contributors to greenhouse gases), it’ll save you money as well. The power company measures how much energy you use (usually in kW-h), and that’s how they compute your electric bill. So, turn the thermostat down, turn the lights off when not in use, replace those incandescent bulbs, unplug those devices that are on standby. And if your coffee gets cold because you turned off the coffee maker, just zap it in the microwave for 30 to 60 seconds. It’s good for the planet and good for your wallet. How good does it have to get?