Debunkifying Energy Myths

What everybody knows about energy and saving money isn’t always true. Here are some common truths that fall into that category.


Myth: Leaving a light on uses less energy than turning it off and on several times.

Truth: Leaving incandescent lights on uses more energy than turning them on and off as needed. But if you're using a compact fluorescent light, it should be left on if it will be needed again within 15 minutes because quickly turning them on and off shortens bulb life.

Myth: Keeping a thermostat at the same temperature uses less energy than turning it down at night and then up again in the morning.

Truth: It takes less energy to warm up a cold home in the morning than it does to maintain a constant temperature throughout the night.

Myth: The higher you set the thermostat, the faster your home warms up.

Truth: It will take the same amount of time for the temperature to reach 70°F whether the thermostat is set at 70° or 90°F. Setting the thermostat all the way up wastes energy and increases heating costs.

Myth: It take less energy to boil water if you start with hot water from the tap.

Truth: You essentially use the same amount of energy whether you use hot or cold water. Using hot water just means you've already paid to preheat the water.

Myth: It is more energy efficient to leave your computer running when not in use.

Truth: Any time you turn off your computer, it saves energy. However, turning it off and on several times a day may shorten its life. Many computers now have sleep modes and other features that save energy when they are idle.

Myth: Cold water will freeze into ice cubes faster than hot water.

Truth: Actually, hot water freezes faster than cold water because it evaporates, leaving up to 25% less water to freeze.
-from Portland General Electric


Myth No. 1: The Clean Air-Filter Swap: Dirty air cleaners increase mpg if they impede air flow.

The Truth: Back in the days of carburetors, changing an air filter could help mileage. But today, a computer controls how much gas your engine gets and knows exactly how much it needs. A small restriction in air flow won't make engines run rich. In fact, today's air filters are relatively large and probably won't affect mileage until they're plugged so thoroughly they trigger the Check Engine light.

Myth No. 2: Myth No. 7: Short-Stop Idling: Quit turning the engine on and off. Just keep the engine idling while you run quick errands.

The Truth: Cars mileage inherently goes down while idling. And once an engine is warm, it uses no fuel to shut down or restart. That's why the engine in a hybrid turns off when the vehicle isn't moving.

Myth No. 3: Fill your tank in the morning: Top off your tank when the weather is cool. The gas is denser and has more energy. The Truth: Cooler gas might be denser and, technically, provide more energy per gallon. But modern cars correct the gas/air ratio so the proportion of air-to-fuel is chemically correct. And although we buy gas by volume and get more, there's little variation in the temperature of gas in underground tanks over the course of a day. Besides, like our modern cars, modern fuel pumps are temperature-compensated to eliminate density difference.

Myth No. 4: Turn off the air conditioner: Turn off the air conditioner, open your windows, and you'll get better mileage.

The Truth: Every car's aerodynamic drag is proportional to the square of its speed. But fuel consumption is proportional to the cube of speed. At low speeds, the six to nine horsepower needed to run the air conditioner will measurably hurt fuel economy and the drag from open windows is negligible. At freeway speeds, the A/C consumes the same amount of power, but the extra gas needed to run with open windows goes up by a factor of eight.

Myth No. 5: The Gadget con: Add this magnet to your gas tank or fuel line, put this high-tech gizmo near the fuel injector, or pour this dust into your tank, and enjoy 90 mpg city and highway travel.

The Truth: The Environmental Protection Agency has tested hundreds of these scams and none work. If such as simple gimmick worked, we would all know about it.

Myth No. 6: Tire inflation: Overinflated tires have less rolling friction and thus boost mileage.

The Truth: Underinflated tires have more rolling friction and waste gas. They also have less traction and stability, and more internal friction in an underinflated tire, all of which wastes gas. Overinflated tires also have less traction because the contact patch becomes smaller as tire pressure increases. The best practice is to keep pressures according to the manufacturer's suggestions.
-from Mike Allen at Popular Mechanics


People use duct tape (fabric-based tape with rubber adhesive) to repair everything from broken plates to cars. It's also been used as a bandage. Astronauts even used it to repair Apollo 13 equipment and get home to Earth safely. But it's not worth a hill of beans when it comes to sealing ducts.

During World War II, the U.S. military bought the cloth-backed tape for making emergency repairs on battlefields. Then after the war, heating and air conditioner contractors began using the tape to seal joints in HVAC ducts. Tape manufacturers even colored it silver to blend in with ductwork and was it soon dubbed duct tape.

But in 1998, scientists at the Lawrence Berkeley National Lab tested duct tape, along with 31 other sealants under conditions that simulated long-term home use. They heated air to 170° and chilled it to below 55° before blasting it through ducts. They even baked ductwork at up to 187° to replicate the conditions of closed attics during summer sun.

And of all the things they tested, only duct tape failed - reliably and often quite catastrophically.

Instead of using duct tape, researchers recommended sealing ducts with mastics, gooey sealants that are painted on and allowed to harden; metal ducts should be held together with sheet metal screws; and flexible duct connections should be secured with metal or plastic bands. But as, all home DIYers know, these methods take time and skill. Duct tape doesn't.
-from the Consumer Energy Center

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