Notes on Energy Efficiency

I arrived in one piece in Hawaii a few days ago, and have been settling in. It is still hard to believe I am here, and I plan to elaborate a bit on why I am here in the near future.

In the interim – and because I haven’t posted anything new in a few days – I thought I would call attention to a story in the New York Times from a couple of days ago:

Energy Efficiency: Fact or Fiction?

You have to be registered to read it (although the Tehran Times has reprinted the first page of the article) but I will paraphrase/excerpt it. The article covers a number of facts and myths around energy efficiency:


1. Screen savers save energy

FICTION — With screen savers, electricity is still pumping to keep your computer and monitor running. In fact, screen savers may even use more energy than a basic blank screen.

2. Your computer stops using energy when in sleep mode

FICTION — Computers still use energy when in sleep mode, but about 70% less.

3. You waste more energy restarting a computer repeatedly than letting it run all day

FICTION — Even though a small surge of energy is required to start up a computer, this amount is less than the energy consumed when a computer runs for long periods of time.


4. No energy is used after you turn appliances and electronics off

FICTION — Many appliances still draw a small amount of electricity when turned off. Solve this by plugging into a power strip that you can turn off.

5. It’s more efficient to keep your refrigerator full than half full

FACT — The larger the mass of cold items in a refrigerator or freezer, the less work is required to maintain the appliance’s chilly temperature. (Of course the more work it then takes to get the appliance to its chilly temperature).

6. Hand-washing dishes is more energy efficient than a dishwasher

FICTION — Dish washing by hand actually consumes more water and energy. People typically leave the hot water running, using up to 14 gallons of water on average. GE Appliances’ Paul Riley says to get the most out of an energy-efficient dishwasher, make sure it is fully loaded with food scraped off the plates.

7. Wash clothing with hot water for a truly effective wash.

FICTION — Heating the water for laundry makes up about 90 percent of the energy used in a conventional top-load washer. Using warm and cold water can be just as effective and can slash your energy use in half or more.


8. It’s better to fill your gas tank halfway because a full tank adds weight and is therefore less fuel efficient

FACT — The lighter your car, the better the fuel economy.

9. If you live in a warm climate, buy a light-colored car.

FACT — The lighter colors reflect the heat, whereas dark vehicles absorb heat and require more air conditioning to cool down.


10. If you live in a warm climate, paint your house a light color

FICTION — A light-colored roof helps dial back the temperature in a home’s attic by reflecting sunlight, but insulation is the key factor when it comes to energy savings. To really cool down your house, focus on proper insulation and plant foliage to block the sun’s rays.

11. Shut the door and vents in unused rooms

FACT — This works only if you close the doors and vents in multiple rooms.

12. Leave the heating or cooling system on all day. If you shut it down when you’re away, the system needs a surge of energy to reach the desired temperature.

FICTION — Switching the thermostat off when you go to sleep or leave for the day will boost energy savings. It will take more energy to bring your house back to the set temperature, but less energy is used during the down times. You can also realize substantial savings by changing the temperature settings. It is estimated that you will realize a 2 percent savings on your energy bill for every degree you cut back.

19 thoughts on “Notes on Energy Efficiency”

  1. I call BS on #6. I've seen a number of studies that claim dishwashers are more efficient than handwashing, but they have all been done by appliance manufacturers and compare the most efficient dishwashers to the most inefficient method of handwashing. Handwashing dishes in an efficient manner is significantly less wasteful than using a dishwasher.

  2. Re. #8, I've heard the opposite. The extra weight of the fuel isn't significant, but some other factor is. It's possible more fuel is lost to evaporation in the tank if you don't keep it full.

    Re. #12, while this appears to be true for air temperature, the problem in my own personal experience is the thermal mass of the house (walls, floors, etc.). If the house is allowed to heat up during a hot summer day, the retained heat will be released all night, forcing your HVAC system to work harder to keep the air temperature down. It's basically your refrigerator effect writ large.

  3. #5 sets off my crap detector.

    Look at the energy flows.

    In Empty Fridge A, there is little thermal mass. When opened for a set time, the air temperature rises from 5 deg C to 15 deg C. ~1000 joules of heat is added to the inside of the fridge, which eventually needs to be removed by the coil.

    In Full Fridge B, there is a lot of thermal mass. When opened for a set time, the combined air temperature and liquid temperature (we're assuming they remain thoroughly mixed) rises from 5 deg C to 5.5 deg C. ~1000 joules of heat is added to the inside of the fridge, which eventually needs to be removed by the coil.

    The only minute difference, if you assume that most of the air inside the fridge leaks out, is that there is slightly less air in fridge B (making it 950 joules instead of 1000).

    It may be smarter to keep a fridge full in order to reduce the temperature flux that food is exposed to, but that has nearly no effect on net cooling required – only on whether the coil has to be activated immediately or can wait until a few minutes later. It still has to go on more than if the fridge hadn't been opened.

  4. I agree with previous posters, but many of these topics are "small potatoes." Filling up your tank halfway…the savings (and Doug is right about potential losses) must be minute.
    You want to save gasoline? Get a high mpg car and drive less.
    Half-tank is just tweety-bird stuff. Darker or lighter car? Again, just not that important.
    I encourage everybody to save energy where possible, but this particualr article seems to have gotten lost in the margins.

  5. #10 – I live in the Washington, DC metro area and have been thinking about house and roof colors for a few months. Our house was built in 1915 and needs a new roof. Currently in renovation, just 3 walls have been opened and R18 batting placed in (we're on a budget). The attic is going to receive R30 blown cellulose, but the roof material is up in the air. We're leaning toward a Galvalume standing seam roof in either natural color or black. Both have the Energy Star reflective pigments.

    It's been a bear tracking down real-world examples of the temperature differential between the two colors. Intuitively black is hotter but, I don't know. Aesthetically, the black will blend in with the thin-film PV panels coming next year. We're trying to figure out which option is best, and whether there is THAT much in energy and dollar savings from the lighter roof considering our climate. Not too worried about the exterior since it will be re-sided with white Hardi-board over the pink foam boards. Any gurus here have advice or web links?

  6. The number one reason for "Dead" Fuel Pumps is running your car too low on gasoline in the Summer. Heat Kills Fuel Pumps.

    The cost of replacing a fuel pump has got to outweigh the savings from running your car with a nearly empty fuel tank. (I think.)

  7. Another silly NYT article. It is a case of penny wise and pound foolish.

    You are not wasting energy if you are enjoying its use. My family derives a great deal of pleasure out of maintaining a comfortable temperature in the house. The cost is minimal. Invest in an efficient heat pump and good insulation, and then set the temperature where you like. Ceiling fans can make a room ‘feel’ cooler and use little energy.

    The reason to use a dish washer is sanitation. If you can, wash your dishes in the summer in the cool part of the day when a window can be opened. Same thing with taking showers. Avoid adding a heat load in the summer. This does not matter in the winter.

    When it comes to driving, avoid aggressive driving. Any extra weight is a larger factor if you see how fast you can pull away from a stop light. Your car will need few repairs.

    The empty space in our freezer is filled with water, milk, and bread. This avoid trips to the store and will come in handy if a hurricane or ice storm makes travel a bad choice.

    Contrary to claims, energy production and use has minimal environmental impact in a modern society like the US. There was a time when houses with no insulation were heated with coal or wood.

    If you are worried about ‘slowly drains electricity, a phenomenon called phantom load’ get a life and enjoy a beer listing to a ball game under a shade tree.

  8. #5 sets off my crap detector.

    I struggle with that one as well. I heard this for the first time about 20 years ago, and I finally convinced myself that it didn't make any difference because of the extra work it would require to bring that extra mass back down to temperature if it varied off of the set temperature. It doesn't take nearly as much work to bring a refrigerator full of air down to the set temperature relative to a refrigerator full of water.

    So it is certainly possible that some of the misconceptions they are trying to correct are misconceptions themselves.


  9. After Katrina,we went with a lighter roof,changed the lights to flourescent or electronic,and bought energy star appliances. We went 16 SEER on the AC. A friend told me there are 23 SEER AC units on the market now. They use about 1/3 the energy of the old 8 SEER models. There are no energy star dryers. Using cold water and extended spin saves a lot of energy. Our water heater is twice as big as the old one,because of the added insulation. We cut the electric and gas bills in half. No telling how much time I've saved not having to change those darn incandescent bulbs. I don't miss 'em one bit.

  10. #5 has to do with the air in the refrigerator/freezer. Cold air is heavier, when you open the door it sinks to the floor and brings in warm air from the room. A fully loaded fridge has less air to lose.

    #6 I've actually made a comparison. Our inefficient dishwasher used about 10 gallons. Our new GE Profile uses 6-8 gallons. I couldn't wash an equiavalent amount of dishes in anything less than about 10 gallons. Dishwashers do it by recycling the dirty wash water more than you would be willing to put your hands in.

    #8 Gasoline weighs about 6 pounds per gallon, so a full gas tank weighs around 120 pounds – no insignificant. Of course, I've wondered why we carry spare tires too. When I was still driving our van I pulled all the back seats out. It lightened the car by 160 pounds and made a 2 mpg difference in fuel economy.

    #12 I've tested this one too. They are right. For A/C though I drop the temperature in the house at night when the outside temp is coolest and then let the house "coast" during the day. Normally my A/C doesn't come on again until around 3pm.

  11. Another myth that we covered here is that "Cash for Clunkers" is an effective way to reduce greenhouse gas emissions.

    On Friday a study by an economist at UC Davis attempted to estimate the costs:

    Implied Cost of CO2 Under Cash for Clunkers

    This study estimated that CfC paid almost 10 times the estimated market price of CO2. Under the best of assumptions, the program cost more than $200 per ton. Estimates to take CO2 out of power plant emissions are anywhere from $90-$150.

    It might work as a temporary stimulus, but the environmental benefits are dubious at best.

  12. “There are no energy star dryers.”

    It is called a cloths line. Front loading washers use less water and less energy. The cloths going into the dryer use less energy because the spin cycle is better. Our front loader is much quieter too. When the old one hit the spin cycle, I would 'yell under way shift colors'. There is also no rule that the dryer has to match the washer. Consumer reports best buy was $200 cheaper for the dryer.

  13. “Any gurus here have advice or web links?”

    Where to start?

    “I live in the Washington, DC metro area”

    The best advice is moving if you want a white roof. Your problem is humidity, not high temperature. A radiant barrier over the attic insulation will make the upstairs feel cooler. When I move to Virginia that is one of the first things I did. I also built a box out of celetex to put over the attack stairs. Do not forget to weather strip the attic doors.

    “we're on a budget”

    Skip the PV, leave making electricity to your utility. Pink(?) foam backed with a radiant barrier (aluminum foil) is a better investment. New low-e glazing in the 1915 windows would also be a better investment.

  14. #5 The volumetric heat capacity of dry air at sea level at zero °C is 0.001297 J/cm3°C. Let's make some worst case sweeping assumptions. Take an empty 12 cu ft fridge and assume that all the air sinks to the bottom like a lead balloon and flows out over your feet when you open the door, to be replaced by 15°C warmer air. You have just added a little over 6kJ to the interior of your fridge. With a typical domestic fridge COP of 5, you need 30kJ of electrical energy to remove that, Let's imagine by comparison our full fridge loses no heat by convection and a negligible amount by conduction when the door is opened. Whoopee! You have saved about 0.01kWH. Open your full fridge a hundred times a day and you may save 10 cent! Avoid, of course, taking out any of the things you filled your fridge with, since those will typically have over 3000 times the heat capacity of the air you replaced with them.


  15. On #6: Born and raised in the US, I thought nothing of running the hot water all the time to wash dishes. But here in Japan that is unthinkable because fuel is more expensive. Although I have not actually measured the amount of hot water we use to wash the dinner dishes, I estimate with confidence that it's no more than 15 liters (about 4 gallons). Here we use wall-mounted on-demand water heaters that supply hot water instantly, and only when you actually need it. Water is cheap and plentiful here; it's the price of energy that is the motivation. Bottom line: cheap energy breeds waste and inefficiency.

  16. #1 You mean won't save the world? 😉

    #7 "Conventional" top-load washer? Must be different conventions where I live — I haven't seen one for about 35 years.

    #8 I've tried this one. Unfortunately I'm overweight by more than half a tank of gas.

  17. “it's the price of energy that is the motivation.”

    Rice, have you ever bothered to calculate the cost? Are people in Japan so poor that they are motivated by 10 cents difference in energy cost. A large mix of electricity generation is nuke in Japan. If the consumers are paying a lot for electricity it is not because of cost of generation.

    The first incorrect assumption is that Americans are wasteful and inefficient. I do not know much about Japan, do not care to either.

    I put my dishes in a dishwasher because it is more convenient and sanitary. Maybe Rice Framer lives in a place with unsanitary living habits but it is more about customs that energy.

  18. I think waste and inefficiency are relative things. Putting it less pejoratively, cost affects average consumer behaviour. I don't see any other way to explain, for instance, the fuel economy of US cars compared to the EU. I think it is customary in the US to claim that EU-style cars are unsafe tin boxes, but I think such customs are price-driven too.

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