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Energy Efficiency

The United States uses a lot of energy: about 25% of the worlds total.  While energy use virtually drives our economy and defines our standard of living, we could certainly use it much more efficiently without making a dent in our standard of living.  In addition to using less, we could also reduce our dependence on fossil fuels (oil, natural gas and coal) by increasing our use of sustainable captured energy (i.e. solar and wind).

The majority of our home energy use is in heating and cooling, although a significant amount is also used in electrical loads, especially as an increasing amount is being used as "phantom loads", the electric used by our appliances when they aren't in use (e.g. to run the clock on the stove or microwave).  There is some confusion between the efficiency of appliances and overall energy efficiency.  All electrical appliances give off heat, and so during the heating season, this excess heat reduces the amount of fuel we need to burn.  Of course during cooling, it make air conditioning equipment work extra hard (see solar section for more information on internal gain)

Most electric in the US is generated by burning fossil fuels (coal, natural gas mainly) in a process that is typically 33% efficient (that is, the ratio of electrical energy output to the heat energy of the fuel).  The electric is then carried over high power lines, often at great distance incurring another 7-10% loss, so by the time the electric reaches your home, about two thirds of the original energy has been lost.  So in this respect, all electric consumption is inefficient.  However if our electric is generated from the wind or photovoltaic solar cells, no fuel is burned at all, and so electric consumption of this sort is environmentally better than burning any fuel. (Note: because hydroelectric generation converts mechanical energy to electric, it is often more efficient than burning fuel in which stored chemical energy is converted to heat, which is then converted to mechanical energy which is then converted to electric.  Nuclear power--which accounts for about 20% of US generation, converts nuclear energy into heat and then electric.  This process is even less efficient than burning fuel, but the stored energy is much greater.)

As a demonstration project an all electric home (including electric heat) was built that is powered only by solar power, and uses no fossil fuels at all, thereby turning the conventional wisdom of avoiding electric heat on its head! 

Understanding Energy Efficiency

For a building, energy efficiency is a simple equation:

  1. Take advantage of whatever solar energy (or shading) you have available
  2. Hold on to the heat (or cool) you have as long as possible
  3. When you do consume fuel, capture as much of the heat value as possible
  4. Use energy efficient appliances & lighting

There are multiple reasons one would want energy efficiency: it reduces our energy bill, it is easier on the planet, and insulated walls increase comfort.  Most people are aware of the first two reasons but not the last one, which comes as a surprise benefit to many and should be fully understood.

The relationship between comfort and temperature
Common knowledge is to set your thermostat to around 70 degrees and you'll be comfortable, yet everyone knows that a 40 degree day outside can be comfortably cool or unpleasantly cold depending on wind, sun, etc.  Human comfort is determined by multiple factors because the temperature we feel is more related to how much heat our bodies either gain or lose.  These factors are the air temperature, the amount of radiant heat we get or lose, the humidity,  the amount of air movement, and of course our level of activity and amount of clothing.

Further complications arise due to the fact that the temperature of both the objects and the air in a room can vary significantly (by 20 degrees or more), especially when there is little insulation preventing cold from leaking inside.  A well insulated house increases comfort by increasing the temperature of the surfaces, reducing the amount of heat out bodies will radiate to those cold surfaces.

To estimate the temperature a person will feel, take the average of the air temperature and the mean radiant temperature.  Unfortunately, calculating the mean radiant temperature is difficult because it is a combination of the temperatures of all the nearby objects adjusted for how far away they are.  Radiant temperature can be measured using an electronic meter that  measures heat radiation in the same way a camera's light meter measures brightness.  So for example, if the air temperature is 72 degrees and the mean radiant temperature is only 60, the temperature a person feels is 66 degrees, not 72.  There is some level of disagreement on exactly how to combine air & radiant temperatures to determine comfort as there are multiple comfort models in use by different organizations.  Proponents of radiant heat will often weight the radiant temperature more significant than the air temperature, combining them so that 1 degree of radiant temperature is equivalent to 1.4 degrees of air temperature.  Often the temperature of the floor is cited as having a more significant effect on comfort than other components of the radiant temperature.  This is due to two possible reasons (1) our feet are generally in close contact with the floor (2) we are psychologically sensitive to the temperature of our feet. Regardless of the details, everyone agrees radiant temperature is very significant and that increasing insulation improves the radiant temperature.

(picture: comfort factors)

In calculating comfort, we must also take into account the relative humidity and the amount of air movement.  Due to other health constraints, it is important to keep the indoor humidity to between 35% and 55% (some sources use 40-60% as the range, but the lower numbers appear to be the latest), and so humidity shouldn't affect comfort.  However, on cold days homes with high air leakage rates tend to get very dry which causes a person to feel colder than the temperature would indicate due to increase evaporation from the skin.  A drafty home will have small air currents, which speed heat loss from the body and so also make a person feel colder than the temperature would indicate.  Again, a well sealed an insulated home will not have these problems.  During hot weather the cooling effect of moving air can be taken advantage of: simply using a fan instead of an air conditioner (which uses much more energy) can increase comfort as the same temperature readings.

A few quick notes on units and terminology

Throughout this section various measurement related to energy efficiency are used.  The most common are described below.

BTU - British Thermal Unit, for those of us not on the metric system.  One BTU is the amount of heat needed to heat one pound of water one degree Fahrenheit

BTU/hr - often you are interested in the rate of heat movement and in this case the typical value is in BTUs per hour.

Kilowatt (kw) - One thousand watts.  A watt is a rate of energy usage, so it can be converted to BTU/hr, but not BTUs.  If you turn on 10 hundred watt light bulbs you are using a kilowatt.  You can think about it like measuring the rate water falls over a dam.

Killowatt-hr(kwh) - This is an amount of energy used, not a rate, equivalent to BTUs.  The conversion factor is that 1kwh equal 3413 BTU.  If you turn on 10 hundred watt light bulbs and leave them on for exactly one hour, you've used 1kwh of energy.  If you turn on 1 hundred watt light bulb for 10 hours, you also use 1kwh of energy.  Using the water analogy, kwh is the total amount of water that went over the dam. 

U value/R value - this is the heat flow rate of a given material, measured in BTU per square foot per hour per degree.  That is the amount of heat that will move through one square foot of surface area of that material in an hour given a one degree Fahrenheit difference in temperature.  The U and R values are inverses of each other, and are discovered by testing the material in a lab.  The only way to find these numbers is to look them up somewhere.  If you still want to know more gory detail, read the fine print below.

CFM - Cubic feet per minute.  This is a rate of airflow, and is how exhaust fans and air leakage are often mesaured.  To calculate heat loss due to air flow you need to know how many cubic feet of warm air escapes, how much heat a cubic foot holds per degree of temperature (.015BTU) and the difference in temperature between the indoor air, and the cold outdoor air that will be replacing it.

ACH - Air Changes per Hour.  This tells you how many times per house all the air in a house leaks out. Like CFM, this is a rate of air flow, but rather than a measured rate, it is relative to the volume of the house.  You can convert one to the other using the volume of the house:   ACH = (CFM*60)/Volume.

Fine print: the R value of materials can change under different conditions, although in most cases the change is small and irrelevant.  Since materials (like manufactured insulation, and natural products) are not uniformly consistent, various sources will give different R values for the same material.  All material absorb heat as well as transfer it, and when the material absorbs a lot of heat, the heat flow equation doesn't apply until it reaches a steady state equilibrium.  In these materials, when steady state isn't reached (always the case with weather!), the effective R-value might be different from the actual R-value, for example in a concrete wall exposed to the sun all day.

The heat flow equation is:

Q = A·U·DT

Where Q is the total energy moved, A is the area in square feet, U is the heat flow value for the material and DT is the difference in temperature between the two sides of the material.  The U value is usually given for a specific thickness of material, so to get the actual U value you just multiple (or divide) by the ratio of the actual thickness to the given thickness.  For example one inch of wood is R1, so if you have 3" of wood your R value is 3 (which is a U value of 1/3 or .333).  To get a U value for a composite material (like a wall), you just add up the R values of all the pieces and then invert it to get the U value.

Presumably that is now more than you ever wanted to know...


To learn more about Energy efficiency, click on the following links:

Building a tight envelope: hold on to our heat (or cool) as best as possible

Solar energy: taking advantage free energy (and keeping cool)

HVAC: systems for burning fuel and delivering heat (or cool)

Water Heating


Zero Energy/Carbon Neutral













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