Energy terminology

Energy Terms Explained

R-values and insulation. R-values are the standard way of measuring insulation. Insulation is the ability of a wall, ceiling, window or roof to prevent, or resist, heat from escaping. Resistance to heat escaping is what puts the R in R-value. The higher the R-value the better the insulation and R2 is twice as good as R1.   A typical insulated roof is about R30, a typical insulated wall is about R12, a typical double-glazed window (i.e., with two panes of glass) is about R2, and a single-glazed window (i.e., with just a single pane of glass) is R0.75. An attic without insulation is about R1. If you double the insulation in your attic, say from R30 to R60 you will halve the heat lost through your roof.

Electricity and energy. The kilowatt-hour, abbreviated kWh, is the standard, if rather clumsy, unit for measuring the energy in electricity. One kilowatt-hour is the energy used by an appliance that uses power of one kilowatt for one hour. The large burner on an electric stove uses about two kilowatts, so if it takes you half an hour to cook supper you have used one kilowatt-hour (two kilowatts x half an hour) of electricity. So you can think of a kilowatt-hour as the energy needed to cook supper. Your utility charges you per kilowatt-hour and the average price in the U.S. is about 15 cents per kilowatt-hour. In Massachusetts it is about 21 cents per kilowatt-hour.

Electricity and power.  The watt, abbreviated W, is the standard measure of how fast an appliance uses electricity. I.e., how much electricity it uses per hour. The Watt is named after James Watt who was a driving force of the industrial revolution. 1 kilowatt is one thousand Watts.

Watts and kilowatts measure the power of an appliance. Power is the energy used per hour. Power is related to energy in the same way that speed is related to distance. If you travel a distance of 80 miles in two hours, then your speed is 40 miles-per-hour because 80 miles/2 hours is 40 miles-per-hour. If the energy used is 80 kilowatt-hours and you use it in 2 hours then the “speed” at which you are using electricity is 80 kilowatt-hours/2 hours or 40 kilowatt-hours per hour. This is more simply stated as just 40 kilowatts.

So power is measured in kilowatts, written kW, and energy is measured in kilowatt-hours, written kWh. The large burner on an electric stove uses power of about 2kW, so if it takes you half an hour to cook supper you have used 1kWh (2kW x 0.5 hours) of electricity. These terms can be quite confusing, but it is far easier than using British Thermal Units (BTUs) to measure energy and horsepower (HP) to measure power.

Heating degree days.  Heating-degree-days are how to measure how cold the weather has been. One heating-degree-day is recorded if the average temperature during the day is 1°F below 65°F. So if it is a cold winter day where the average temperature is 10°F, that day creates 55 heating-degree-days because the average temperature is 55°F below 65°F for 1 day. Or, 65°F minus 10°F is 55°F, multiplied by 1 day, is 55 heating-degree-days. Heating-degree-days are widely used to remove the weather as a factor when analyzing the use of energy for heating.

Triple-glazed windows.  I deliberately use the term triple-glazed windows, even if it is a little clumsy, because other terms like triple-pane and tri-pane are confusing. Some manufacturers refer to a double-glazed (double pane) window with a storm window as triple-pane. It does have three sheets of glass, but the gap between the storm window and the double-glazed panel is not sealed and does not trap air. It is only trapped air that can provide the insulating benefit. When I say triple-glazed window I am referring to three sheets (panes) of glass with an air gap of about ½” between each pair of sheets of glass. Each air gap is sealed and the spacer running around the edge of the glass panes is insulating. Early generation windows had spacers that conducted a lot of heat through the edge of the window, which lowered the insulation value.

R-Values and windows.  R-values are the standard way of measuring insulation. Insulation is the ability of a wall, window or roof to prevent, or resist, heat from escaping. Resistance to heat escaping is what puts the R in R-value. The higher the R-value the better the insulation. R2 is twice as good as R1, and R4 is twice as good as R2. If you double the insulation of your windows, say from R2 to R4, you will halve the heat lost through your windows.

R-Values and u-factors. I have described the insulating value of windows here in terms of their R-value. This is not how it is described in the industry. The window industry uses the u-factor to describe the insulating value of windows. The u-factor can be converted to the R-value by dividing it into 1. In other words a u-factor of 0.2 is equal to an R-value of 5 because 1/0.2 = 5.

I used the R-value here because I wanted to compare the insulation of our windows to that of our walls and roof so I could find out where the energy sieve was leaking the most.

If you had to design a way to make it hard work for consumers to get to a zero-carbon footprint, using different units to measure the insulating property of walls and windows would be a great way to do it!

Low-E means low emissivity. Emissivity is a concept that is best left in the physics textbooks. What it means to the homeowner is that the glass (not just the air) is now contributing to the insulating properties of the window. Low-E is achieved by coating the surfaces of the glass with extremely thin, and invisible, layers of metals that can reflect either ultraviolet light or infra-red light without reflecting the visible light we can see. This keeps the room bright and sunny, but much better insulated. Modern low-E triple-glazed windows are an engineering marvel, on a par with solar panels and heat pumps. They are the unsung heros of the zero-carbon revolution.

Air conditioners, refrigerators and heat pumps.

Air conditioners (also called just AC units), refrigerators and heat pumps are often confused with each other. Technically they all use a heat pump to pump, or move, heat from one place to another. But in common language the term heat pump is used specifically to refer to a unit that can heat your home rather than cool it. The heat pump that cools your house is usually called an air conditioning unit, the heat pump that cools your milk is usually called a refrigerator and the heat pump that heats your house is usually called a heat pump. Modern heat pumps for heating your house will also cool it in the summer so, in effect, they are both heaters and air conditioners in one unit.

Single-glazed, double-glazed and triple-glazed windows.

A single-glazed window has only a single pane of glass, double-glazed windows have two panes of glass separated by an air gap of about ½”, and triple-glazed windows have three panes of glass with two air gaps, each of about ½”. The gaps in modern windows are filled with argon or krypton gas rather than air because argon and krypton are just as clear as air but are even better insulators. It is these air gaps that create most of the insulation. Having the air gap in a double-glazed window is like putting on a thick sweater. In a sweater, it is not the fibers of wool that keep you warm but the air trapped in between the fibers. Air is a great insulator. This is why double-glazed windows are more than twice as good at insulating as single-glazed windows. When you go from double-glazed to triple-glazed windows it is like putting on two thick sweaters because triple-glazed windows have two air gaps not just one.