Sensible Home: Do the math when looking at energy-efficiency claims

Question:

Whenever I shop to buy an electric or gas appliance, I hear and see many "efficiency" claims. It gets confusing trying to determine which are legitimate.

How can I compare the real efficiencies of products?

- John K.
Answer: It's important to buy energy-efficient products, but it can get confusing. Depending on the item and the fuel it uses, efficiency ratings can mean different things. Also, some manufacturers and salespeople play fast and loose with the accuracy of efficiency claims.

A basic definition of efficiency for any energy-consuming product is the amount of usable energy (heat, light, sound, etc.) output divided by the energy input.

The energy input is usually an electric plug in the wall or a gas or propane line coming into the house. Electricity input is typically measured as wattage, gas or propane input as Btu per hour (Btuh).

For products that do not consume energy, efficiency is more difficult to compare. Most home products, such as doors, windows, skylights, etc., base their efficiency ratings on some industry testing standard. Keep in mind that the conditions at your house may be different from the test standard, so you should seek the advice of reputable local contractors.

With windows and doors, efficiency is related to how airtight they are and their U-values when tested to the industry standard. (U-value is a measure of heat flow through a material; the lower the U-value, the greater a product's resistance to heat flow and the better its insulating value.)

Always ask who did the product testing. It should be an independent laboratory or an industrywide trade association. Check with Energy Star (www.energystar.gov) for listings of the most efficient products.

Most plug-in electric heaters, from the cheapest to the heavily advertised units that cost $400 each, are nearly 100 percent efficient. All the electricity consumed ends up as heat inside your house. Actual differences in operating costs depend on how you use the heater and whether its design is the proper one for your specific needs.

Always read efficiency claims thoroughly when comparing products. For example, some lower-quality gas space heaters may claim a very high 90 percent "combustion" efficiency. This refers only to how efficiently the gas burns, not to how efficiently heat is transferred into your house. The actual heating efficiency may be only 70 percent.

For electrical appliances such as window air conditioners, calculate the efficiency yourself by dividing the Btuh cooling output by the electric wattage shown on the nameplate.

For major appliances such as washers and refrigerators, rely on the yellow energy labels that come attached to them. A consumer's usage habits often have more impact on the overall efficiency of an appliance than the design itself.

For central heating and cooling equipment (furnaces, air conditioners and heat pumps), the manufacturer's efficiency ratings should be accurate. Look for these indicators: for furnaces, AFUE, or annual fuel-utilization efficiency; for air conditioners, SEER, or seasonal energy-efficiency ratio; and for heat pumps, HSPF, or heating seasonal-performance factor.

Q: I have a large microwave oven that has a glass turntable in it. I have heard I should not remove the turntable. I want to put a large rectangular casserole in there. Can I remove the turntable?

- Pat L.
A: The purpose of the round glass turntable in a microwave oven is to rotate the food for more even cooking. Many smaller and older microwaves do not have turntables.

If you remove the turntable, the only problem you're likely to have is that the hub, which normally sits in the bottom of the turntable, will still be turning and may rub on the bottom of the casserole.

Use some plastic spacers, not metal, to raise the casserole a little.