Dear Jim: Whenever I shop to buy an electric or gas appliance, I hear and see many “efficiency” claims. It gets confusing to determine which are legitimate. How can I compare the real efficiencies of products? – John K.

Dear John: It is important to buy efficient products, but it can get confusing. Depending upon the type of product 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 your house. Electricity input is usually measured as wattage and gas or propane input as Btuh (Btu per hour).

For nonenergy-consuming products, efficiency is more difficult to compare. Most home products, such as doors, windows, skylights, etc., base the efficiency rating upon some industry testing standard. Keep in mind, the conditions at your specific house may be different from the test standard, so also seek the advice of reputable local contractors.

For windows and doors, the efficiency is related to how airtight they are and the U-value (heat flow characteristic) when tested to the industry standard. Always ask about 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 ones.

Most plug-in electric heating devices and heaters, from the cheapest to the heavily advertised ones for $400, are nearly 100 percent efficient. All of the electricity consumed ends up as heat inside your house. The actual differences in operating costs depend upon how you use one and if the 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. Combustion efficiency refers to how efficient the gas burns, not how efficiently the heat is transferred into your house. Its “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 other major appliances, such as washers and refrigerators, rely on the yellow energy label. Your usage habits often have more impact on the overall efficiency than the appliance design itself.

For central heating and cooling equipment (furnaces, air conditioners and heat pumps), the manufacturer’s efficiency ratings should be accurate. They are AFUE (annual fuel utilization efficiency for furnaces), SEER (seasonal energy efficiency ratio for air conditioners) and HSPF (heating seasonal performance factor for heat pumps).

Dear Jim: I have a large microwave oven which 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.

Dear Pat: The purpose of the round glass turntable in a microwave oven is to rotate the food for more even cooking. Many smaller and older microwave ovens do not have a rotating glass turntable on the bottom. The only problem you may have is the hub, which seats in the bottom of the turntable, will still be turning. This will rub on the bottom of the casserole. Use some plastic spacers, not metal, to raise the casserole a little.

Send inquiries to James Dulley, Bangor Daily News, 6906 Royalgreen Drive, Cincinnati, OH 45244 or visit www.dulley.com.