Household Energy Use

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When the need to conserve energy in your house is considered, then the focus should be mainly on heating and cooling processes. They are the major uses of energy. Hot water heating is also a sizable use of energy, as is the cooking process with surface unit and oven. The use of energy by a refrigerator is significant, and the lighting process for a whole house becomes a significant energy use. Electronic appliances on the whole use a small amount of energy, and are not a major part of energy conservation initiatives.

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Electric Hot Water Heater

Commonly used electric water heaters in the U.S. use an electric resistance element to heat water directly. Some commercial water heaters employ the heat pump principle and are therefore more efficient by perhaps a factor of three, but they are not available in the residential market. Sometimes electric utility companies, in overenthusiastic marketing strategies, claim almost 100% efficiency for these water heaters. This is misleading because of the thermal bottleneck at the electric power plant; about three units of primary fuel is used to produce a unit of delivered electric energy, so the real efficiency is more like 33%. Natural gas water heaters at 60-70% efficiency are then about twice as efficient in the use of energy resources as the electric water heater.

The energy necessary to heat water is determined by the specific heat of water:

cwater = 1 calorie/gm °C = 4186 J/kg°C = 1 BTU/lb °F

A typical U.S. residential water heater will be taken as one which has a capacity of 40 U.S. gallons (= 320 pounds, 145 kg, 151 liters). The typical heating range will be taken to be from 60 °F to 140 °F (15.6 °C to 60 °C). The energy required to heat the water can be determined from the specific heat relationship.

Q = cmΔT

The energy required to heat one tank of water over the specified range is then

(1 BTU/lb °F)(320 lb)(140°F - 60 °F) = 25,600 BTU

or

(4186 J/kg°C)(145 kg)(60 °C - 15.6 °C) = 26.9 million Joules

Since a kilowatt-hour is 3.6 million Joules, this energy amounts to about 7.5 kWh of electricity. Taking an electric energy cost of 9.5/kWh, it would cost about 71 to heat one tank of hot water with an electric hot water heater assuming all the electric energy went into heating the water.

If you use gallons of hot water

and your electricity rate is /kWh

then your energy cost will be $

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Household Lighting

Although there are a large number of lighting options, the majority of lighting in homes is done by either incandescent or fluorescent lights. Fluorescent lighting has a considerable advantage in energy efficiency over incandescent lighting.

Other options for lighting include high pressure mercury lamps, metal-halide lamps, and sodium lamps. With rapidly increasing efficiency, light emitting diodes (LEDs) may become a lighting option for specialized applications.

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Energy for Cooking

An oven and surface unit for cooking consume a lot of energy when they are used, but the total use depends upon how much you cook. Pictured is an electric double oven and an electric surface unit. Alternatively, the oven and surface unit may be either natural gas or propane. The choice is generally made on the basis of preference, but from an energy point of view, the gas is more energy efficient. The reason is that it takes about three units of primary fuel to produce one unit of electrical energy because of the thermal bottleneck in electricity generation.

The power ratings for appliances are required to be displayed on an Underwriters Laboratory label somewhere on the appliance. The voltage for these appliances is 240 volts, 60 Hz AC. The labels will sometimes give the current, from which you can calculate the power using the power relationship. Since these appliances are almost purely resistive, then the DC power relationship (i.e., just voltage times current) can be used to a good approximation. These appliances were labeled with power of 7.4 kilowatts for the surface unit and 7.2 kW for the oven.

Operated for a full hour at full power, the surface unit would use 7.4 kWh of energy, which at 9.5/kWh costs just 70. It would be rare to operate at full power, so it becomes clear that the cooking energy cost is not comparable to that of household heating.

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