Water Bed

In a water bed, the mattress is filled with several hundred liters of water – without insulation. To avoid being cold in bed, you need a heating pad to heat the water. In the example shown in Figure 5, the pad is rated at 200 watts, but a thermostat limits the consumption to an average power of 50 watts.

Figure 5: The heat pad in a waterbed can be limited to 50 watts by a thermostat.

The measurement from Figure 5 reveals two effects. The sleep period started at the time 10 hours. The subject awakened at 17 hours, after which the bed was aired for several hours to show the effect more clearly. When a person is lying in the waterbed, the temperature of the bed is balanced with the person's own body heat; which means that the waterbed consumes practically no electricity during this time. But airing the bed, that is, leaving the mattress exposed without a cover, is energy intensive.

To save energy, a waterbed should be well insulated. Little can be changed on the underside due to the design. To save a few dollars a year, you need to avoid leaving the mattress exposed for too long during the day.

Kettle vs. Microwave Oven

A kettle not only heats water, but also the vessel that holds the water. Microwaves heat the water, but not microwave-safe vessels. In a microwave, only part of the electrical energy is transferred as heat. This means that, for large quantities of water, a kettle is more energy-efficient, but for smaller amounts, you will want to use the microwave.

To close in on the values of interest, I first heated some water in the microwave. The temperature increase for a given amount of water and time determines the heat energy absorbed. Its ratio to the electrical energy of the microwave, in turn, defines the efficiency: Approximately 60 percent of the electrical power is consumed by the whatever is being cooked; the remaining 40 percent is consumed by electronics and cooling. A kettle converts 100 percent of the energy into heat, but a fixed portion of this is spent on heating the vessel itself. Figure 6 illustrates the numerical ratios: A 200-milliliter cup of coffee in a microwave-safe mug can be heated more efficiently in a microwave.

Figure 6: Comparing the efficiency of kettles and microwaves for heating water (diagram).

Conclusions

A regular check of the household meter shows the daily electricity consumption. In order to save energy in a targeted way, it makes sense to take a look at the consumers you have connected to the power grid. Some devices run permanently. The electricity costs per year are directly related to the average power consumption, for example, EUR100 for a refrigerator. The cost of upkeep for a water bed will drop if you leave it well covered during the day. Other devices only incur costs when in use. A wash cycle with the washing machine costs less than 50 cents thanks to modern detergents. Compared with this, even state-of-the-art clothes dryers have an energy hunger that is a whole order of magnitude higher.