British Thermal Unit (Btu): the amount of energy to raise 1 pound of water 1 degree Fahrenheit
Therm: 100,000 Btu
DekaTherm (DKT): 1,000,000 Btu
Natural gas contains about 1 DKT of energy in 1000 cubic feet of gas.
ELECTRIC POWER and ENERGY
1 Watt = 1 Volt*1 Amp in purely resistant circuits
1000 Watts = 1 Kilowatt (KW) (this is Power)
1 KW* 1 Hour = 1 Kilowatt-Hour (this is energy)
Solar Domestic Water-Heating Systems
To make economic calculations, we will use a family of four consuming 303 liters of hot water a day. The hot water temperature is set at 120 degrees F, the cold water inlet temperature is 55 degrees F, and the hot water tank has a heat loss of 2,147 Btus a day.
The total daily heat energy demand is calculated to be 45,081 Btus.
Given that the energy in 3,413 Btus of heat is equal to the energy in 1 kWh of electricity, we can compare the cost of using fuel oil, natural gas, or electricity as the energy source to heat the daily hot water demand in this example.
Fuel oil is sold by the gallon and on average contains 139,000 Btus per gallon.
Natural gas is sold sometimes by the MCF (thousand cubic feet) and sometimes by DekaTherm (DKT).
For the purposes of this example, both are 1,000,000 Btus.
Keep in mind that the efficiency of converting from one source of energy to another is not usually 100%.
For this example, the use of electricity to heat water can be considered to be 100% efficient because all of the energy from electric resistance heating goes into heating the water, so there are no losses.
Fuel-burning appliances must be vented, and some of the energy from burning the fuel carries the products of combustion out of the house.
About 70% of the energy content of both natural gas and fuel oil is used to heat the water in the water tank, and the remaining 30% escapes up the flue. Because some energy is lost up the flue, more Btus of gas or oil must be purchased than Btus of electricity.
The annual Btu demand for heating the hot water in this example is 16,454,565 Btus.
16,454,565 Btu / 3413 Btu per KWh = 4,821 KWh 16,454,565 Btu/70% Efficiency = 23,506,521 Btu input of natural gas, propane, or fuel oil.
There are other reasons that people install solar energy systems, such as reducing greenhouse gases, reducing the need for more power plants, reducing consumption fossil fuels and reducing environmental destruction.
There are other economic analysis methods that can tell you more about the value of installing a solar energy system, but the final analyses are only as good as the input data. For example, it is difficult, if not impossible, to estimate what the future cost for fuels and interest rates will be. In the long run, the simple payback comparison is a quick, easy way to compare between systems using present day costs for fuel and equipment.
