This has a great deal to do with your local rates for electricity and other fuels. The comparison involves the efficiency of the device, the type of fuel used and the cost of that fuel.
Commonly used heating fuels have the following approximate heating content:
Fuel oil - 138,000 Btu/gal
Propane - 90,000 Btu/gal
Natural gas - 100,000 Btu/therm (1,000 Btu/ft3)
Electricity - 3,413 Btu/kWh
A common index of the cost of heat is "dollars per 1,000,000 Btu of useful heat." In order to calculate useful heat (heat actually delivered to the house), it's necessary to adjust for the efficiency of the heating device and the cost of the fuel. The following equations can be used for this purpose:
| Fuel oil | 7.25 x $/gallon
efficiency | Efficiency Old - 0.65 New std. - 0.78 Moderate - 0.84 High - 0.92 |
| Propane | 11.1 x
$/gallon efficiency | |
| Natural gas | 10.0 x
$/therm efficiency | |
| Electric resistance | 293 x $/kWh | |
| ASHP | 293 x $/kWh COP | COP Warm climate - 2.5 Cold Climate - 1.8 |
| GHP | 293 x $/kWh COP | COP Warm climate - 3.9 Cold Climate - 3.1 |
As an example, let's look at a location in a moderately cold climate when the fuel costs are as follows:
Electricity, $0.07/kWh; fuel oil, $1.05/gal; propane, $1.20/gal; and natural gas, $0.60/therm. This would result in the following useful heat costs:
| $ per Million Btu | |
|---|---|
| Fuel oil | 9.06 |
| Propane | 15.86 |
| Natural gas | 7.14 |
| Electric resistance | 20.51 |
| ASHP | 9.54 (2.15 COP) |
| GHP | 5.86 (3.5 COP) |
Obviously, it is necessary to know the total amount of heat required for the year to calculate annual savings. The above values, however, provide an indication of the percentage savings to be expected from a GHP system compared to other options for heating.
Savings are also generated during domestic hot water heating and cooling. These will be small compared to the heating savings in all but southern climates. See the next question for some examples.