When shopping for appliances the salesman told us that the local energy costs were such that there was no advantage for paying more for a gas clothes dryer since the utility rates were about the same.
I was skeptical that he actually understood the actual energy cost so did a little research (basic stuff any high school physics class should teach.) This page shows my utility costs and the equivalent energy content of both natural gas and electricity. If you would like to do the same you should be able to use this page as a guide and get the rates either from your bill or your local utility company(ies).
| --- | Sold In | Unit Name | Typical Cost/Unit | Energy/unit | Cost/unit-energy | Other Costs |
|---|---|---|---|---|---|---|
| Natural Gas | 100 Cubic feet | Therm | 0.7062 | 105,480,400 J | 6.695e-9 | not shown - 60-80% efficient burn |
| Electricity | kWh | 0.0702 | 3,600,000 J | 18.944e-9 | not shown 100-300% efficient heat pump |
Note, in the rates above (taken from Colorado Springs Utilities rate charts) there are 2 components to a rate. The "access charge per kWh" and the "Electric Supply per kWh". The rate used is the sum of these two rates (0.0351 and 0.0351). (Natural gas has "Access Charge per CCF" and "Natural Gas Cost per CCF"). There are additional "Access charge per day (0.2200 vs 0.3008) which I neglect since they are independent of the amount you actually consume.
Also note: I believe the access charge from that site is incorrect. It should be 0.0351 rather than 0.351 (I sent 2 emails and they claim they will be correcting the site.)
I found the wikipedia water heater thermodynamic discussion interesting. I've attempted to recreate the calculations here using my utility rates: (calculations for gas water heater)
| Calculation | Notes |
|---|---|
| 50 Gallons * 8.3 Pounds/Gallon = 415 Pounds | -- Water Heater Size |
| 415 Pounds * 55°F = 22,825 BTU's | -- hot water @ 105°F - ground temp 50°F = 55°F |
| 22,825 BTU's / 100,000 Therms/BTU = 0.2285 Therms | |
| 0.2285 Therms * 70.62 cents/Therm = 16.12 cents | -- rate from above |
| Assume 70% efficient burn = 23 cents | |
| Assume 50 Gallons/day = $84 / year |
| Calculation | Notes |
|---|---|
| 50 Gallons * 8.3 Pounds/Gallon = 415 Pounds | -- Water Heater Size |
| 415 Pounds * 55°F = 22,825 BTU's | -- hot water @ 105°F - ground temp 50°F = 55°F |
| 22,825 BTU's * 1054 Joules/BTU = 24,057,550 Joules | |
| 24,057,550 Joules / 3.6×10^6 Joules/kWh = 6.683 kWh | |
| 6.683 * 0.0702 cents/kWh = 46.9 cents | |
| Assume 100% efficient heat transfer from element to water | |
| Assume 50 Gallons/day = $171 / year |
If you assume a 60% efficient burn rate natural gas works out to 1 cent per Mega Joule. (not all the gas will burn and when it does not all the energy is released as usable heat.)
Electricity which is effectively 100% efficient for any heating application costs 1.9 cents per Mega Joule.
So in summary we decided to go with a gas clothes dryer which should cost almost half as much to operate as an electric clothes dryer.
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