For gas heating systems:
1. get a stable home temp (turn the thermostat to 65F and let it alone for about an hour).
2. turn to “pilot” or “off” all of your gas appliances except the furnace (water heater, dryer, stove, fireplace, etc). Off is better, but it can be hard to relight the pilot on some units.
3. Record the readout on your gas meter
4. Turn the thermostat to 70F, and wait until the home heats up (this can be a long time in a drafty house)
5. Read the setting on the gas meter. Subtract the number you got in step three from this number, and you get a snapshot of the amount of gas (in therms) that it takes to heat your home up by 5F. Remember, it will take more fuel in cold weather, less in warm, so this isn’t a definitive number but more of a rough guide.
6. Look at your gas bill and see what you are paying per therm. Multiply these numbers, and you get roughly the cost of raising you homes temp from 65F to 70F during that day’s particular weather.
For electric heating systems, in step 2 turn off everything that uses electricity in the home (except for whatever heats your home). In step 5, read the electric meter, and in step 6 look at your electric bill and multiply the difference in the two numbers by the cost of electricity.