You were getting 80% of the gas energy as heat with your gas furnace. You are now getting 40% of the gas energy as electricity from the natural gas generator. That 40% is then extracting 4 units of heat from the ground for each unit of electricity. So your net is 40% X 4, or units of heat energy per unit of natural gas. Since originally you were producing units of heat energy, you have effectively cut your CO2 emissions in half even assuming the same fossil fuel is used to generate your electricity as was used for your furnace. Of course, if you switch to a Green electricity supplier at the same time that you switch to geothermal, you’ll not only eliminate all CO2 emissions from your heating and air conditioning system, but you’ll probably still wind up saving on your monthly bills even with the premium green electricity suppliers charge for their fossil-fuel-free electricity.