Calculating comparative heating costs
I live in Richmond, British Columbia, Canada and have basically two choices for heating my home: natural gas, or electric. My home has been heated with natural gas since it was built about 30 years ago. I recently did a calculation comparing electric space heating to natural gas to see if it might be worth switching. I was interested in comparing both cost, and greenhouse gas emissions. Here is what I found out. These results are specific to my home and my region.
|Electric (per kWh consumed)||Gas (per kWh consumed)||Gas (per kWh used)|
|CO2||0.0055 kg||0.2 kg||0.33 kg|
Experimenting with electric space heating
Empirical data always trumps calculations, so I performed a simple experiment over a couple of consecutive winters to test the theory.
My home was originally built with a natural gas furnace and forced air heating system. In the winter of 2006/2007 I heated my home “normally” with my gas furnace (plus the heat from electrical equipment in my home). In the winter of 2007/2008 I turned my furnace off completely and heated my home entirely using electric space heaters. I do not use significant electric power outside the home (no Christmas lights for me) so it’s a reasonable assumption that all my electrical usage ends up as heat inside my home. What little gas usage is shown in the second chart was for my hot water heater. The reason I compared only winter months is so that I could be certain all the windows and doors were closed for both tests.
The data below shows the results of my experiment.
These first two charts show average continuous power consumption in kW. I determined the average power consumption in each month by taking the total energy consumed in kWh and dividing it by the total time in hours. Power is a more useful measurement than total energy consumption because, in theory, the temperature difference you maintain between the inside of your home and the outside should be directly proportional to your continuous power consumption. The two charts seem to indicate strongly that heating my home with electricity is more efficient than with gas. It clearly required less power to maintain the interior temperature in the winter of 2007/2008. However, these charts don’t tell the whole story since the temperature difference being maintained could have been different between the two years. The data can be normalized by looking at the temperature difference (T_inside – T_outside) per kW of continuous power consumption. The average outside temperature during each month is provided on my gas statement, and I monitor the inside temperature myself. Here is a plot of temperature difference per kW of continuous power consumption for both winters.
Â°C/kW is technically a measure of the thermal resistance of my home’s building envelope. It should be roughly constant over all months since it is a property of the materials and geometry of my home. However it isn’t constant in the chart. The reason is that there is an additional heat source, solar, that I haven’t accounted for. Solar input power is the reason both lines rise at either end of the chart. Higher solar input in these months results in less power being required from electrical or gas sources to maintain a given temperature difference.
To reduce solar effects that might vary from one year to the next, lets compare the data in December. Using only electric heat, I could maintain a temperature difference of approximately 4Â°C/kW. Using mostly gas heat, I could maintain a temperature difference of only 2Â°C/kW. Therefore, my above estimated gas heating efficiency of 60% was actually high. This data seems to indicate a gas heating efficiency of less than 50%.
My total energy consumption and cost for these two consecutive years agree with this assessment:
|12 months ending May, 2007||12 months ending May, 2008||Change|
|Gas consumed||24393 kWh||2522 kWh||-21871 kWh|
|Electricity consumed||10900 kWh||19765 kWh||+8865 kWh|
|Total Cost||$1973 CAD||$1616 CAD||-$357 CAD|
I was able to replace the loss of 21871 kWh of gas consumption with only 8865 kWh of additional electrical consumption. Assuming similar temperature differences being maintained in each year, that indicates a gas heating efficiency of only around 40% compared to electric space heating. Note then that my “real” cost of heating with gas (without replacing my furnace) is around $0.1075/kWh (that’s kWh into my home and not up my chimney) compared to $0.071/kWh for electric. Note also that the $357 difference represents about an 18% savings and more than covered the cost of the space heaters I purchased.
Replacing my current gas furnace with a higher efficiency gas furnace would likely make heating with gas more economic than heating with hydro. However, taking into account the capital investment of a new furnace, the additional greenhouse gas emissions, and the fact that gas prices are rising faster than hydro, I will happily continue heating my home with electric space heaters for the time being.
Having made that decision, my only gas appliance in the house is now my hot water tank, which is also a horribly inefficient beast with an open chimney and a continuous pilot light. Converting that to electric will not only reduce my water heating costs, but will also allow me to cancel my gas account saving me about $120 per year that the gas company bills regardless of whether I consume any gas. I’ve devised a plan for converting my current tank to an electrically heated one on a timer. Expect that project to be posted soon.
I converted my gas hot water tank to electric. See how I did it here: Convert your gas hot water tank to electric.