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)
There are many units of measurement for energy (ex kWh, GJ, BTU, Therms, etc). For ease of comparison, I’ve used kWh for both electric and gas, even though gas is more commonly quoted in GJ. Notice I’ve included two columns for gas. The one with values “per kWh used” takes into account the efficiency of my gas heating system. Electricity is inherently 100% efficient (or close enough that system losses are negligible). This isn’t true of combustion based heating systems. Some of the energy consumed simply goes up the chimney and never heats your home. Even if a gas furnace were 100% efficient, gas heating systems tend to route heating ducts through non-living spaces like crawlspaces and attics, resulting in additional losses that electrical space heating systems don’t experience. I estimated my gas heating system efficiency at around 60%. Electrical CO2 emissions in the above chart were determined by referencing the data at Carbon Monitoring for Action (CARMA) which gives CO2 emissions for major power companies around the world. You can search for your power provider and see exactly how green your power is. My power provider is BC Hydro. CARMA data shows BC Hydro currently emits 285,620,000 kg of CO2 per year and produces 52,400,000,000 kWh of electricity. Thats 0.0055 kg/kWh.Combustion of natural gas produces around 58 kg of CO2 per GJ. One GJ is equivalent to 277.8 kWh. Therefore combustion of natural gas produces about 0.2 kg of CO2 per kWh consumed. I divided this by my estimated efficiency of 60% to get 0.33 kg of CO2 per kWh used.Costs in the above table are based on actual statements for my home over one full year (total cost divided by total kWh consumed). This is more accurate that relying on figures quoted by gas or electrical companies which may not include hidden costs. To get gas cost per kWh used, I divided the gas cost per kWh consumed by my estimated efficiency of 60%. These results indicate that for my home, heating with electricity is best in terms of CO2 emissions and about the same in terms of cost. Installing a more efficient gas furnace would likely bring the cost per kWh of gas below that of electricity, but the CO2 emissions would still be much higher. A better alternative could be to install a ground source heat pump. This would be even more efficient than straight electric heat and would produce even less CO2 per kWh used. This is something I’ve been considering. The main barrier is the large capital investment.
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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
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.
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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.