revolutions

I was thinking about carbon emissions and calculated some rough estimates for our house.

Between December 11, 2020 and December 12, 2021, we used 2031 cubic metres of “natural gas” (mostly methane) to heat our house and provide domestic hot water. (The heated area is about 120 square metres, the house is double brick and largely uninsulated.)

This seems to correspond to about 21 MWh or 75 GJ of energy, and as a rough approximation 4400 kg of direct carbon dioxide-equivalent (CO₂eq) emissions. (Numbers rounded due to uncertainty in conversion rates, emissions values, and exact properties of gas being delivered to our house. Extracting and transporting the gas also causes emissions, but this seems to be at least within order of magnitude.) Our gas bills for this period were about $1000 (all dollar figures CAD).

Electricity Map / Tomorrow gives CO₂eq emissions goal to meet Paris Agreement objectives of limiting global warming to 2°C by 2100 as below 5 tons per person per year by 2030, and below 2 tons by 2050. Atmosfair gives 1.5 tons CO₂ per person per year until 2050 for a 1.5°C warming goal.

With three people in the house, our per-capita emissions from burning gas are around 1.5 tons, using up 30% of our 2030 yearly carbon budget and most to all of our 2050 yearly carbon budget.

At a very simplified glance, replacing gas with purely resistive electric heating would drop emissions by at least half, and possibly up to five times: Ontario’s electricity carbon intensity usually varies between 30 and 100 kg CO₂eq per MWh, depending on grid load and how windy it is. (Currently highest intensity is during summer due to cooling load, but with a wider shift to electric heating the winter peak might also approach these levels.) 21 MWh of electricity would then cause about 600 to 2000 kg of emissions for the house per year, or about 2.5 to 3.5 tons CO₂eq less than gas.

With our effective electricity price of about 12 cents per kWh (excluding flat fees, but including per-kWh delivery and taxes), and effective gas price of about 43 cents per m³ or 4 cents per kWh, swapping to purely resistive electric heat would cost us a premium of about 8 cents per kWh, and higher use would push us into a higher price tier adding another 2 cents per kWh. 10 cents per kWh would then add up to $2100 extra per year, or average of $175 per month. Roughly calculated, this is about $0.70 per kg of CO₂eq avoided.

Of course, there are much better options than purely resistive electric heat. Heat pumps aren’t a perfect fit for our house, because our old cast-iron radiators work best with high water temperatures, and heat pumps work best with lower water temperatures. Ontario’s winter low temperatures can also push air-source heat pumps to limits of their efficiency. Ground-source heat pumps are an option, but at higher capital costs and substantially more installation work. But on the whole, it should probably be feasible to do all domestic hot water heating (if using a tank heater) and probably about half to two-thirds of space heating using electric heat pumps, supplementing with resistive electric when it’s particularly cold.

Outside of heating season, our use — which was then entirely domestic hot water — seems to average out to about 1.5 m³ per day, which is about 15 kWh or 54 MJ per day. Since our hot water use doesn’t change much over seasons, our yearly hot water use seems to be about 5.5 MWh or 20 GJ, meaning about 15.5 MWh or 55 GJ is used for heating.

A basic heat pump used for space heating would likely be about 3 times more efficient than resistive heating. (Assuming HSPF 9.8 for a system based on a MXZ-3C24NA2, which is primarily aimed at cooling, but this should be a decent initial reference. Resistive electric has HSPF 3.4.)

Using a heat pump like this to provide 5.5 MWh for domestic hot water and two-thirds of 15.5 MWh for space heating would then use about 5.3 MWh of electricity, and the remaining one-third of space heating would add about 5 MWh, for a total of roughly 10 MWh of electricity per year. This would bring our house electricity emissions down to about 300 to 1000 kg CO₂eq per year (3.5 to 4 tons less than gas), and our cost premium compared to natural gas to about $1000 per year, or average of $83 per month. The operating cost per kg of CO₂eq avoided is about $0.27. (There would also be a capital cost of the new devices and installation.)

For further comparison, carbon offsets from a respectable offset provider like Atmosfair cost about 23 euro per 1000 kg of CO₂, which is about $31, which is about 3 cents per kg offset, or about nine times cheaper. But we’ll have to lower our own emissions at some point. A realistic solution would improve our house insulation, sealing, and heating efficiency before diving into heat pumps, but this post offers some baseline numbers for more comparison later.