I’ve become the token environmentalist in my family, and I was recently asked about the impact of buying an outdoor heater.

There are two questions:

  1. If you buy a heater, what’s the relative impact of buying propane vs electric?
  2. How bad is the total impact?

First, let’s understand where the impact comes from.

Direct heat output

For both propane and electric, you are putting heat directly into the atmosphere. In the climate change literature this is known as “waste heat” or “anthropogenic heat flux” (AHF). This seems to be a relatively small effect on a global scale, but possibly more relevant in cities and over short time scales. Overall about 1% of humanity’s climate forcing is due to waste heat. So we’ll ignore its effects here.1


The propane heater burns propane to generate heat. When you burn propane, you release CO2 into the atmosphere, so it’s just a question of how much you’re burning. The propane heaters we were looking at generate about 40k BTUs/hour2, and according to the EIA, propane produces 139 lb CO2 per million BTU. So these heaters generate 5.7 lb CO2/hour.


Electric heaters are slightly more difficult to compute, because it depends on where you get your electricity! If you’re just pulling from the grid, the EPA has a tool where you put in your ZIP code and it gives you your average CO2 intensity and mix of sources. The house in question is in the New England (NEWE) region, which is powered mostly by natural gas and nuclear and has an average emission rate of 522 lb CO2/ MWh. If we were in a region with more renewables, we might need to think about time of day as well, but we’ll ignore that here.

The electric heat lamps use about 1500 W (5100 BTU/hr), so we get about 0.8 lb CO2/hour, or 14% the emissions of the propane heater.


How bad is this? The relevant comparison for almost everyone is driving. Burning 1 gallon of gasoline creates 19.6 lb of CO2. So you’d have to run the propane heater for 4 hours, or the electric heater for 28 hours, to have the same impact as burning a gallon of gas.

If they work equally well, buy the electric one. But either way this seems relatively small to me, as long as you’re using it infrequently. If you don’t worry about the environmental impacts of having people drive to your house from 30 minutes away, then this shouldn’t bother you on the margins either.

  1. I remain confused about how the climate scientists think about this. The paper that seems to be most-cited measures waste heat in terms of W/m^2 across the planet’s surface. Watts are a rate of energy transfer (joules/s) so this is measuring how much the flow of waste heat, rather than its cumulative effects over time, which I think is what you would really care about. I couldn’t find anything that expressed waste heat in terms of CO2-equivalent over longer timescales. 

  2. Heating and cooling devices typically list their “BTU” output, but BTU is actually just a unit of energy, like Joules. Apparently the convention is that they mean BTU/hr. This is stupid.