I’m kinda out of the loop on this stuff, but it seems like suddenly out of nowhere everyone around me is converting natural gas heat into electric heat pumps—specifically a thing called a “mini-split” which can be both a heat pump in the winter and an air conditioner in the summer. It consists of one or more indoor units connected to one or more outdoor units by a small-ish (well, small compared to an air duct) tube of flowing liquid refrigerant (which transfers heat between the indoor and outdoor unit).

An electric heater converts 1W of electricity into 1W of heat. A heat pump converts 1W of electricity into >1W of heat, because it pulls heat from the outside air—it’s basically an air conditioner pointed backwards. The ratio of heat to electricity is called Coefficient of Performance (COP).

The COP for heat pumps gets worse (albeit still >1) as the outdoor temperature goes down, basically because the outdoor air is colder so it’s harder to extract heat from it.

I live in a pretty cold climate; I figure the weighted-average outdoor temperature (weighted by how much heating I'm using) is 30°F (~0°C). A quick search suggests maybe I can expect COP of 3-3.5. The highest figure I found anywhere for that outdoor temperature was 5. I could be wrong here. (Update: in this comment I used a different method based on the "HSPF" spec and concluded that I should definitely expect weighted-average-COP below 3.5 where I live, unless I messed up the calculation, which is entirely possible.)

Meanwhile I also need to account for my gas boiler not successfully transferring all the heat from "burning gas" to "warm interior" (as opposed to losing heat out the chimney or whatever). I think 80% is a reasonable guess for our not-particularly-well-tuned boiler. It should go up a bit, maybe towards 90%, when I get it tuned up or replaced at some point in the future, which I need to do anyway, it's super old and the plumber says it could catastrophically break any day.

I looked up my marginal electricity price and marginal gas price from my most recent utility bills, and did the unit conversions to a common unit of gigajoules (GJ). (I'm using marginal price because it's not feasible for me to rid of gas altogether, for other reasons.) It turns out that 1 GJ of electricity costs me almost 10× as much as 1 GJ of natural gas! (I fully expected the electricity to be more expensive...

Matthew Nolin

Matthew Nolin

Matthew Nolin

Matthew Nolin

Matthew Nolin

Matthew Nolin