At 952 Heating Degree Days (HDD) this past January was the coldest month in the three years we've lived here. With the help of the wood stove, we only used 1012 kWh for heating (more on that later in this post).
We used a total of 2065 kWh so about 49% was for heating. The heating cost was about $91 (1012 kWh at about 9 cents per). All other items were pretty typical for winter use.
The Economics of Heating with Wood - Part 2
In December and January I burned roughly 1 face cord of mixed wood each month. As you might expect, both months showed a reduction in electric use. With two months of data I can now compare the results against past months that had similar heating loads.
First, I'll explain a bit about the two month experiment. If you're heating with propane (for example) the comparison is easy. You can just compare the cost of propane to the cost of an equivalent amount of firewood.
Liquid propane produces 91,500 BTU per gallon. If propane costs $1.60 then one therm (i.e. 100,000 BTUs) will cost about $2.19. Mixed firewood produces about 24,000,000 (yes million) BTUs per cord. If you can buy the firewood for $250.00 per cord that comes to about about $1.25 per therm. At these example prices the firewood is clearly less expensive than propane.
Note that firewood, propane, natural gas, etc will always produce a consistent amount of heat regardless of the outside temperature. In other words, a gallon of propane always delivers 91,500 BTU no matter how warm or cold it is outside.
Heat pumps, however, are more efficient at warmer temperatures and less efficient at colder temperatures. In simple terms, this means that the colder it gets, the more expensive it is to run. The following chart shows the efficiency of the Diakin mini-splits taken from the Diakin Engineering Manual.
The chart is a little hard to read but it is showing efficiency from 5 degrees F (on the left) up to 60 degrees on the right. The solid orange bars show the actual efficiency at the Diakin engineering test points of 5, 14, 23, 32, 43, 50 and 59 degrees. The efficiency is about 1.5 at 5 degrees and is an amazing 2.82 at 59 degrees.
The hollow orange bars are from my computer model. I have decreased the efficiency by about 10% in a rough attempt to account for defrost cycles.
The next chart shows the cost per therm at outside temperatures from 5 degrees to 60 degrees. This chart assumes that electricity costs about 9 cents per kWh and firewood costs $250 per cord.
At 25 degrees they are equal and at temperatures above 25 it is cheaper to run the Diakins. I should also point out that even at 5 degrees the Diakins are less expensive to operate than burning propane!
Based on the theoretical performance outlined above I decided I would use the stove only when the temperature is below 30 degrees. I used about .75 face cord in December and about 1 face cord in January (which was colder). We have a small pile left for February.
By comparing to previous months it appears that this saved between 30-40%.
For example, in January 2018 we had 952 HDD and used 1012 kWh. The closest previous month was January 2016 at 912 HDD and 1507 kWh. January 2018 was 5% colder but used 1/3 less energy. The wood stove saved abut 550 kWh.
December of 2017 showed similar results. December 2017 had 785 HDD and used 799 kWh compared to February 2016 where we had 747 HDD and consumed 1139 kWh. Here again the savings is about 1/3.
Although the results are good, it shows that we are only saving $45-50 per month from an amount of firewood that would cost about $80 if I had to buy it. I suspect that my free firewood is pretty low quality and that all oak or hickory would do better. In any case, it's a free resource so I will continue to use the woodstove in the future.