BEWARE major thread drift (since others have already started it )
It's not incredibly hard in theory to come up with justification for mini split versus wood, it is just very tedious to build up the spreadsheets. And depending on your spreadsheet skills you may need to spend some time relearning various built in functions, included the dreaded array functions (that take me hours to get right if I havent used them lately)
First thing is do a manual J calculation heat load calculation with outdoor temp as a variable and if the space is not heated 24/7 then consider adding indoor temp as a variable. There are multiple sites on the internet where you can download local temperature data usually from a nearby airport. You can get really detailed but it comes down to how many columns of data you want to generate. So lets assume you use a daily average ambient and disregard direct heating from the sun, its indirectly in there based on average ambient but its a stretch if a house is optimized for solar gain.
Now knowing the daily ambient you can do a look up table for a particular heat pump which is KWhr input for heat output. The best source of info is NEEP
https://ashp.neep.org/ They are big enough that they have
enough clout to get low temperature performance data that is not normally published.
https://ashp.neep.org/#!/ NEEP has an advanced calculator that does some of the heavy lifting based on accessing local weather data and it generates some real nice graphs but as far as I can see, there still is a need to upload the data to a spreadsheet and do the KWh calculations. Note that some companies do not participate in all Energy Star ratings, I was surprised that Mitsubishi does not appear to participate in the Energy Star V6.1 Cold Climate category (while Fujitsu and others do) so I skipped that category and grabbed an arbitrary Mitsubishi model and pasted it below.
For now, lets skip the cooling. Take a look at the COP which is the amount of electric power required to put out the heat load you need. You will notice it varies depending on load and outdoor temp. Keep in mind the COP for an electric heating is effectively 1.0 so the extra efficiency gained by heat pump is COP -1.0. A wood stoves COP is at best 0.8 and an old smoke dragon is probably 0.4. Luckily wood is cheap per BTU compared to electric power so COP is less important for wood at least initially. Keep in mind 1 kW= 3412 btu. These are also best case numbers, many cold climate units have electric pan heaters to keep from freezing up the outdoor unit and that is not counted in the COP.
Notice the kW line. It is showing you how many kW are required to put out the heat load you need. It is related to COP, so in the -13 F case to get 2,830 Btu you are putting in 0.54 kW. For an hour of heat that is 0.54kW/hr. Notice that if you need more heat, the COP goes up, so the unit looks to be more efficient. I (could really confuse things at this point on how the efficiency is calculated but lets not and save that for the advanced course)
. Ultimately you need to plug in a KW demand for your load that is accurate for the ambient temperature.
With this variable efficiency you need to figure how you are going to deal with what happens when your unit runs out of capacity. So now your spreadsheet needs to have some "IF" statements on how you want to split the heating loads between multiple units or do what most smart designers do, write off really cold temperature operation and use a backup heating source.
I know I am pissing into the wind on this compared to the PR being pumped out to the public but not many folks are willing to pay an extra $5 to $6K each for a one or two extra mini splits to cover the coldest temps. Even if they do, they are probably paying a premium for power in the winter (obviously depends on who you buy your power from). Down at those low temps, those higher electric costs may offset any savings running multiple heat pumps. Far better to crank up the backup fuel paid for by the savings of not buying the extra units. In a cold area like where I live, I would need four 17,000 Btu units to heat my house in the coldest temps and that is lot of power My 5 to 6 K was for a 12,000 BTU unit so lets use 7K for swag at cost per unit. I can heat my house a long time on the $21,000 needed to buy 3 additional units to cover my worst case load especially if I am burning wood. I need backup as my ambient temps go below -13 F on occassion but most folks live farther south so its not a consideration.
On the other hand, I have a 12,000 Btu rated mini split and that covers a lot of my shoulder season heating load down to about 20 F overnight temps ( I have a small house and set the temps way back on my second floor at night), it is an older cold climate unit and I do not even have a rating sheet as NEEP was not around. My wood boiler with storage is not very efficient during shoulder seasons, as I need to heat up a lot of mass and water to heat up the storage. The storage has pretty good insulation but the 1000 pound plus steel boiler and 40 gallons of water (320) pounds inside it is effectively not insulated. The boiler is in a basement so everytime I heat up the storage, I am effectively throwing away a portion of that 1320 pounds for warming up the ground and floor outside the basement. The other aspect is my house is somewhat well optimized for day time solar heat gain, I have some terrain blocking my house at low winter sun angles so I do not get full sun until 9 AM this time of year and I lose it around 3:30 PM but I do get 6 hours of free heat where I really do not need auxiliary heat if the temps are over 30 F and the sun is out. I do have R5 double cellular blinds with side seals on almost all the double pane windows which I close at night but around 5 PM its time for auxiliary heat, so on those days I will probably run the minisplit during the day and keep the storage warmer until the evening when I am normally recharging it.
Going back to the table, take a look at COP at load heating load versus high between 47 and 17 F. Sure the COP is great if its idling but when cranked up it has gone way down. Still twice as efficient at electric baseboard but keep in mind the colder it gets the more time the units going to run at high output.
It is important to note that woodstoves are going to have similar variations in efficiency. At steady state operation they do have some limited turn down range where the efficiency stays up but during start up and shut down they are far less efficient. If the stove can idle along during long periods of no heat demand like my 6 hours a day, that is great but I expect the efficiency at idle is zero during that time if there is no need for the heat.
So feel free to run the calcs, the data is available and for a spreadsheet guru its probably an interesting challenge. as far as I am concerned, I will keep my minisplit on my main floor and am considering replacing a 15 year old cooling only minisplit on my second floor to a cold climate unit and running my wood boiler when the heating demand goes up. The strange thing is, I have surplus solar credit so I could probably heat full time down to -13F with minisplits but I hate how they run down at low temps. Defrost cycles happen more often and the heat coming out the register is cooler than forced hot air. Yes its warm but not hot and the air flow and noise is higher. I also notice far more room stratification with warm ceilings and cold floors. I know I personally have to keep the temperature setting higher with the minisplit on than if I am running baseboard.
