A heat pump is actually the reverse of a heat engine. A heat engine uses the difference in temperature to generate work. A heat pump uses a motor to move heat in the opposite direction. The ideal Carnot cycle is reversible!

if the cost of gas ($/therm)/29.3 is greater than ($/kWh) / COP, you should go with a heat pump otherwise use gas. COP is how much heat you get per unit of input.
1 therm of gas = 29.3 kWh. Not sure why we’re still using British imperial units.

Note that a forced air furnace uses about .4 kWh for the fan in addition, and it’s not included above.

The minisplit I have in my shop does about a 10 minute or so defrost cycle. The nice part is that it’s a hot gas defrost so no cold air is blown inside. Mine is a Zokop which is also sold as Traiden and many other brands. It’s a 19 seer unit.
I need to look at it again but I think there is a way I can either disable the defrost or lengthen the time between them. It kept the shop at 65* in the 14* nights and warmer during the day when I was working out there. (that was before I found the low temp heat mode) There has never ever been any sign of ice of any amount at all on the fins. A little on the fan but not on the fins.

The issue with conventional US built systems is that they don’t use hot gas but revert to AC mode.

But in a cooling dominated climate solar peak matches cooling peak or at least get a start on providing nighttime cooling.

In cooling dominated climates they should be located in a garage. This is very common in Florida.
Basement would be the “worst” place. Worst as in not ideal.
Large utility room on a living floor but these units have been known to have noise complaints so YMMV.

In a warming dominated climate I would probably skip it unless you have a quiet room in a conditioned area of the home. Just remember that some of the heat generated to warm the house will be used to heat the hot water heater so there’s a small conflict there where you’re paying twice to heat your water ($$ to heat the house, $$ to pull that heat from the house into the water tank).

My two cents.

Heat stripes in the Air Handler

I would be interested to know how heat pump efficiency varies with the delta T from room temperature. Also at what outside temperature is a heat pump simply not practical, either because it would need to be excessively large or so inefficient that it consumes enough electricity to negate its advantages.

When I installed our HVAC I put in a gas furnace as gas was the cheapest to heat with at the time. But the coil I used can be run with a heat pump condenser with a different expansion valve. Now today I would like to swap out the regular condenser for a heat pump and move it to the side of the house vs the back yard so the back yard is quieter. And also the newer scrolls are quieter and have les vibration than the ones from a couple years ago. Many scroll compressors have an issue where they impart a vibration through the lineset. There are bandaids like flexible linesets at the condenser but I’d rather swap it all out at some point since now a heat pump is cheaper to run than gas but I still have the gas backup.
Always have a backup source of heat and cooling for your house. Never depend on one system. It often can take a week to get a programmed board from Trane. Do you want to wait that long without HVAC…

I also have a 1 ton Minisplit in my shop. It’s to supplement the 2 ton window unit in the summer. Over time the window unit will be replaced with another minisplit. Heating so far it’s wonderful and very cheap to operate. We will see how it does this weekend when it’s down to 7*.