The answer depends upon your climate and house size. 18,000 BTU/hr for heat is a pittance where I live. Western NY. (6600 Heating Degree days per season is typical.) In this area typical homes, 2000 sq ft, have at least 75000 BTU/hr for heat and either one or two 5000 BTU/hr air conditioners for the 4 days of the year when we need it. Our large wood burning stove is rated at 73000 BTU but it will only keep my house comfortable if the temp stays near freezing or above. If it gets below that, I fire up the second stove or let my oil furnace kick on.
If you live in Tennessee with heating and cooling seasons near 2200 HDD and CDD respectively, that size unit might be large enough if you have some back up for extreme days, like a wood burning stove, gas heat, electric space heater, etc.
If you live in a warmer climate see if you can install the compressor/heat exchanger unit on the shade (cooler) side of the house. If you live in a colder climate install it on the sunny, warmer, side of the house.
Remember, they get more efficient when the temperature difference between outside and inside is not great.