My units are 220v. The larger BTUh units all have to use 220v, due to their higher power usage.
Units rated for 12,000 BTU or less can run on 120vac, but some use 220, so they can have
common parts with the larger BTU units in the same product line.
Plus, people in other parts of the world use nothing but 220..
And it's easier to build them with just one type of power supply..
If the 18,000 has good specs, you could use it. If your living space ever needed a full
12,000 BTUh to keep it comfy, that extra 6,000 BTU would be good to have,
just in case..
My den never gets real cold when we BBQ and we are in and outdoors 20 times..
That 36,0000 BTUh can handle it easily..
Here's some basic ASHP info.. A mini-split has some small differences, but the theory is the same..
http://youtu.be/14MmsNPtn6U
Here's how they say it in the UK..
http://youtu.be/g39nM7GbSJA