Assuming, instead of 180sq ft, your is 500 sq ft instead with 1.5HP air cond. If you use Inverter, you will actually ended up using 10% more electricity than a non-inverter air cond. So using wrong size air cond will not save money.

---

Added on June 28, 2012, 10:40 am

I think you need over 30,000 BTU.

If you only fit one unit of 24000 or 27000btu, just get the non-inverter.

If you are going to fit 2 units of 18000 + 18000, inverter should work for you.

My 2 sen.