[jeff5may]

To answer your question, an air source mini split will level your heating and cooling bills.

Without improving your home at all regarding energy efficiency or leakage, a 3/4 ton unit will do a little more cooling than your window shaker a/c unit. How much less it costs to run depends on the unit. A window unit usually comes in around 10 SEER or less on the energy scale, so a higher SEER mini-split will either run less often (saving money) or cool more of your home (for the same price).

During heating season, you can predict how low the unit will go regarding outdoor temperature by relating to the current window a/c unit's summer performance. For example, if the current unit does a good job up to 98 degF during the summer, the mini-split will do roughly the same at heating down to around 38 degF, asuuming a setpoint of 68-70 degF. Below this temperature, a "cheaper" constant-speed outdoor unit will begin to drop off and may not satisfy your heating demands. A variable-speed "inverter" unit may or may not keep up with demand, even if its heating capacity does not drop off. Depending on the home, the heat may leak out faster than you can afford to supply it. A larger unit may or may not be necessary to avoid using backup heating.

Right now is the best time of year to do your own site survey. Shut the house up tight and turn on an exhaust fan (bathroom, range hood, etc.). Light up a stick of incense and start checking windows and doors first. If you have a large leak, you won't need incense to find it: the cold airflow will be obvious. Rooms that are colder than the rest of the house (unless kept that way) are that way because they are leaking heat. And that heat costs money.