Why is a mini split more efficient?
Although they're relatively new in adoption to the U.S., mini split air conditioners (and heat pumps) have been around for 50+ years in Asia. Why? American families traditionally aspire to live in large, separated houses. More folks in Asia live in more densely-populated settings like high-rise apartment complexes where there wasn't the ability, or need for, installing a whole-house unit. So, these mini split units were developed and refined in Japan. But mini splits are surging in popularity in the U.S. now since they are more efficient than whole-house units in 3 primary areas...
1. Mini splits focus heating or cooling only on one room, and as such, they allow homeowners to keep their main unit at a higher/warmer thermostat setting in the summer or a cooler/lower thermostat setting in the winter (assuming that they are heat pumps--able to do both heating and cooling). Or, that one room may indeed be an area that had little or no cooling in the past, such as a garage.
2. Mini splits don't have duct work, and thus avoid duct losses, estimated to be north of 20% in most homes. These losses come from both true leaks in the system (from old adhesives splicing together duct joints, for instance, drying out and becoming unsealed) and from friction/transport losses of the air. Mini splits take air from the room at which the inside unit is installed and processes that air for cooling or heating on the spot--no such losses.
3. Mini splits now very often come with variable-speed compressors and fan motors. Why is this important? First, it avoids the surge/inrush current of on/off systems, which is typically 4 - 6x higher than the running current. And, more importantly, variable speed means that the unit is designed to slow down when it approaches your setpoint. It doesn't jolt past it, sit around for 10 minutes, and then come on to jolt past it one more time, making you go from too hot to too cold. These variable-speed mini splits run more, maybe even all of the time in the hottest or coldest months, but at slower speeds using less electricity. So, double goodness of more comfort and less energy used.
As the installation of these units becomes easier and as prices come down, unfortunately in concert with utility prices for most of the country increasing, their prevalence is bound to increase.