I wouldn't even think about this. There is no "Add-on Module" you can put in to do this. All the millions of old air conditioners will be of thousands of different types and you have to obtain, then redesign, the control circuits for each one.Inverter types control the power to the compressor to control temperature and conserve power once set temperature is reached. Older type just switch the compressor off and on to control temperature. The gains in efficiency can be considerable, up to 50%, but it would not be worthwhile converting old units.Incidentally my inverter unit - a few years old now - has inverter control on cooling, but on heating it still switches off and on.