Before looking at more recent units, your high power usage may be a symptom of older and lower standards of installation, or unwanted leakage. How your home is constructed, insulated and orientated will also influence the heating/cooling load, hence power consumed.
A newer inverter to replace what you are saying is also an inverter is likely to make only a small difference to the power consumed. The power draw when the thermostat is off is mostly the circulating fan in operation. Dirty ducts, filters, directing vanes, and grills all create extra loadings on the fan.
For refrigerant air conditioners the efficiency is not quoted as a percentage. The effectiveness is expressed as a COP. There is a COP for cooling and a different and slightly higher COP for heating. The higher the COP the more energy efficient a refrigerant air con will be. You will need to find the COP for your existing unit to compare with any replacement.
There is a discussion of split systems in the following that may be of further interest.
