What's An Inverter?

An inverter is basically a variable speed controller that allows the heat pump compressor to speed up and slow down as needed. This provides a low and high heating output or somewhere in between meaning only the energy needed is used. 

Older heat pumps or fixed speed systems as they are known, simply started and stoped constantly to control the temperature which is a good deal less energy efficient. An inverter will have a minimum and maximum output and a nominal capacity as well. Normally this is expressed something like as follows

5.0kW Cooling (1.4kW~5.8kW)
 6.0kW Heating (1.4kW~7.2kW)

A heat pump will always have a greater heating output than cooling because the electrical energy used to power it ends up as heat on the heating cycle but of course that's a negative on the cooling cycle.

Please note that model numbers of every brand of heat pumps always reflect the cooling capacity. Some manufacturers use model numbers reflecting the imperial measuring system (btu's) and some metric model numbers (kW) . A model number with 18 in it will be 18000 btu's or 5.0kW