1. A system comprising: a compressor operable in a refrigeration circuit and including a motor; a first sensor detecting current supplied to said motor; and processing circuitry storing a first relationship between a current variable and a condenser temperature variable and a second relationship between a compressor capacity variable and said condenser temperature variable, said processing circuitry using current data from said first sensor to determine a condenser temperature value based on said first relationship and using said condenser temperature value determined based on said first relationship to determine a capacity value of said compressor based on said second relationship.