Self-sufficient and autonomous robots necessarily make their own decisions. As designers of robots we need ways of assessing their behaviour. A functional method, called the basic cycle which is based on the utility of the robot owner, is proposed to assess the robot's performance within an environmental niche. The method allows judgements to be made on the robot's use of time or energy, as well as being able to compare alternative robots' performance. A mechanistic method of implementing robot behaviour sequencing - having reason to perform comparably to a utility maximising method - is proposed, and demonstrated to behave opportunistically in both a computer simulation and an actual robot experiment.