The 2 main battery suckers on the Watch are screen and radios (wi-fi, bluetooth, cell). For the screen as its been said here darker colors with less screen area lit would consume less. For the radios/connectivity some complications pull data from the internet and they do that with background activity sessions so those complications would drain the battery the most. As for complications in general they are refreshed by the system for the most part and also via app background activity 4 times an hour for short period of time if needed. Overall the way Apple implemented complications is very power efficient (sometimes to the detriment of end users complaining of them being unresponsive) and should not have any discernible effect on battery life if they use on device data and dont fetch or push anything over Watch radios.