I wouldnt go any less than 18 with autos but lots of people run 24 with no issues. Nice thing about 24 is that if you live in a dangerous area, you are less likely to be identified by your power provider as having unusual electricity usage, as long as your bill isnt too high. Some providers run software now that will identify scheduled use that is unusual, like a 500 w draw on a timer, with the same on and off times daily. Better to use 500 watts continuously than have it turn on every day at 8am and off at midnight or whatever.... Thats suspicious!