Our company charges our flat hourly rate 75/hour plus materials etc. We will charge a minimum of 1 hour typically (you don't want to make a person feel stupid by turning on a breaker). I have found that if you do that you should tighten "connections" or something or else they will not feel any value so try and fix something or you will never hear from them again.

It does not matter if you win or lose but how you lay the blame :-)