Most service guys do take the hourly approach plus primary material usage.
So $100 and hour plus primary material used, such as pipe, wire, etc. The $100 an hour covers time, fuel, small parts, tools, wear and tear, utilities, billing and customer service time, taxes, etc, etc, etc. This is a dead simple way of pricing your business but it takes quite a bit of time to figure out average costs to utilize the simple hourly billing approached.