Welcome to the forum.
Someone could write a book on starting out, calculating labor rates, mark up, dealing with customer issues, etc. just with all the information here. There are also a few books out there for the EC including for sale here.
Many have different tecneques to figure what to bill out for a job. There have been some long posts and disagreements about the matter. It can be hard road figuring it out. One one end you work for nothing and lose money, the other end it's hard to make a sale.
Without going into too much detail I will say this. Know your cost to do business. Most start-ups say the same thing they have no overhead. They are lieing to themselves. Some even have a hard time distingushing between personal and company money and costs.
An example of some costs for a part time start-up:
Insurance
local contractor registration fees
Business cards
Office stationary (paper, ink, envelopes, stamps, staples, lables, pens, etc.)
Fuel
Truck maintence
Truck registration
Cell phone
Busness phone line
Business fax line
Internet connection
New equipment (ladders, drills, cords, testers, etc)
Replacement tools (drill bits, lost, broken tools)
New inventory
Material not returnable (1/2 spools of wire, 1/2 boxes of fittinga, damaged parts, etc.)
Basic web site
Accountant
Before you know it you could be at $800 a month ($200 / week) with a nothing overhead and paying yourself $0. You miss a week of work and then you need to make up $400 the next week to cover a nothing overhead. Or only 1 job a month you need $800 to cover your OH. The above leaves nothing for advertising (besides cards), buying trucks, computers, desks, etc.
But the same plan if you worked and sold 20 days of labor would be $40/day.
I'm not tring to say what to charge, I don't have the real numbers. The point is OH cost per hour is relitive to the amount of hours billed. For a start-up you wont be working every day. For service calls efficency may be 4-6 hours billed in a 8 hour day in the truck. This does not even include office time when you get home.
To figure your costs you need to figure every expence for the year and then divide it down to the month, week, day, and billed hour. The reason you need to go by the year is there are many costs that happen once or a few times a year. Such as an accountant charging $500 for figuring you basic taxes, or $100 to do quarterly taxes. Truck registration for $75 once a year. City registration $50 onece per year. Etc.
It may seem like I'm listing small dollar items that don't matter to someone starting out. But that is where a combined portion of you money goes. Every month I would go to the office supply store and spend $50-$100 for a few small items. Every month there was an unexpected cost. Windshield broke, meter broke, had to pay an attorney for a small deal, material lost or stolen, too many minutes on cell phone, truck got stuck in snow and needed a tow, the list gos on.
