i tried this system for a few months and it didn't work out for me. What a pain in the butt having to fire up the computer, type the bill in the truck, and waste 20 minutes at the end of every job. You save a step for sure and for a 1 man show it may be fine. I was always worried about the laptop being stolen, it took up so much room, sometimes the printer had issues and with more than 1 truck on the road unless you synched up the computers at the end of the day you have to enter all the info into a central office computer anyway.
Now we use this system - it works well for us.
A 2 part invoice where the electrician writes in detail job, location, description of work, labor, material, total price, notes, etc.
Then has the customer sign the invoice, gives them a copy, collects a check, turns in to office at end of day the 2nd copy with check and material slip from supply house if has one, and any notes to follow up.
Then it goes into the computer system in the office.
A lot of larger jobs are billed from our office anyway, so the guy in the truck also has a daily log where he records times, addresses, notes, materals, etc which he turns in to office as well. He doesn't need a computer for that anyway.
But, you should be able to buy a small power inverter at radio shack or box store to plug into the cigarette lighter to power AC.