I know some work out of a PU but for residential service calls I would recomend something bigger like a 12' box truck or step van. You never know what you need so it's good to have a good inventory.

There are so many electrical items that cost less then $1. If you don't have it you can waist an hour or more for that 1 thing. Or you find it but pay dearly for it. Things like trim plates, devices, garvins, finish blanks, fittings, screws, bolts, lags, togals, & cauck.

We cary togal / duplex and decora in ivory and white.
GFI's, dimmers, fan controls,
combo switches.
Light bulbs - many types
6" cans 6 New construction / 6 Remodel & trims
Pipe 300'+ 1/2, 100+ 3/4, few of other sizes
Tarps
Small Garbage can
Dust pan & 2 types of brooms
Most types of steel boxes
Fan boxes 4 pan cake, 2 easy brace, 4 bar hanger
Phone / CATV tools, wire, plates, etc
4',6',8',10' step ladder
Many types of breakers
We use Siemens panels so at least a case each of 15a & 20a, one of each 2 pole, and some minis
Pullchains, keyless
Lugs, split bolts
underground splice kits
grounding fittings
100-200' 3/8 & >100' 1/2 Flex & fittings
>50'1/2 & 3/4 sealtite & fittings

In the shop we keep:
metal stud / steel beam things
1 1/4, - 2 1/2" Nipples, & fittings (mostly 2")
Other residential service stuff
More cans
Larger wire
extra stock

Tom