I'm with Andy on this one. Although water and electricity don't mix, I have never encountered an issue. Wiring problems are probably best uncovered at this stage, anyway.

Tips:
Get a can/bottle or two of a good degreaser. If you can be very careful not to get it on you or your plastic/wiring, oven cleaner is mega-powerful and works well on thick coatings of nasty oily kaka. Of course, it is nasty stuff so use gloves and hold your breath when spraying (not so bad after it has "worked" awhile) Common household dish detergent (liquid, not powder) is a marvelous grease cutter.
Get a couple scrub brushes you don't care to keep. One toilet bowl brush and a smaller dish-scrub brush (both with stiff plastic bristles) work well and are cheap.
Get the engine warm first.
Don't spary water with the engine running.
Covering distributor with bag and twist-tie when power washing is a very good idea.
Disconnecting battery NEG lead (once parked) also a very good idea.
Never had any kind of problem with the alternator. If you are worried, you can spritz it with WD-40 (that's what it was made for).

Honestly, brushing in a decent de-greaser is the best way to go as the power wand sprays nasty kaka everywhere... They work really well for the underside, however. I use them at a distance (a foot or two) for the rinsing operation in the engine bay.

The commercial ones don't have nearly the jet they used to because of legal troubles when people did stupid things and cut/injected themselves!

Mike!