Not sure how but the top of my engine(mostly the battery/electric motor) is dirty. I have about 20,

you should see mine! i even have pebbles sitting atop my engine. since i have under covers i can only guess it is coming through the grill.
i want to wash but worried about water in the componets. i just brush it off. no problems to date.

 

Brush it off with an old paint brush? You can also use a shop vac on blow, to blow junk out of the corners. (or my favourite, a leaf blower ) One of those cat tail type duster wands is good too.

I'd avoid spraying it with any sort of liquids, just my 2 cents.

Assuming it's similar to a regular Prius, you can also pull off the ornamental engine cover, clean behind that.

 

Low pressure water from the garden hose will hurt nothing. I've done it twice a year since new. What do you think happens in there when it rains. Just don't go to the self serve car wash and blast every nook and cranny with high pressure and you will be fine.

 

I usually wipe my engine bay and accessories clean every car wash or every other. That way there is never any major buildup / gunk that will have to be dealt with later on. Minor maintenance goes a long way.

 

Here's mine after 10K miles. Just wipe it down every once in awhile (every other car wash) and it will still be like new. Definitely helps when you sell the car private party down the road.

I've had cars at 200k that look showroom clean in the engine bay. I've got california weather to thank for that too.