Since it's mostly plastic covers you're looking at under there, a damp rag will get the job done. That's what I do about once a month. I could do it more often, as the engine bay gets dirty fast, but it's only worth so much of my time. You can also use compressed air to blow all the loose bits(leaves and pine needles and the like) out of the all the hard to reach areas.
I wouldn't call it a "powerwash" but I have on many occasions just used the garden hose sprayer and gave everything a good spraying. It seems to work well. On previous gas cars I think there was so much oil residue under there that the hose didn't seem to have much effect because all of the dirt was just trapped. But on our Volt and Leaf I spray out the engine bay and it clears all the dust out just fine.
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