personally I never like using a pressure washer, due to they have the ability to push water into places it shouldn;t be, and or blow out seals and such,
I think you are way better off removing the heavy build up by hand first, then buying some good cleaning solution , (still pay attention to electrical conncetions and plugs) spraying down machine, allowing it time to work and then using a basic garden hose
there are very good cleaners on the market now, that remove things and with less risk of pressure washer worries!
seen far too many folks have issues with using pressure washers to again recommend NOT using them
yes they do remove stuff, but often cause mroe issues than wanted!