powerwashing...how much to charge
If you are starting out why don't you try a cost plus approach until you find what would be a good profit margin.
For example perhaps 25/hr in your area is not too high, make out a contract that states you'll be paid your hourly rate plus any materials you supply. Make sure to put in there that you are not resposible for existing conditions! Say if you're cleaning siding & hit a rotten board, it's best to have it in writing that you can't be held liable for replacing that board.
Once you get the feel of how long it will take for various projects you'll be able to get that down to a sq ft cost you'll be able to quote & use.
Another idea is to call around with say a 10x10 deck and ask what the others in your area would charge to clean it for restaining?