Hey Late,
Welcome to the board.
If your planning on washing 2 or 3 houses a week, you will have the money pretty quick.
If you really think you can get two or three houses a week, go to Home Depot and buy a three hundred dollar cold water rig, then go to your farm supply store and buy a $150 shurflo type chemical sprayer. Mount the shurflo chemical sprayer on a home depot two wheel metal dolly. Mount a battery jump pack up above it like Ron said and your nearly in business. That is if Houses is all you plan to wash.
This is not exactly the best way to do it, but it is how a lot of us got started.
If it were me and I had it to do all over again, I would save up and do it right. If your Bank is able to float a loan, borrow enough to get a hotwater machine. You can do almost anything you need to do with a hotwater machine that has at least a 5 gallon per minute (gpm) and 3000 psi.
My suggestion through experience is to study this board like a good book you can't put down. Search, Search and research everything you can and ask questions when you don't understand.
Once you finally feel confident, buy the right equipment and do a lot of free work. Mom's driveway, your driveway, your sisters driveway and house, your in-laws house and try it out on anything you have permission to wash. You'll know when the time is right.
Once the time is right, make sure you learn how to price your jobs right. Don't low ball the power wash guy down the street, listen to everybody willing to help and never fail to give the new guy help if he asks for your opinion.
There is a lot of ways you can get started, but the main thing is learning the business before you hit the road. If you don't, you could regret it. This and other boards or design for people just like you and I and there is probably an average of 8 answers to ever one question you ask. You just have to use which one fits your situation and budget.
Have Fun. This is a great business if you have the stamina and back bone for it. Otherwise it can make an old man out of you really quick.
Anybody who tells you this is not hardwork and that there is no art to this business probably isn't a true Power Washer.
Remember, the word Power in the Power Washer doesn't always mean from the machine. I have found more musles than I ever knew I had. After your in it a while, you'll know where ever joint is in your body.
Take Care
Richard R.