To give you a correct answer we need a few more details – you say you are going to produce 400 to 600 watts of power that is great but at what voltage?
600 watts of power is about what is used in a domestic twin slice toaster.
Most water heating elements are rated in Kilowatts which is as I am sure you already know 1000 watts, so given that fact you are not going to be able to purchase a NORMAL conventional water heating element commonly installed in a domestic electric water heater with such a low rated element.
Now having said that you might want to check some of the pet shops out that sell water heating elements for fish tanks – they may have them for the wattage you are playing with.
Hope this helps.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians