The charging system of any small engine, as well as the horsepower rating, are determined at wide open throttle.
The cooling supplied by the fan attached is maximized only at wide open throttle; remember that oil is the primary coolant in small engines so ALWAYS ensure you have enough in the crankcase. (DO NOT OVER FILL THE CRANKCASE)
In other words, with an electric start cold water machine and a smaller charging system you need to operate it at wide open throttle for optimal performance and maintainance of the battery charge.
The difference is the 12V units require a high amp charger because the blower motor runs off the battery. You need to be sending 14V to the battery to maintain the 12V supply. At ~11.7V the blower motor will slow down enough to change the burn and start sooting the coil. At ~10.8V the blower may stall out but the fuel pump could continue to put fuel into the chamber and potentially cause an unburnt fuel situation in the coil. This is why you should not idle down the engine on a 12V hot water machine.
In either case running an engine at wide open throttle ALL the time is the best bet for longevity and performance. If you want lower pressure then use a different tip; DO NOT idle down the engine nor adjust the unloader.
The unloader is a mechanical SAFETY device that is factory preset to prevent overpressuring the unit and to release excess into by-pass when the trigger gun is closed. Adjusting it will cause it to fail prematurely.
Pumps (well most of them) are positive displacement and will pump the same amount of water at a given RPM every time. The orifice in the tip provides the pressure; exactly like placing your thumb over the end of a garden hose. Be sure to not exceed the ratings of the pump, see the dealer for details.
Go to
http://www.giantpumps.com/nozzle_chart.html for a chart to determine how different tips affect pressure or flow. Or better still contact your local professional pressure washer dealer for assistance.