Some where on the ho****er system there should be a plate with the watts of the heater element, most heating elements in hot water systems are 2400Watts. If that is the case then around 3Kva generator.
The way it was put in the van I cannot see those figures, it is a 25 litre Rheems main pressure and was in van when we bought it, works fantastic when on power but we will be doing some free camping and wondering if it is worth buying a genie just to have a quick shower.
It shows the element is 2.4Kw which means you would need a 3Kva gennie. Apart from being bulky and heavy, that would be a pretty expensive way to have a hot shower.
Here is the cost of a 3Kva Honda generator (albeit the Honda is the dearest quite probably)
Seeing you have all the Rheem details ring Rheem and see if they make a 1600watt element, if so all you need to do is change the element and then you can buy a cheap 2000watt generator.
Seeing you have all the Rheem details ring Rheem and see if they make a 1600watt element, if so all you need to do is change the element and then you can buy a cheap 2000watt generator.