i wouldnt bother unless your going to be getting it installed into a new van (that your planning in keeping for at least 5 years or so)as its expensive for the heater/charging units/batteries/ fitting and running costs(around £100 a month on diesel)
however i love mine now...no worries about hoses/pumps freezing on cold nights as the frost stat takes care of that and obviously working with hot water is better than cold water window cleaning.easier scrubbing and rinsing,supple warm hoses and the windows dry quicker...
having used DIY L5 and L10 gas shower heaters before i reckon a professionally installed diesel heater is much better,more consistent heat and a lot safer....it looks cool too if you get the retro fit from grippa...
id go for the 9kw heater mate.dont forget in winter on very cold days the water at brush head is only around 25-30c with lots of hose out on freezing ground(and thats with the 9kw heater on full whack).which is fine...
obviously when it warms up that heat rises dramatically to 45-60c which is super duper for the spring/summer months for all that bird muck,slug trails,pollen spots,bug poo,etc,etc that we all get on the glass and add on jobs are easier to clean....