My chosen method, for warm to hot water, has been the 5 gallon Sunshower bag with the screw cap.
A large 'S' hook, Beefing up the bar on top, and an extension for the shower wand, and a method to hook the showerhead higher than the shower bag, are pretty key in my opinion.
Making use of the black dashboard facing south, can really heat the water quickly, placing it on black asphalt in direct sunlight, is like a 250+ watt heating pad.
And I do have a ~45 watt 12v heating pad I can and do place the bag on, either to slowly warm it, or maintain the heat. Well insulated, It does not use too much battery power overnight to maintain ~100F.
I get the black PVC always in the sun at roof height is appealing, but what happens in cold weather, or when one wants warm water in the morning, or in cloudy weather? How about filling it? Seems like a pain in the ass.
While i have only done so once or twice, I've draped the hot water bag over my legs when I was cold and threw it under my covers and slept with the darn thing too.
I am on my 3rd one of these in the last 8 years:
https://www.amazon.com/Advanced-Elements-gal-Summer-Shower/dp/B005CB7XCK
I've bought longer clear tubing and a barbed brass fitting to extend the wand length, replaced the plastic tube at top with a wood dowel, and reinforce this area with fiberglass saturated with 'amazing goop'.
The tearing of the bag at the top has been the demise of previous bags. The clear plastic does also turn cloudy with UV exposure and increases time to heat up water which is a bummer.
I have no issue throwing 5 gallons of showerbag on my raised fiberglass roof, but I know this is not within everyone's capabilities. I do hang the bag on my passenger side windshield visor too with door open, and just get a little lower to shower by passenger door, with rear barn door acting as somewhat of a privacy shield. The red rubber coated S hook is pretty key to hanging for both filling and showering ease.
There is more than one way to heat water for showering, this is just what has evolved in my usage as a van dwelling surfbum who can really appreciate warm water, in the morning.
I've been using this 12vDC heating pad:
https://www.amazon.com/ObboMed-SH-4...3_3?ie=UTF8&psc=1&refRID=EYPSGQWVQEESCQK6CMF6
Its footprint is larger than the summer shower. The actual wattage it consumes varies with the voltage it receives.
Bypass the Ciggy plug and splice in some fatter wiring, and one can get it to consume 55 watts at 12.8v and 62 to 65 watts at 13.8v. Experiments to lower the current via PWM, were failures resulting in much lower water temps but barely any less electrical consumption.
It will only heat to about 100F, and it does cycle on and off to keep it no higher than 100F or so.
On top of a few layers of reflectix and covered with light jackets and clothes, whatever is handy, it Consumes about 28AH of battery capacity to heat 5 gallons of 65F degree water to ~100F. Surely insulative improvements could be achieved to lower this basic number.
Previous to this 12v heating pad I used a 53 watt 120Vac heating pad powered by a 400 watt PSW inverter, and old school one without the Emm Effing safety timer. its footprint was smaller and it could get the water in excess of 114.5 degrees, which I find to be the limit of heat I can handle. It did consume more battery power too, and eventually failed.
I've been thinking some 12v seat heaters would be better suited for the task of heating water more efficiently:
https://www.amazon.com/Carbon-Fiber...75350872&sr=1-4&keywords=12v+seat+heater&th=1
Since I can consume electricity to heat this water, I have also been viewing hot water as additional energy storage. So I have ways to hold shower bag tightly to black south facing conversion van windows, or place it on asphalt, generally get it as hot as possible, and then insulate it when sun starts going down. it usually resides on front passenger seat, and I will place hot bag on heating pad and insulate it and plug in pad to maintain heat. If I get it over 100F directly from the sun or sun heated surfaces it is place upon, the heating pad rarely powers up and does not consume much battery power to keep it above 100F, but that depends on ambient temps and the insulation covering it.
A hot shower can really be a luxurious mood improver. So I try to keep some degree of hot water on hand, always.
Another nice thing about the shower bag is I have heated water on my stove and used a funnel to pour non boiling water into the bag, hanging via S hook on seat back, to get it to the 100 to 112f area.
