Oh, that is an excellent question! I've been using the shower over the last couple of weeks, and each afternoon it's a little bit better. Still, the temperature is far from being warm. Also, right now it's cloudy and rainy, so it's not even warming up. And yes, in fact, we've been talking about building a solar panel for it ( black copper tubes under glass in an insulated box). We might get around to it, possibly as a side project along the earthship build.
Oh, and I'm so happy you liked the details of my post. What makes me appreciate this even more are the incredible circumstances I wrote and published it under. Yeah, my WiFi situation is really not the best here. So thanx!