So… Shower with no power. Instant gas heaters are the best option. Cheap to run, steady temp, no electricity needed.
Well, once in one place I rented I had to change the 9v battery that was used to light/spark it, but otherwise, no electricity needed at all.
Don’t have household gas? Just connect it to a gas bbq bottle. For me, I had to swap it every month, but for a family, I’d probably get a bigger canister. They’re all fairly cheap to refill-swap.