I am looking at different water heaters to supply my darkroom ... I have ruled out connecting to the house gas hot water system because of the extra water heating costs. So i have settled on a mains water supplied instantaneous electric type - either a shower unit or a handwash heater (like the type occasionally seen in public conveniences or workplace type wash basins).

Wiring in a shower unit seems to be relatively complicated for me in that I have no extra space for new residual current breakers in my fuse box, so will need to get the electricity board out to fit new stuff all costing +++! So, i need to know whether I "need" to have a similar set up with a hand wash type water heater? I cant find any info about fitting these.

How have other darkroom owners supplied heated water to their darkrooms?