I have an electric water heater (40 Gallon) that is hooked to 120V. 240V is
run to a shut off switch (fused) just above the heater (3 feet away) but for
some reason the installer ran 120 to the heater (black and white wires, the
red is disconnected, and yes I can measure voltage). The main fuse panel has
2, 15 amp fuses for the circuit.

I was thinking of changing the water heater to 240v for the purpose of
quicker recovery time. Is there a reason the person used only 120V? If I did
this what would the fuse size in the panel need to be?