But a 120 v. electric water heater would have such a pathetic recovery rate that the tank should be larger to have some chance at providing enough capacity.
". . . when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; . . ." Sir William Thomson (Lord Kelvin)
So let's put a some numbers to it.
bigdcarter said he needed to support 2 or 3 showers per day in the summer.
Putting in a 240 Volt circuit is a good idea if bigdcarter wants to run a 30 Amp circuit to the "glorified shed", with enough additional power to run the rest of the house. But if he doesn't want to run that circuit then it can probably meet his needs with a 120 Volt circuit.
Putting 120 Volts on a 240 Volt heater gives about 7.6 gallons per hour at 60 degrees temperature rise. That is a reasonable temperature rise for the summer. If he sets the thermostat at 150 F it will store the same energy above 60 degrees in the water as a 60 gallon heater at 120 F.
That is enough to provide three 5-minute showers at 2.5 GPM, and more if one uses low-flow shower heads.
It can provide even more hot water over the day, since 7.5 gallons per hour x 24 hours is 180 gallons. That is more than most people use.
A point of use heater is a small tank unit (2.4 to 4 gallons) that is suitable for a sink but not very good for a shower.
An instant heater would be completely unsatisfactory. You might get
1 QUART per minute on a 120 Volt circuit.
