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The math is pretty simple using BTUs and ΔTs.
Shower is 110°, groundwater is 50° so ΔT is 60°.
8.3 pounds of water per gallon x 2½ GPM max x 60° x 60 minutes = 75 MBH
3413 watts / MBH if you want to calc the cost of an electric shower.
Anyway, the drain would probably only be about 80° on average over the course of a shower depending on how much mass is there like granite tiling that will absorb BTUs before it goes down the drain. Plastic showers would be best here. Anyway, with the incoming water temperature of 50° that gives a ΔT of 30° x 8.3 x 2.5 x 60 min which is 37½ MBH. Half the ΔT, same flow half the BTUs required.
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