[probsolver]

Hey 12vman, awesome job putting some calculations from Gary in. Builditsolar is a great resource!!
If you use his calculations, and super-insulate your tank (we're not talking about an attic here), you could heat 30 gallons from 52 to 120 at only 82% efficiency.
Insulation (heavy) is the key and made possible only after and because the vent on top is no longer used.

And yes to the next post . . . any simple solar pre-heater would go along way just as 12vman mentioned because it takes less energy to heat wamr water than it does to heat cold.
You don't have to mortgage the house to make an improvement, or make a difference.