The heat needed is eventually the heat radiated away.

The heat radiated away depends on

- size of the surface

- is the tank closed on all sides or open on top? (diff heat loss)

- on the amount of liquid in the tank (the lower part will loose more heat)

- the delta with the outside temperature. (yeah it is a differential equation in the end)

So proper insulation might help keeping costs (and CO_{2} emission) low

If the tank is full the contents is 1/4 * 80 ft * 80ft * PI * 85 ft ~ 3,200,000 gallons

That is quite large, is it for wine or ?

The outside surface is 80 ft *PI * 85 ft + 2 * 1/4 * 80 ft * 80ft * PI = 31,146 sqr ft

assuming top is closed