On Tuesday, January 19, 2016 at 1:59:21 PM UTC-5, Ian Field wrote:

A well insulated tank costs more to heat up from cold than keep topped up by
the thermostat.

Not as bad heating half a tank from cold, but I'm guessing it would still
cost slightly more.

Except the rate of heat loss is dependent on the delta T between inside and outside. Colder tanks lose less heat, if they haven't already reached minimum, than warm tanks being kept warm, so it takes less energy.