Have been slightly racking my brains trying to remember the basics of this, but have a quick and probably pretty simple heat transfer problem. Was in a discussion Two standard 1000 liters square enclosed plastic HDPE containers (1m x 1m x 1m). One is filled up 800 liters (some air present at top) One is filled up only to 200 liters (lots of air up at top) Each tank is wrapped in a 2kW tank insulating heater blanket (around the sides). Tops and bottoms are also insulated (although no heat applied to those portions). What are the heat transfer physics at play here? Which tank will heat up the fastest, but more importantly, scientifically why? My prediction was that it would be about equal due to the fact that although the filled tank is more mass, there is more surface area being heated through conduction of HDPE to water.