I can't really say because it is the original tank installed in my home. The way the tank is built however would lend me to believe that it is very efficient. It is basically one "tupperware" tank blown inside another and then the ~3 inch void between the two tanks is foamed. Also, because of the woodstove coil connected to the Marathon the water inside the tank can get very hot but the outside of the tank still feels normal to touch. I've also found out through my solar peers that the Marathon tanks were often used for storage in thermal solar designs.
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