Btu is potential energy. It's defined as amount of heat required to raise the temperature of one pound of water by one degree... I believe they are measuring the amount of heat that this thing can put out. note little giant has a lower flow rating. This is saying at 5 to 7 gpm 130 is your rise. 500 psi with jets you may be using 1 to 2 gpm. I'll write more tmw when I'm on computer.