electric heating elements for aquariums generate around 3 BTU's per watt . And a BTU is the amount of heat required to raise one pound of water 1 degree. water weighs around 8.33 lbs per gallon . I also don't like having just one heater in case of failure . They usually stick in the on position so like using a bunch of smaller ones like 300 watt each . I took all my electric heaters out and ran a coil off my boiler because I have heard so many stories of tanks being nuked by electric heaters either sticking on , voltage leaking or its contents leaking in to tank . But there is the formula