For each watt of energy consumed, the fixture will emit 3.4144 BTU's per hour. Keep in mind that S4 and S4 Jr fixtures absorb more of the infrared light spectrum than most typical fixtures, and therefore transmit less heat to the stage.
so...
a 575 HPL * 3.4144 = 1963.28 BTU's per hour
and...
a 750 HPL * 3.4144 = 2560.8 BTU's per hour