Well for anyone that does not like to estimate or guess, 1 watt = 3.412142 BTU/hr. (Look it up, its a fact)
And Watts = Volts x Amps (Also a proven fact!)
Look at the nameplate on your ballast. Read the amp draw or FLA (Full LoadAmps)
If ballast is outside of the room multiply the FLA amps times the efficiency of the ballast, or just multiply by 85%
Measure the voltage supplied to your ballast.
Ballast inside room ...BTU/HR = 120 volts x 10,0 Amps (nameplate) x 3.41 = 4,092 btu/hr
Ballast out of room... BTU/HR = 120 volts x 10.0 amps x 3.41 x 0.85(lowefficiency) =
Typical non digital ballast not in room>>>> 10.0 Amps x 120 Volts x 3.41 x 0.85 = 3,478 BTU/HR
Typical non digital ballast located in room>>> 10.0 Amps x 120 Volts x 3.41 = 4,092 BTU/HR
So.. Ballasts in room....4.1 times bulb wattage = btu/hr for lights
And ballasts outside of room....3.5 times bulb wattage = btu/hr for lights
Ballasts in room 4,100 btu/hr for each 1000 Watt bulb (for non vented fixtures)
Ballasts outside of room 3500 btu/hr for each 1000 watt bulb (for non vented fixtures)
This is only to remove the heat load of the lights. Additional cooling will be required for other heat sources in the conditioned space. If the heat source is due to an electrical device you can estimate the cooling load with the simple formula
1 watt = 3.41 btu / hr