put it this way dump the glass heaters and get a titanium heater i just switched all of mine and they work absolutely great compared to the glass i dropped my electric bill by 60 bucks a month just switching the 400 gallon pond from 2 300 watters down to 1 250 w titanium and i keep it right at 81-83 degrees and the heater light is barely ever on when i walk in there the other 2 were alwys on so just my 2 cents
It's not about BTU. It's about energy consumption. Having a heater run 24/7 and could not raise your tank temp to desire level, not only a waste of energy, soon, your heater will be wasted, too! 
I work in HVAC and people surprise me on this forum and aquaria central every day.