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I'm a homebrewer and i'm trying to find the boil off rate in gal/hour when using an electric heating element in my kettle.

If i'm using a 5500Watt heating element at 100%, i'm inputting about 312 BTU/min...Using this site and the table for the losses due to evaporation and radiation of the water surface and the losses from the vessel, I can calc the losses in BTU/min for temperatures LESS than boiling, and use the total BTU's from the element less the losses to determine the BTU's available for heating the water and the associated ΔT(°F)/min...but i'm sure these losses change while boiling and i'm not properly accounting for them and i cant find any examples of this problem online.

Now what i am looking to understand is the losses when i'm boiling the water/wort so i can determine the net BTU's available for converting water to steam (at 970 BTU/lb, i want to boil off 1 gal per hour, 8.34 lbs/gal, so i would need 8092 BTUs to boil one gallong in one hour)

Any information on how i can determine these losses while boiling would be greatly appreciated!!

(PS, i understand the losses are not static and dependent upon how vigorous the boil is as well, I'm guessing the element will need to be fired around 65% to overcome losses and input enough BTU's to boil at that rate/hour)

Thanks!!