This table lists BTU/hr loss per square foot in a steel tank with no insulation, assuming the temperatire outside the tank is 70F.
So, using basic stats & the Engineering Table I found, I can calculate heat loss in BTU / hr, assuming you are boiling 12 gallons of water.
The graph has the temperature across the bottom and the loss along the left side.
The first thing you probably notice is the fast growing yellow line - all the way to 9000 BTU / hr at 210 F.
This is also close to the BTU / hr you will need to maintain a boil.
The next thing you probably notice is the fast growing blue line - this is loss due to evaporation.
This also shows that keeping the lid on your hot liqour tank and boil pot will make a huge difference!
A typical 10 gallon grain bill for me is 20 pounds & I will heat 7 gallons of mash water to 170F and mix with my mash which will drop the temperature to less than 130F. I'll use my RIMS tube to bring my mash the rest of the way to 145F.
Using the weight of the keggle (30 pounds) + 7 gallons of water, I calculate about 61.98 BTU / hr to raise the temperature one degree F.
So, the total heat needed to raise the water and keggle from 60F to 170F should be 6818 BTU / hr.
Assuming no loss, here are the times for each heating element wattage to heat 7 gallons water to initial strike temperature.
But let's look at the same graph with heat loss added in.
You can see that heat loss affects the lower wattage elements the most.
Almost no time is added to the 5500 Watt Element and the 1650 Watt Element is pushed out to 140 minutes!
In other words, you don't want to use a 1650 Watt element for 10 gallon batches.
And at 90 minutes you probably don't want to use a 2000 Watt Element either.
A typical 10 gallon grain bill for me is 20 pounds & I will heat 9 gallons of sparge water to 170F to rinse the sugars from my grain bed.
Using the weight of the keggle (30 pounds) + 9 gallons of water, I calculate about 78.66 BTU / hr to raise the temperature one degree F.
So, the total heat needed to raise the water and keggle from 60F to 170F should be 8653 BTU / hr.
Assuming no loss, here are the times for each heating element wattage to heat 9 gallons water to initial strike temperature.
Almost no time is added to the 5500 Watt Element and the 1650 Watt Element is pushed out to 185 minutes!
And at 120 minutes you don't want to use a 2000 Watt Element either.
For a typical 10 gallon I usually cut off my sparge at 12 gallons. This gives me enough volume for a 60 - 90 minute boil.
Assuming no loss, here are the times for each heating element wattage to heat 12 gallons water from 150F to boiling temperature.
And the 1650 Watt & 2000 Watt Elements never reach boil!
At about 174 degrees F for the 1650 Watt & about 187 fedrees F for the 2000 Watt Element the heater just keeps up with heat loss.
Ten minutes are added to the 5500 Watt Element and 20 minutes are added to the 4500 Watt Element.
The 3500 Watt Element is pushed out to 75 minutes to boil.
In other words, you can't use a 1650 or 2000 Watt element for boil.
You probably don't want to use a 3500 Watt Element either.
I mentioned earlier that most of the heat loss is through evaporation. Go back to my BTU Loss graph and you'll see that evaporation loss becomes the largest heat loss past 180 degrees F.