I'd be real careful using that spreadsheet.
Insulation and diameter of opening on the lid can have huge effects on efficiency. If you are just heating mash water, you can achieve pretty high efficiency if well insulated and the lid is on. Almost all of the energy goes into the water. A little lower power means it just takes longer to get to temp.
However, for doing a full boil, consider that heat rises. Most of the heat will go out the top. And you gotta keep the top off to eliminate DMS.
The larger the diameter of the pot, the worse it gets. Remember, area = pi*R^2, meaning the surface area that loses heat goes up with the square of the radius.
So it's the boil that's most critical, so when comparing with another system, check the radius of the boil vessel. If yours is larger than the system you are comparing to, you'll need more watts.
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