| Both, and more. In my experience, the surface area where evaporation occurs has the most impact. If I know (and obviously I do every time, just measure it) the evaporation area size, that's what I use to dial in how much pre-boil wort I need the first few times I use a set of equipment. But heat energy is the second most important factor. Sea level, salt/mineral levels, and other factors are - in my experience - not worth considering. But I've not tried to brew beer on the top of a mountain. I've brewed over 300 batches of beer in vessels of various dimensions (including surface area and evaporation area), and with various amounts of heat energy, applied in various places in the vessel. Ranging from 2kW to 18kW, sometimes heat applied under the pot, sometimes distributed throughout for example 3 heating elements at various heights. 2kW for all from 15L to 60L vessels, 6-18kW from 25L to 120L, and 18kW (IIRC) for 500-800L. When brewing beer, the boil usually lasts for 60-90 minutes, and the boil off rate is extremely important for hitting your target gravity and volume, and getting a consistent result if you're brewing the same recipe again. What you do as a brewer is you test the equipment on the first 2-3 batches and then after that you can hit your target boil-off with 99% accuracy every time, or 100% in an enterprise-grade professional setup. I've never been more than 5-10% off my estimated boil-off rate for the first batch, and I only account for surface area in that estimate. Even with 6kW for 25L. |