Evaporation rate

So, in order to prepare for my next batch of beer, I decided to test out my evaporation rate. I filled my kettle with 3 gallons of water, put it on my outdoor burner, and cranked up the heat. Once it started to boil, I turned it down until it was just able to maintain a low boil. I boiled the water for 15 minutes, turned off the heat, then cooled it down in an ice bath for awhile until it was cool enough to handle. I measured 2.69 gallons.

That's a 41% evaporation rate. How the heck could it be that high? I've read all over the forums that it is normally along the lines of 10%-15%. Am I doing something wrong? I did leave the kettle uncovered the whole time, even when it was heating up. Should I leave it covered while it is heating?

I think gallons/hour is a better way to think about boil off than percentage of volume boiled off. The amount that boils off is really a product of temperature, time and fluid:air surface area, total volume boiled doesn't seem to play as large an impact for reasons others can explain far better than I.

I've read that argument on several other threads using the forum search, and I think I'm inclined to take that side as well. That would be 1.24 gallons per hour for me, which falls in line with some other people's numbers.

Depending on how much juice I use on my system, I can get between 1-1.375 g boil off in 60 minutes. I think right around 1.2 is about ideal for me. I get a great boil, good hops utilization and can hit some high gravity beers with ease with that kind of boil off. Your number sounds good, but I would do a full volume boil for 60 minutes to make sure it is spot on. Your numbers will vary slightly between seasons, temps, wind factors, and humidity.