With the lid on mine will heat 3-5 gal. to strike / sparge temps in about an hour. They work great for an hlt if you sort of set and forget and do a few other things around the brewery / house. If you watch it they are painfully slow!!!
I use mine to heat water as well as a kettle supplemented w/ a heat stick. I think they are 1800 w +-, not real aggresive and not nearly enough for a full 6 gal. boil.
However, supplemented w/ an additional 2000w heattick, a kick a$$ little boiler. I paid $50 shipped and thats what it is worth. H. Depot clearanced these things out about a year ago for 30-40-50 bucks.
I think where these things would really shine is in making a low country boil....corn, chicken, sausage, new potatos, shrimp and some old bay, and dump on picnic table w/ some homebrews. Thats what I want to try w/ mine...but thats anotheer topic.