I bought some 10w LEDs off eBay for dirt cheap: I got 10 for just over $3, so I had my expectations set very, very low. I did initially test how well balanced they were, and, to my surprise, only two had dead chips, the rest were, as far as I could tell, perfectly balanced. I used one of these balanced ones for the testing that follows.
The seller lists them as 10-12v, but does not list Amps. I tested them today. I set my power supply to 12v, but I was really confused when they only drew just over 200mA. I then tried increasing the voltage until they reached 10w. But, after about the 14v mark, they actually began to dim, and the colour temperature changed. After realising I was going way past the recommended spec of the LED, I dropped back down. After some experimentation, I found that the peak brightness was at around 13.5v, drawing 380ma. I was getting just over 5w, literally half of what it should be.
I guess I got what I paid for. For future reference, can anyone give me some more info about these LEDs, and why they are not actually 10w?