So, If you have been reading my other project logs you may have noticed that my LED grow light uses thirty 3 watt LED's.  But that I am using two 36 watt power supplies.  And honestly I am confused also about this... What I know for sure is that none of the "3 watt" LED's that I have actually pull three watts of power. 

I have two different types of LED's on the grow light.  450nm Blue LED's and 660nm Red LED's.  The power draw for each type of LED can be calculated easily using the following formula.

P = V * A

So Power is equal to Voltage times Current.  I took a volt meter to measure the voltage drop across each type of LED and here is what I found.

450nm LED Voltage drop = 3.74 Volts

660nm LED Voltage drop = 2.60 Volts

Since the LED's are connected to a constant current power supply outputting 700mA of current it simple to compute the power.  So using the formula above...

450nm LED power = 3.74 Volts * 0.7 A = 2.618 Watts
660nm LED power = 2.60 Volts * 0.7 A = 1.82 Watts

In the case of the 450nm LED's they are close to 3 watts.  But the 660nm Red LED's are using almost 1/2 of the "Rated" 3 watts.  Looking at other "3 Watt" LED's they all have similar current and voltage usage.  So I am not really sure why they call them "3 Watt" LED's... But in any case, my LED grow light has 62 Watt's of LED's which I feel should be enough for plants I plan to grow.