Ya that is right, you can have a 1 watt LED that only consumes about 200-350 mA.

I model and make Radio Controlled helicopters, I am buying 1W Luxeon compatibles and have to tune them down with resistors or they will burn out if left on for more than a minute at the full 5 volts. Different colors need different resistors too, so you would have to calculate how many mA total and I betcha you will find it isn't using the whole 15W for all 250.

Remember they do not need huge amounts of voltage to run, they are very efficient

LED technology has advanced a great deal of late. Though I am perfectly pleased with incadescent lighting, I am noting the LED world. IF you are an off the grid person this could allow you to grow over not be able to grow with lights.