What do you want to do with your lighting? Are you keeping live plants, or just looking at your fish?
Small LEDs like you typically find on eBay don't show wattage because they don't use much power. They also don't put out enough light to sustain plants. If you want to calculate the power: Watts = Volts * Amps.
If you're looking to support live plants, you'll need some more powerful LEDS. I'm working on a fixture using Cree XR-E Q5 LEDs. They're rated at 3.7V at 1A for a power rating of 3.7W.
None of this information will tell you how much light the LED
is emitting though, just the power dissipated through the LED
. Some of it is converted to heat. At the very least you'll need a lumen rating and even that is sketchy. For aquariums, the best way to determine an LED
's suitability is to set it up and measure it's PAR (photosynthetically available radiation) at the bottom of the tank. PAR meters are expensive though, so I'm basing my design off the designs of others who have had success.
If I get the chance and I'm feeling motivated, I'll post some pictures of my fixture when I'm done with it in the DIY forum.