Ceramic Infrared Heat Emitter watt

I've been looking at Ceramic Infrared Heat Emitter and was wondering what watt to use for my 20 gallon long tank?

That's going to depend on:

• the ambient temperature of the room where the enclosure is/will be
• the heat-loss of the enclosure
• the output of any other bulbs or heat producing items in the enclosure

First let me ask why do you think you need one?
Would a hotter basking light do the trick?
If it's for nighttime, what's the nighttime ambient temperature of the room where the enclosure is/will be?
If we're talking about a glass walled terrarium type tank, have you considered insulating it? Half-inch rigid foam insulation (white, pink, or blue) works very well. Insulate the back, sides, and top (except where the lights and ventilation are). Assuming we're talking about a baby dragon, this is also a less expensive option for when you upgrade the enclosure when the BD gets larger in a few months.

Once the other question have been addressed, finding the best wattage is usually a matter of trial & error to get the temperatures in the tank right, i.e. you may have to buy more than one and settle on the one that does the job. The same trial & error applies to finding what wattage basking light works best.

As far as brands, I would check out the options someplace like Amazon, and look for large numbers of positive (5 star) reviews.

I would consider using a heat projector. They produce a good amount of heat under the bulb, but they don't get nearly as hot to the touch as a CHE does which reduces burn and fire risks. http://www.lightyourreptiles.com/arcadia-deep-heat-projector-50w-sale-25-99/