The wattage needed for the light depends upon various factors, such as the air temperature where the brooder is located, the type a material the brooder is made of, the size of the brooder, etc. I recommend that you setup your brooder, including the bedding, and test it a couple of weeks before you get the chicks. This gives you time to make any adjustments. For testing, turn on the heat lamp and leave it on for at least a couple of hours to allow time for the brooder to warm up. I completed setting up my new brooder this last Sat. I had the light on for about an hour and found it was not heating up enough. I lowered the lamp (only a couple of inches) and covered most of the top of the brooder with plywood. I let it set with the lamp on for a couple of more hours. The temperature is 96 degrees under the lamp. I will plug it in a couple of hours before I get my chicks to warm it up and then make any needed minor adjustments based upon how they act. I am using a 250 watt red heat lamp. My brooder is in the garage. Last year I used a metal tub smaller than my new brooder and it required (2) 250 watt heat lamps for the first couple of weeks.
I have about one more week to wait for my chicks...