Wattage for green screen lightning
Wattage has nothing to do with lumens, as different instruments provide a wide spectrum of output lumens based on lamp type. For instance, a 1.2K HMI ArriLite would put out more light than a 5000W tungsten lamp. LED, and flourescents are similar, higher output, less watts. 

What is critical for greenscreen work is that you use a monitor with a simple waveform scope. When you light your greenscreen, on your waveform monitor you will see basically a horizontal line. 

You want to get that line as straight and as thin as possible, at 70-72 IRE. Its fairly challenging the larger the screen (you won't be able to do this with the naked eye). 

After your screen is "flat lined". Put in your on camera talent and light them for picture, using dimmers, silks, scrims etc, to cut the light. 

If you do it the other way around, you will drive yourself crazy trying to get the screen at flatline 72.

This will give you the best possible key. If you have variance in your IRE (darker corners, even 5 IRE) you will have to use a looser key (meaning more edge artifacting). 

I recommend for post KeyLight for AE, Lightwrap for AE. 

Also, using a premium screen really makes things easier. Hollywood Rags makes an Ultra Key cloth (order by size) that is ultra bright, the material they use, therefor you need less light to get it to 72 IRE. It really pops. 

Last tip most people don't know, based on your Kelvin, use the same Kelvin for ALL lamps. For your hair light; hit your talent from behind, and gel it using 1/8 magenta CTM.   The magenta is complimentary to green, and will give you a clean edge, specifically on the hair. 

Hope this helps!