Someone did a simple analysis of the way Mr. Amano lights his tanks pictured in his books. http://www.fitchfamily.com/lighting.html
. They did an analysis of watts per gallon (and liter) and also of watts per surface area. The results are interesting and they produced a calculator on the page which will give you a watts needed for a volume.
I think that there might be a slight fault in the analysis as it wasn't taken into account that some of the tanks were planted with only low light plants and so Mr. Amano would only put what was needed for those plants. The other bias would be that Mr. Amano likes to restrict stem plants more to the smaller (shallow??) aquaria (AquaJournal Mag quote) because they need frequent pruning and leaning over a large tank for several hours isn't a good use of time. The result being he installs higher lighting over these tanks. Hard to say but the calculater said I should have 65 watts over a 10 gallon tank and 109 over a 30, which is close to what I have over my tanks anyways.