A 10G is 20"
The WPG rule does not apply to small tanks or very large tanks.
For 10G's ~30watts of good lighting is close to 2wpg on medium tanks IMO(55G ect..).
After that point you need CO2 and ferts.
Its not clear cut, but I'm running 2x 20w CFL's on my 10G with very minimal ferts and no CO2. No since its poorly designed lighting, it prob compares to ~30w of T5 or PC IMO or less.