Figuring on full power efficiency - a/b will be around 60% and D will be around 75% so...
800/.6 = 1333 1000/.75 = 1333
Pretty close I'd say.
Overall though, the class D will be less draw to the alt because their efficiency is relatively linear at all output levels whereas a/b are most efficient at full power and get less and less effecient as volume nears zero.
