Assumming you are talking flourescent?
Provided your math is correct, (I did not calculate it), the output from the ballast will be relative to the input.
The lamps will operate sufficiently. The ballast is a XFMR. What goes in, is reflected by what goes out.
At lower voltages the lamps lose brightness. Since this is an outside application, temperature plays a major role in the brightness of the lamps.
I always recommend high output ballasts for outside use. At the lower voltage it may take some time for the lamps to achieve effective lighting. Even high output ballast's struggle in cold weather. If possible, leave the lights on 24-7.
I would run #10 copper.
