Okay, So I understand the simplicity of the OP's question but I have a similar question.
Say I have a 4 tube 48" flourescent fixture. Is it cheaper to only put 2 tubes in or does it use the same amount because the ballast is still trying to power the missing tubes?
This is pretty simple
Shut all electric consuming devices off except this fixture. Light it with 2 tubes and observe the meter for one minute. Light it with 4 tubes and then observe the meter for one minute. What did you conclude??