Stick with watt hours MrWizard as it shows total energy delivered to your batteries.
Uh oh, another what hath god wrought suggestion...
Go to a home supply store and purchase a sheet of foam insulation that has a very shiny aluminum vapor barrier on one or both sides. Place one half of a cut sheet on each side of the panel(s) and then tilt it up and have someone keep an eye on the amps or watts delivered meter. Use judgement to not block off too much low angle sunlight access but once you get the angle right woo-eee, cheap turbocharging. Photons R Amps.