007 a good link but his math is wrong and his understadning of how an MPPT a bit shaky. No solar controller I am aware of puts out less that 13V to be at all useful.
To quote HB, "All full timers should use either what I have (Tristar PWM) or spring for the Tristar MPPT. I personally only recommend MPPT if the system is approaching 600 watts, if the solar panels are those high voltage 44 cell units so that MPPT is needed to use the excess voltage, or if the existing wiring is too small and series wiring the panels can be done to improve voltage drop."
I have a panel that puts out over 50V open circuit and will put out 35V on a cloudy day, It was meant for a grid tie system.