The angle isn't the only thing that matters; hours of daylight change too. Nearer the Equator not so much so. Up here ... egad! Ten hours difference!
To that end, changing the panel angle nearer the Equator does not gain you as much daily power as it does when further away.
In fact when you get to the Equator it actually reverses; Summer daylight is slightly shorter than Winter.
At least we have the cold temps to raise panel Voltage! Come visit the Great White North some Winter. Be sure to wear insulated everything.
Mac's Lab panel angle calculations (good explanation of the business, but is geared for grid-tie): http://www.macslab.com/optsolar.html
Calculate length of day where you are with this US Navy site: http://www.usno.navy.mil/USNO/astron...duration-world