Sensing resistor calculations; solar energy variation

   As a precursor, let me reiterate, these blogs are for intelligent people with a university background and courses in mathematics, engineering, physics and other of the hard sciences.

   The use of a charge controller, generally, obviates the need for blocking diodes, but not always.
Their installation is a safety measure.
   Along with blocking diodes, bypass diodes need to be installed. Bypass diodes allow energy to flow around panels in series-parallel. If one panel in a string gets shaded, the diodes allow energy to flow around the panel, leaving the other panels in the string operational. If three panels are connected in series, and there are several such strings connected in parallel, shade can reduce the output of an entire string when only one panel is shaded.
   Current sensing resistors allow detection and measurement of each panels output. They serve to measure and verify the efficiency of the charge controller. An additional wire also lets the voltage be measured for each panel and the power can then be calculated.
   Knowing what's going on with each panel, and not just the array as a whole, improves safety and reliability. If a panel starts dropping in power under full sun, then operator can do a manual inspection for stray objects, leaves, or snow. If no objects are responsible, then the panel is wearing out or going bad and needs to be replaced. Each panel varies from specifications to some degree. Panels can be better matched if their real parameters can be measured.
   The current sensing resistors used need to be of a very low value. A value less than one is common. In my case, i'll be using 0.01 ohm 5 watt resistors. The maximum current for the type of panels I'm using is around 8 amps. The typical power output for a fully illuminated panel is 100 watts at 12 vdc. The resistor power rating can be calculated : 8 amps x 0.01 ohm = 0.08vdc. And 0.08vdc x 0.08 amps is 0.64 watts. My choice of 5 watt resistors will allow me to use the same resistor for an overall measure of the array output of about 600 watts. The array is composed of two parallel strings of three panels, in series, each, for an array voltage of 36 vdc. Two parallel strings results in a typical current of 8 amps per string, therefore, 16 amps for the array as a whole. Given 16 amps x 0.01 ohms, the power dissipated is calculated: 16 amps x 0.01 ohms =  0.16volts; 0.16 vdc x 16 amps = 2.56 watts, leaving enough headroom to insure the resistor does not burn up during surges.
   The output of a solar panel will vary throughout the year since the highest position of the sun in the sky during the day, changes with the seasons. My array is at a geographic latitude of 36.34 degrees. This means the sun will never be directly overhead, and therefore, at its hottest. The energy of the sun's rays is reduced by the atmosphere and the light has to pass through more atmosphere when the sun illuminates the panels from an angle. The amount of atmosphere the light rays pass through is least when the sun is directly overhead. The adjustment of the position of the panels with respect to the sun, ameliorates the effect to some degree. The effect of the combination of incident light angles for the sun and the panel can be minimized. The only way to reduce the effect of the sun's lower zenith in winter, is to move to the equator. Even then, the sun is further from the earth and that, too, will reduce the intensity of the sun's rays. If the nutations of the earth are taken into account, the sun will, still not reach a point directly overhead at thie latitude mentioned.