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Sizing the Array:  Now that the battery bank has been sized, calculations must be made that allow the PV array to provide this power. The equation above that calculates the number of Amp-hrs to be delivered to the battery bank. This value then needs to be adjusted for dirt and mismatch that may occur on the PV array. The amount of power that the PVs should provide before this derating is generally 5% more than what is needed at the battery bank. Assume 5% derate for dirt and other mismatch.

To find:            Ah from PV

Equation:         Ah from PV = (Ah to batt.) / Dirt

Where:             Dirt = 0.05

Insolation at the location’s latitude is required to calculate the number of modules that should be included in the array. The same Amp-hrs required in Darwin could be provided by a PV system that is much smaller than one in Hobart because of the higher intensity of the sun at lower latitudes. A list of insolations at various locations throughout Australia is available here. The following equations calculate the required rating of each module, and the number of modules and how they should be wired. Recall that 6 kWh/m2-d is equivalent to 6 hrs of sun @ 1kw/m2 (1-sun).

To find:             PV rated Amps

Equation:          PV rated Amps = (Ah from PV) / (hrs @ 1-sun)

Where:              PV rated Amps = Amps(A) needed @ 1-sun

                         Ah from PV = Amp-hrs produced by PV array before derating

To find:             Mparallel

Equation:          Mparallel = (A needed @ 1-sun) / rated current (A)

 Where:            Mparallel = number of modules wired in parallel

                         A needed @ 1-sun = Amps needed to be produced by array

                         Rated current (A) = individual module’s rated power in Amps

The product of this equation should be rounded to the next highest whole number in order to provide sufficient power. This is the number of panels that should be wired in parallel, or with positive terminals wired to positive terminals and negative terminals to negative terminals. As with the battery bank, it is assumed that the system’s voltage is 24V in order to keep the current under 100A. The majority of PV modules are 12V, so two strings of modules wired in series must be connected to produce a 24V system.

To find:             # of strings

Equation:          # of strings = Vsys / Vmodule

Where:             # of strings = number of PV strings wired in series

                         Vsys = 24V, system voltage

                         Vmodule = 12V, module voltage

The product of the number of modules wired in parallel and the number of strings wired in series will give the total number of PV modules that are required to produce the desired power.

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