This is for a purely offgrid system, not a hybrid system in which the batteries back up a grid tie system.
Understand daily power needs
Daily kWh : for each appliance that is being used, multiply ( Number of Appliances) * ( Wattage) * ( hours/ day) * ( days/ week)
Max Watts used during peak : Figure out which appliances are used during the peak usage hours, and sum on the wattage of all of these appliances in order to find the maximum wattage that will be used.
Receive information regarding the site :
Sun hours (ideally these will be modified based on shading, tilt angle, etc)
Min T , Max T : can use the average and the average low T at the location
Other Power Losses : Caused by soiling, shading, voltage drop, module mismatch, power tolerance. In the SKG we assume that this = 0.9
DC Voltage of the system : in the SKG we assume 48V, which is the most popular choice
Batteries :
Batteries will be arranged in an array of a few battery strings. In general, we try to design for no more than 2 battery strings.
In general  batteries are calculated based on the total daily kWh needs, and the required days of autonomy.
For a specific battery, this is how the number of batteries in the array is calculated:
Number of battery strings = number of batteries in parallel = Round up[( Total Ah) / ( Battery Ah rating at c/20)] , where
Number of batteries on each string = number of batteries in series = ( DC voltage of the system) / ( Battery voltage)
Total number of batteries = (Number of battery strings) * (Number of batteries on each string)
Charge Controllers and sizing the panel’s array :
The array of the panels consists of a few identical strings of panels. The min number of panels on each string depends on the bulk charging set point of the battery, and the maximum number of panels depends on the charge controller that was selected, and the Voc of the panel at the lowest temperature. For a given selection of battery, panel, and a given low T, some charge controllers won’t be possible  if they dictate a maximum number of panels on a string that is lower than the minimum number determined by the battery.
Other considerations when selecting a charge controller are to make sure that the maximum power and current that the charge controller allows are larger than the maximum current and power that is needed for the solar array.

Solar Array Configuration :
Minimum Number of Panels on a string = Minimum number of panels in series = Round Up[( Battery bulk charging set point) / ( Panel V mp at High T) * 1.1] , where
• Panel Vmp at High T = V mp * ( 1 + T kVmp / 100 * ( High T + Delta Ground T  25) ,
• And Delta Ground T = 30 if this is a ground mount, and 25 if it’s a roof mount
• High T is in degrees C
Maximum Number of panels on a string = Maximum number of panels in series = Round down[( Max Charge Controller PV V oc) / ( Panel V oc at Low T)] , where
• Panel Voc at Low T = V oc * ( span style="fontsize: 14.6667px; verticalalign: baseline; whitespace: prewrap;">1 + T kVoc / 100 * ( Low T  25)) , and
• Low T in degrees C
These will give a range of string lengths to choose from, typically this range is not very large (e.g. between 3 and 4 panels per string), and may rule out charge controllers that dictate a maximum number that is smaller than the minimum number. After selecting number of panels per string:
Total number of panels : • Should be a multiple of the number of panels that we selected per string • Should allow for enough power: Array Wattage > Min wattage needed , and: > Array Wattage = Number of panels wattage per panel > Min wattage needed = ( Daily kWh output needed) / ( Sun Hours * All Losses) , where > All Losses = Battery efficiency * CC efficiency * Losses at High T * Other Losses > CC efficiency = charge controller efficiency > Other losses = as defined above (in the site info section) > Losses at high T = 1 + ( T kVmp / 100 * ( High T + Delta Ground T  25)) • Note: it is possible that the sun hours during the winter are so low, that a customer would like to use the sun hours during the summer for this calculation, and substitute with a generator during the winter
Number of Charge Controllers : will determine the maximal power and current allowed. Need to make sure that both are larger than the power and current of the solar array that we have selected.
The needs of the array:
Array Watts = Module W * # of modules
Array Amps = Module Isc * # of strings in parallel
The maximums, determined by the charge controllers:
Array Max Watts = Max output power of the charge controller *1.1 * Number of charge controllers
Array Max Amps = Max output current of the charge controller / 1.25 * Number of charge controllers
Off Grid Inverters :
Number of inverters : multiplied by the inverter output gives the total amount of power the inverter system can provide at any given time
should have a size that will allow all of the consumption that you want to run in parallel at the same time (Max Watts used during peak that is defined above)
not connected to the PV system size
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