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                           Basics of How to Size a Photovoltaic Solar Power System Array



There are two ways to size a photovoltaic solar power system array. The easy way is to use PV Watts, or similar online solar power system array calculators. These nifty tools are free and will spit out a fairly accurate array size based on standard equipment, location and system integration conditions.


Even though some of the data used for system sizing using programs like PVWatts is universal, an accurate sizing can also be done by hand. This will take into account specific component configuration, site and environmental conditions that will affect the actual performance of the array.


As an example, let’s assume you want to size a photovoltaic solar power system that provides 50% of the total annual Kwh of electricity demand for a specified location.


For this example, we’ll also assume optimal site conditions, including an unobstructed Southern facing mounting area, plenty or square footage for mounting the array and a clean power source from the utility grid. Our test solar array will be located in New York City, a location with readily available solar data.


Annually 20,000 Kwh of electricity is used at this location, but since we are only trying to offset 50% of the total yearly electricity use with our solar array, our baseline number is 10,000 Kwh. The average yearly sun hours per day for this location are 4.08. We’ll round that down to an even 4 to make the math a little easier to follow.


The average sun hours for any geographical location can be found using this interactive sun hours per day/solar insolation map.


First, we’ll need to calculate the daily electricity use. To do that we just divide 50% of the annual electric use by 365.

10,000 Kwh year divided by 365 = 27.4 Kwh per day.


Next, take the daily Kwh usage and divide that number by the number of avg. daily sun hours.

27.4 divided by 4 = 6.85


Next, we need to account for inefficiencies associated with all solar power systems. In other words, we need to take into account that many components of the system will vampire some of the electricity away before it actually reaches the load. The term used for that current loss is the “Derate Factor.” The Industry standard derate factor is .77.



6.85 divided by .77 = 8.9 Kwh (rounded).


Now we need to simply convert Kilowatts (Kw) to watts with the calculation:

8.9 X 1000 = 8900 watts.


Next, we take the total anticipated energy production and divide that number by the panel size in watts. Our test system will use 205 watt solar panels. The final calculation for the array size in number of panels to use is below.


8900 divided by 205 = 43.4 panels.


The final array size choice would probably be rounded down to an even 40 panels.




By: Rick Contrata





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