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Properly calculating production for a solar array is critical to appropriately design a system and calculate the economic value of that system.  Overestimating production can give the system owner an inflated estimate of their system’s value, while underestimating production can cause the system to produce excess power that may be of little or no value to the system owner.  However, even the best calculations and estimates can fail due to varying sunlight from one year to the next.  It is not out of the realm of possibility that the total sunlight in one location could vary by 10% or more from one year to the next.  Still, taking the time to calculate production as closely as possible is very important.  We will explore the different factors that make up a system’s production:


Solar Pathfinder


Location – Weather station data is the starting point for calculating production, but is also the biggest variable.  Even though the sun can be measured, it cannot be controlled and so there may be variations in one location from one year to the next.  When choosing a weather station, it is important to consider the number of years that data has been collected from that site, as well as the proximity to the site of the solar array.  Our calculations use data from TMY3 datasets.  

Tilt – Typically, the closer the array tilt is to latitude for a particular location, the greater the solar production.  However, slight variations in tilt will only have minimal effects on production.

Orientation – An orientation of 180° (true south) typically provides the optimal production for a solar array.  An orientation of east or west can reduce production by 15%, although that can vary significantly due to the array tilt.  For arrays that face east, west, or north, it is more advantageous to have a lower tilt.  PES will consider installing arrays at any orientation that provides value to the system owner.

Shade – Shade is an enemy of any solar array.  Calculating the reduction in production due to shade can be very challenging, since it depends on the time of the day/year in which the array is shaded, as well as what module and inverter technologies are used.  PES uses complex 3D modeling to calculate the effects of shade.

Soiling/Snow – This is an unknown on any project, since it is difficult to know in detail how much snow will fall at any given location on any given year, and how much soiling will stay on the modules vs. how much will be washed off by rain.  PES uses a soiling factor of 5%.


Solar Panel


Rating – Modules are rated in watts, but they also have a power tolerance percentage.  If this percentage is negative, the module could actually produce less than it is rated for.  For example, a 300 watt module with a power tolerance of -3% could actually produce as little as 291 watts at Standard Test Conditions (STC).  All modules used by PES have a positive power tolerance, which means that they will produce their rated amount or more at STC.

LID – Light Induced Degradation is a loss of performance arising in the first hours of exposure to sunlight.  PES uses industry standard 1.5% LID losses for performance calculations.

Mismatch – Although modules are rated for the same voltage and current, they may experience a small amount of mismatch, particularly over time.  Module Level Power Electronics (MLPE) solutions can resolve this issue.  PES uses the industry standard 2% losses for string inverters and 0% losses for inverters utilizing MLPE solutions.


Solar Inverters


Efficiency –  Inverter specification sheets show 2 efficiency ratings, max efficiency and CEC efficiency.  Max efficiency is the maximum efficiency in which the inverter can convert power from DC to AC, while CEC efficiency is more of an average efficiency.  PES uses complex software to calculate the inverter efficiency based on the sunlight and electrical characteristics of the modules connected to each inverter.  This is significantly more accurate than using the CEC efficiency, as efficiency can vary depending on DC voltage and inverter utilization percentage.  

Clipping – Clipping refers to the power that is produced by the solar modules that is greater than the capacity of the inverter to harvest.  Clipping is common on cool sunny days, when the array is producing at peak capacity.  Again, complex software is required to properly account for clipping over time.  As solar module prices continue to fall, more and more systems are designed for higher percentages of clipping to properly utilize the inverters and other equipment installed for the solar array.   


Solar Array Wiring

Wiring and Connections

Due to heat and voltage drop, even a system with properly sized wire and connections will lose a small percentage of production in the process of moving power from the modules to the system owner’s loads or utility meter.


Most systems will experience some down time when the system is not producing at all or is only producing at partial capacity.  Down time can includes power outages, as well as module and inverter failures.  
At PES, we take production calculations very seriously.  We understand that our customers depend on accurate production estimates over the life of the system, up to 30-50 years or more.  As a result, PES has a 10 year performance guarantee for all of the new projects that we install.  Because of our high confidence in our production estimates, if our systems under produce we will pay the difference between the production that we guaranteed and what the system actually produced times the customer’s initial electric rate.  Learn more about our Triple Ten Guarantee.





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