Inverter Output Clipping with Oversized PV Arrays

How much energy is lost due to inverter clipping when the power form the PV array far exceeds the inverter output rating? And under what conditions do the losses occur?

Today’s interactive inverters allow high PV to inverter size ratios. An example 7.7kW residential inverter allows a PV array STC input rated up to 12.32kW; that is a 160% oversizing. Some utility scale central inverter design allows up to 200% oversizing of the PV array.

Two issues come to mind with this level of PV array oversizing. How does this affect the life and performance of the inverter and how much of this energy gets clipped (wasted)?

  • The answer to the first question is easy. These inverters are transformerless and operate at high efficiencies so there is very little heat created when they are running at full capacity. Clipping is a process of electronic pulse modulation which limits current output and therefore current input; this process eliminates most of the heat that would be generated if the current was converted from input to output.
  • The answer to the second question is a little more complicated and depends on many factors but primarily it is the result of irradiance on the array throughout the day. For a fixed PV array, there is only one time of day when the irradiance is highest; for a south facing array, that would be noon; for a SE or SE facing array, that would be around 10:30am and 1:30pm; for an east or west facing array that would be around 9:00am and 3:00pm. With an east-west tracking array, the peak irradiance will be at noon. In the following paragraph we will look at each one of these PV array direction scenarios and examine how much net energy is actually present at the array during its highest irradiance time of day.

First, we will look at the other derating factors. Temperature is going to reduce net energy on average by 5% to 10% depending on the quality of the PV cell technology. PV cell degradation in the first year will be 1% to 3% depending on the quality of the cell technology. Electronics conversion losses will be 2% to 3%. Variances between module output will cause 1% to 3% losses. Soiling will cause 3% to 5% losses depending on the cleaning maintenance cycle. Voltage drop will reduce net energy by 1% to 3%.

  • The best quality modules and inverters will net out at around 87% of the solar energy received by the PV array. The lower quality modules and inverters will net out around 73% of the solar energy received the PV array. This is considered the net power conversion efficiency (PCE).

Second, we consider the actual irradiance that the PV array will receive during the peak hours of the day. This is highly variable depending on the location of the PV array; high irradiance locations with consistently clear skies are at the extreme end of the irradiance spectrum such as the Southwestern U.S. deserts; low irradiance locations with variable cloudiness are at the other end of the irradiance spectrum such as New England. Most of the U.S. falls into some range between these two extremes.

  • Let’s assume that the highest irradiance occurs in April or May when the temperatures when the skies are clearest and the sun is close to its highest point in the sky. In the SW desert, we may be seeing 1200W/m² for one hour at noon if the array is pointed directly at the sun. (That would be a 10° tilted array facing south.) The net energy output would be 120% higher than STC.
    • Using the PCE de-rates with best net of 87% and the worst with 73%, that would bring the net to 104.4% and 87.6% respectively. With the highest rated module and inverter example, that means anything over the rating of the inverter will be clipped. With the lower rated module and inverter, everything over 87.6% of the inverter rating would be clipped. A PV to inverter ratio of 114% would not clip any energy.
    • This is an extreme case and this only occurs at one time of year during one part of the day. The average irradiance at this location would be 900W/m² for this month and less for the other months of the year. That gives a net of 87% for the highest rated equipment and 73% for the lower rated equipment. A PV to inverter ratio of 114% and 137% respectively would on-average not clip any energy. This is only for the highest output month of the year.
  • In New England, the irradiance at noon may be 1000W/m² if the array is pointed directly at the sun. (That would be 20° tilted array facing south). The net energy output would be 100% of STC.
    • Using the PCE de-rates with best net of 87% and the worst with 73%, that would bring the net to 87% and 73% respectively. With the highest rated module and inverter example, that means anything over 87% the rating of the inverter will be clipped. With the lower rated module and inverter, everything over 73% of the inverter rating would be clipped. A PV to inverter ratio of 115% and 137% respectively would not clip any energy.
    • This again is an extreme case and this only occurs at one time of year during one part of the day. The average irradiance at this location would be 800W/m² for this month and less for the other months of the year. That gives a net of 70% for the highest rated equipment and 58% for the lower rated equipment. A PV to inverter ratio of 142% and 172% respectively would on-average not clip any energy. This is only for the highest output month of the year.

Most of the time the sun is not at zero incidence angel to the PV array and the skies are not perfectly clear. The average irradiance in most months is anywhere from 20% to 40% less than the peak month. These factors prevent the PV array from producing energy that exceeds the rating of the inverter. Some clipping will occur when the PV array is sized greater than 140% of the inverter rating but only during one or two month of the year and only for an hour or two during some of the days.

The following graph shows the average daily clipping during only one month of the year, April, with a PV to inverter ratio of 160% located near Atlanta GA. The net conversion efficiency is 84% before irradiance variations are factored in below.

Inverter clipping

Generally speaking, the PV to inverter size ratio for interactive residential systems should not be greater than 140%. The ratio for interactive commercial should not be greater than 160% and the ratio for utility scale systems should not be greater than 180%. You may instead refer to the specific inverter data sheet for the maximum PV array size for the inverter.

 

Kelly Provence

IREC Certified Master PV Trainer
Solairgen
www.solairgen.com

Dunkelflaute

At first glance I thought this term was describing a dark German beer that had lost its carbonation. I was wrong but it is actually almost as bad. Dunkelflaute translates to “dark doldrums” and refers to a period when there is no sunlight or little wind for renewable energy generation. This is particularly serious in areas where solar and wind energy provide a substantial portion of electrical energy generation and is even worse at latitudes above 40˚. Germany faces this problem on a regular basis but are very creative in the ways they deal with it. The first solution may appear to be batteries or other means of storing electrical energy which is a partial solution; they also store energy by other means and incorporate smart energy efficiency.

There are solutions that don’t include conventional energy sources such as nuclear and fossil fuels, and thermal energy (steam driven generators) has been the workhorse of the electrical energy industry for the past 100 years. Electrical energy from renewables can be converted to thermal and stored for a short period of time. Electrical energy can be converted to any number of energy stores such as water reservoirs for hydroelectric and compressed air for air turbine generators. It is also possible to convert renewable electrical energy to hydrogen through electrolysis. This process is fairly easy but storing it as a condensed fuel is energy intensive. Smart energy efficiency is another way to reduce dependence on electrical energy when it is in short supply.

Smart energy efficiency sounds good and it is very doable in this age of smart computers but in order for this to work, we have to give up at least some privacy. Every appliance that uses energy will need to be electronically visible and controllable by the smart energy network (SEN). In the U.S. we are seeing this occur with the new listing for interactive inverters. They will be smart in their own accord with electrical energy fluctuation and controllable by the utilities for further control features to make the electrical grid more reliable. California and Hawaii have already implemented these rules and the rest of the U.S. will follow within a few years. Home monitoring of electrical circuits is becoming an affordable norm. It is reasonable to assume that these devices will eventually be required to communicate with a smart energy network. Everything that uses energy will communicate with the network. This may sound futuristic but it’s not and I don’t think it will turn into some dark dystopian landscape of existence where machines control humanity.

A more conservative but less desirable option is to use conventional energy sources such as nuclear or fossil fuels for base level energy demand. Nuclear would get the vote if CO2 emission reduction is the primary concern, but I think what we are going to see is a blend of renewable energy, energy storage, smart energy management and clean conventional energy production.

There is no way to prevent Dunkelflaute but we can progress to a cleaner and stable electrical grid – and the best way to prevent a good beer from losing its carbonation is to drink it faster.

Reliable Resources for New Solar Contractors

Online information is abundant these days and misinformation tends to dominate many or most search parameter queries. This seems normal and the information appears to be free but it really isn’t. Usually it’s either written to attract people to buy a product or to attract people to someone’s misinformation-laden ego. Either way, it is difficult to filter the good information from the bad. The purpose of this blog is to provide some basic sources of reliable information for the new solar contractor.

Below I’ve listed the resources needed and information to be a well-informed, competent solar contractor.

  1. Training for the solar contractor. Knowing the design and installation trade and standards is most important and it can be achieved through good accredited training providers and certified trainers. If the training provide is not IREC accredited with IREC certified trainers, don’t waste your time with them, https://irecusa.org/. The only exception to this is manufacturers of solar equipment. They will provide good training, but only on their own equipment.
  2. Suppliers of solar equipment. There are many suppliers of solar products and they all differ in the range of products and depth of support they provide to their customers. Small to medium scale PV contractors will need suppliers who offer a wide selection of products from several manufacturers. Large scale contractors may go straight to the manufacturers. Follow the link provided below and select your country. They list wholesalers and distributors of solar products. https://www.enfsolar.com/directory/seller

  3. Leading solar equipment. One way to determine which solar equipment to purchase is to look at a leading resource such as EnergySage. They have a solar panel database with consumer ratings, https://www.energysage.com/solar-panels/. They also have an inverter database with consumer ratings, https://www.energysage.com/solar-inverters/ and a database on batteries as well, https://www.energysage.com/solar-batteries/. If you would like a more comprehensive list of solar equipment, go to the California Energy Center’s (CEC) data base, https://www.gosolarcalifornia.org/equipment/index.php. You can also review the products that the leading suppliers carry; this may be best since this will be your primary resource.
  4. Solar energy resource data. There is only one source for historic solar resource data and that is the National Solar Radiation Database (NSRDB). It has been developed from data collected by NREL, NASA and NOAA over the past 50 years. There is no other resource that can provide this data. The best tool to calculate solar irradiation for a tilt and azimuth of a specific site is PVWatts developed by NREL https://pvwatts.nrel.gov/. Several companies and organizations use this data in their shading analysis tools. If you use one of these tools, check to verify that their irradiation data is from the National Solar Radiation Database.
  5. Financing for the customer. The list of lenders can be long since the solar industry is growing at a rapid pace and has been providing owners a good return on investment. A good place to start is with the list on the EnergySage website, https://www.energysage.com/solar/financing/loan-providers/.
  6. Certification and licensing. These terms should not be confused. Certification is an industry merit that is earned by very competent solar workers; the organization that tests individuals and issues certifications is the North American Board of Certified Energy Practitioners, https://www.nabcep.org/.  The certification adds value and credential to the individual and solar contractor. Certification is often required by solar PV system owners and/or utilities who are offering a financial incentive to their customers. Licensing is a requirement by the state and is a separate issue. The purpose of licensing is to make sure contractors have met the state requirement for competence in their field of construction. A licensed electrical contractor is responsible for all the electrical work performed on the solar installation. The license holder must be a permanent part of the solar company such as an employee or partner.

Kelly Provence
Solairgen School of Solar Technology

 

Residential Solar PV Ground Mount System

Starting a Solar PV Installation Company

Like any new adventure, starting a solar PV design and installation company seems simple at first and then as you learn more about it, the more complicated it gets. If you are already a construction contractor, it is a lot easier to see where you are going; if not, it can be a difficult undertaking.

The first major obstacle to overcome is knowledge about the solar PV industry. Most people start by browsing the internet. There is an almost endless abundance of material to be found when you start your search. As with all internet searches, some of the information is good, some bad and some has nothing to do with what you really need to know.

Costs are a major consideration. The initial investments can be high. You must purchase the necessary equipment and tools to get the job done. You will learn which tools are essential when you are taking the training classes. The basic tools can start as low as $1000 and go up considerably depending on the scale of solar PV systems you plan to install.

It isn’t a bad idea to work for someone else for a while so you can learn the ropes under the supervision of someone who has already gone through this process; this is a good idea if you are not already a contractor. Even if you are a licensed contractor, it can be a good idea to sub-contract the first few installations to an experienced solar contractor. You can learn a lot from this method even if you only break even on the jobs.

I recommend finding an accredited school that offers an introductory course to PV design and installation. Some state technical colleges have solar PV included in their electrical programs but most do not. The best source of education in this field is a school that is accredited specifically for the solar PV technology through the Interstate Renewable Energy Council (IREC). If the school is not accredited, don’t waste your time with them. Taking this entry level course should provide you with enough information about the industry so you can determine if you want to continue, and in which direction you will want to go next.

Once you have completed the basic solar PV course, the next steps will be more obvious to you. If you are a contractor, you are already registered with the city or county as a contractor. If not, you can get a business license with the city or county where you plan to do business; it is a simple process and the fees are usually nominal. Once you have your business license, you can apply to one of the many distributors who sell all the solar products you will need. Not all distributors require you to have a business license to purchase from them, but the better ones do.

Most states require that you or someone within your company be licensed through the state for the type of work you will be providing to the customer. A licensed general contractor meets the requirements, but you will need to hire a licensed electrical contractor before the installation begins. If the owner of the business is not a licensed contractor, either general or electrical, they must meet the state requirements by hiring a licensed contractor. Most states require this license holder to be a permanent employee, not a subcontractor.

Now that the local and state license requirements have been met, you should consider insurance options and requirements. If you hire employees, worker’s compensation is required by law. If everyone who works in the company is a partner, it is not required. However, if you sub-contract under another contractor, they will require it even if it doesn’t cover you.  Liability insurance is also a good idea and is required by most customers. The amount of liability insurance should be balanced with your actual liability if something should go wrong on one of your jobs. However, some commercial contracts will specify the minimum amount of liability insurance.

If you have gotten this far, you will want to advance your knowledge with solar PV system design. If you are a contractor, you know that there are two ways to learn advanced principles, through formal education and by making mistakes. I recommend formal education to lessen mistakes. There are two primary sources for advanced level training; they include advanced online and hands-on courses offered by IREC accredited schools and manufacturer’s training resources.

Manufacturer’s provide design and installation videos and webinars for their products alone, ignoring other products on the market. They often combine the sales aspect with the technical design during webinars and a lot of their installation videos are very educational but, limited to their own best interests.

The quickest and best way to get comprehensive advanced training of design and installation principles is by taking advanced level courses from an IREC accredited school. The benefit is they are impartial when it comes to the market products, so your education is broader which will build your confidence and abilities for when you are on your own.

Your first installation is the point where the risks jump to a high level so make it easy on yourself and first do an installation on your own property or the property of an associate. You are bound to make a few mistakes with the first few installations, so it is a good idea to keep the stress and liability as low as possible in the very beginning.

In review, first estimate the financial investment to get started, second educate yourself on the basics of solar PV design and installation, third get a local business license, fourth address the state contracting license requirements, fifth secure your insurance needs, sixth get advanced level training and experience on solar PV systems and finally acquire the necessary tools and equipment to properly perform the job.

Kelly Provence
Solairgen School of Solar Technology

 

The Benefits of Achieving NABCEP Certification

Overall, the benefits of NABCEP certifications are to the industries they serve. The North American Board of Certified Energy Practitioners (NABCEP) was founded in 2002 as a 501(c)(6) non-profit organization with the stated goal to develop voluntary national credentialing programs that will promote renewable energy, provide value to practitioners, promote worker safety and skill, and consumer confidence in the industry. But how does certification help your career? Consumers need to be confident of their choices.

Consumers at all levels need to know as much as possible about the products they purchase. However, it’s next to impossible to know all that is necessary to make the best choice, so they depend on third-party grading and rating systems to help make decisions. This works out very well for the consumer if the grading/rating organization is doing the hard work of vetting the products and services provided, like NABCEP.

But it also works out very well for the product/service provider. This is especially true when it comes to certifications held by installation contractors. A certified installer’s high standards and achievements stand out so the consumer can see there is a quality difference with minimal research. NABCEP has done a great job of serving the solar industry by providing this credentialing and certification process and standard. They now offer several levels of credentials and certification.

The entry level NABCEP certification is the Associate Credential; the requirement to sit for this exam is to complete a solar PV course from a NABCEP approved training provider and then pay the registration fee through that provider. Achieving this NABCEP Credential provides the holder with more visibility in the industry to employers and potential customers. The credential holder’s name is placed on the NABCEP web page under NABCEP Associate Directory.

The highest level of certification is the NABCEP Certified PV Installation Professional. The minimum education requirements to sit for the exam are 58 hours of solar training with 40 of those hours being advanced level, solar PV specific, provided by an accredited school. A minimum 10-hour OSHA card is also required. Verifiable experience in PV design or installation is also required to sit for the exam. The value of this certification has many benefits in the solar industry and is often required by entities such as utilities or governments. Many developers will specify and require that the solar contractor must have this level of certification. Showing potential customers that you are a certified solar contractor will give you the advantage over a contractor who is not certified. The certification holder’s name is placed on the NABCEP web page under Certified Locator.

For professional sales people, the achievement is the NABCEP PV Technical Sales Certification. The education requirement varies widely from 0 to 60 hours of solar training depending on the person’s background. However, it would be a good idea to take some training courses to prepare for the exam. The first person a customer meets is usually the sales representative; if the sales person is certified, it provides the customer with confidence and an element of trust, elements that are an essential part of winning an installation contract.

NABCEP now has three Specialist Certifications: PV Installer Specialist, PV Design Specialist and PV Commissioning and Maintenance Specialist. Each of these require 24 hours of advanced training from an accredited school. Verifiable experience in each of these specialist categories is also required to sit for the exam. A minimum 10-hour OSHA card is also required. Each of these certifications is focused on the individual’s specific area of expertise. The greatest value to the individual holding the specialist certification is companies who employee certified specialists gain a level confidence with this individual and it also improves the company image to the potential customer.

Another level of certification is directed to building inspectors. Many of the building inspections are now contracted to professional inspectors. These inspectors are usually certified in as many fields of building construction as possible. The NABCEP Certified PV Inspector certification provides these inspectors with this credential that indicates they possess a sufficient level of knowledge and expertise with PV system inspection requirements. No training or experience is required to sit for this exam. However, it is highly recommended to sufficiently educate yourself prior to registering for the exam; it requires extensive knowledge of the IBC, NEC and PV system design.

Certifications are key to any industry’s growth and success. The solar industry is still young and it cannot afford failures due to poor quality installations. One effective way to guard against these failures is to support industry certifications. NABCEP has done a great job of providing their widely accepted certification program to many from different backgrounds. When you become certified at any level, you enhance your opportunities for personal success, you help raise the standards of the industry and you help provide consumers with the ability to make smart choices.

Solairgen PV specific training programs are NABCEP approved and IREC accredited solar PV training programs, designed in support of industry certification and professional excellence.

Kelly Provence
Solairgen School of Solar Technology
IREC Certified Master PV Trainer
NABCEP Certified PV Installation Professional
NABCEP Certified Technical Sales Professional
Master Electrician

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Jobs in the Solar PV Industry

Image of Solar Installer on Roof of a Virginia HomeThe solar workforce in the U.S. has grown by over 160% since 2010 for a little over 93,000 to well over 250,000. Solar accounts for less than 2% of all electrical energy generation in the U.S. However, the solar industry employs twice as many workers as the coal industry, almost five times as many as nuclear power, and about the same number as the natural gas industry.

The largest sector of jobs is in installation of the PV equipment and systems at 51%. The second largest sector is in manufacturing of equipment and materials at 16%. Project development is third at 14%, sales and distribution is fourth at 12%, and the remaining jobs are in various sectors at 7%.

These figures indicate that the best job opportunities will be in the installation sector and that would be correct. This will require a certain amount of training to make yourself attractive to an employer and the solar industry in general. The most difficult positions for employers to fill are, (1) Sales Professionals, (2) Electricians and (3) Installers. If it is possible to get retrained to fill the roll of one of these professional groups, your prospects for employment are very good.

It may also be possible that your present skill will be attractive to an employer in this industry. Every solar contractor or developer needs workers who possess a variety of skill sets. Aside from developing skills listed in the previous paragraph, here are some job skills that are always be required by some of the personal within each company:

  1. An understanding of IP and computer communication are necessary skills since all solar electronic equipment is now built to communicate within its own network and with the internet.
  2. A proficient understanding of computer programs and the ability to bridge computational and drawing programs with DOE and Google type programs is essential to most solar companies.
  3. Writing and editing skills are always necessary when communication with other people and groups is common; this is an undervalued skill that can can make a big difference in the success of a company.

The best way to get into this industry as an employee or as a contractor is to first look at the skills you now possess and see where you would fit in. The next step is to get enough training with solar sales and design to get you started and continue training as you move up the ladder to success.

Kelly Provence
IREC Certified Master PV Trainer
NABCEP Certified Professional PV Installer
Master Electrician

Solairgen School of Solar Training

Solar Training Education and Learning Text

Training for Changes in the Solar PV Industry

Why We Keep Up with Industry Changes

There is no argument that the only constant in the world seems to be change. This is even truer for the solar industry as changes in the solar market happen fast. The only constant appears to be the solar resource itself, our sun.

Training courses must be updated regularly to meet those industry changes and to prepare students who are either just entering the solar business or professionals who are currently working in it. Solairgen is constantly editing its courses for these changes. When necessary, we’ve been known to completely rewrite courses to address industry changes due to product innovations, code changes, regulated utility interconnection requirements, and rapid industry growth.

Here are the most recent changes to our training curriculum:

PV301 NABCEP Associate Installer Credential Exam Prep:
Recently we added a NABCEP Associate Credential exam prep course to follow our PV201 Introduction to PV Design and Installation online course.  PV301 is free to our PV201 students and costs just $185 for anyone who has not taken PV201, but has met the training prerequisite to sit for the Associate Credential exam but needs exam prep study material.

PV202 Solar PV Technical Design
Our online PV202 course now focuses on the technical design aspects of residential, commercial and energy storage systems. It is the next step for designers and installers after completing PV201.

PV210 PV Sales and Cost Analysis
We have introduced a new course, PV210 PV Sales and Cost Analysis that focuses on training students to focus on the marketing aspects of solar: identifying the solar customer and their energy needs, system costs and financing methods, the value of the investment, environmental impacts and proposal writing.

PV221 – PV224 Advanced Online Series
The advanced training workshop and advanced online series have been updated to address new solar products and changes to the NEC codes. These courses include PV221 PV System and the NEC, PV222 Interactive PV System Configuration, PV223 PV Maintenance and Troubleshooting and PV224 Energy Storage PV System Configuration.

Solar is growing faster than any other energy industry so change means growth, and we believe you will enjoy being a part of it. The changes happening today are exciting innovations that make the solar industry stronger and our training produces strong professionals in the field.

Kelly Provence
IREC Certified Master PV Trainer
Solairgen School of Solar Training