PV Electrical Codes, Time of Use and Inverter Clipping Considerations

Dr. Sean White takes on student questions about National Electrical Code compliance, solar panel Time of Use (TOU) optimization and inverter lifespan. 

Student 1: Why do some AHJ’s (Authority Having Jurisdiction’s) still allow installations under a previous edition of the National Electrical Code? Safety being paramount, is there a time limit for adoption of the latest code?

Dr. Sean White: I figure there are different reasons why the new version is not adopted right away, even though it is usually safer.

1) The electricians have time to learn how to comply with the new Code.
2) If you are making plans for a big project a year in advance and do not know what the new Code will be, it can be unfair.
3) It gives time for the manufacturers to come up with new products that comply with the new Code.

Sometimes it takes years for manufacturers to come up with reliable products that will comply with the new Code. When the 2011 Code came out, there was a new provision for DC arc-fault protection. There was no equipment that would meet the requirements. Whenever a new version of the Code comes out, you hear talk in Massachusetts (where they adopt the Code right away) of people citing NEC Article 90.4 Enforcement, which says that if there are no suitable products available, then the previous version of the Code may be used. The only state that adopts the NEC right away is Massachusetts and the rest of us have some time to figure out how to apply the changes. Often the authority having jurisdiction (AHJ) will let us use a newer version. It is always exciting in MA every 3 years when the new Code is released.

There is no time limit for adopting the code, it is up to the state and some states will have different jurisdictions that adopt at different times.

Student 2: Regarding areas that have TOU (Time of Use)… there may be incentive to ensure you are producing the most during on-peak periods. For example, if your on-peak was in the later afternoon and corresponding with the highest energy prices, then there may be an incentive to try to ensure maximal output at this time. Does that make sense?

Anyway, the question that I have is around this concept of the inverter “clipping” the excess. If the system is oversized and regularly doing this, does it reduce the life of your inverter? Can/does it affect the manufacturer’s warranty?

Sean White: Regarding your TOU question– Time of use rate schedules, when implemented, give utility customers an incentive to produce solar energy then the demand is greatest, which is typically in the afternoons and more often summer afternoons. This gives customers more of a reason to put some west-facing (or southwest facing) orientation into their arrays. For sloped rooftops that are east or west facing, I tell people that a good reason for choosing the west-facing rooftop is because there is a good chance that TOU rate schedules will be implemented over the long lifespan of their PV system. As electric vehicles become more popular, people will want to come home and charge their cars. Having a TOU rate schedule will encourage people to wait until everyone has gone to sleep. Time of use rate schedules are more common on the West Coast of the US where there is less humidity. When there is more humidity, the heat is trapped on the surface due to a greenhouse effect and air conditioners are often left on all night in the summer, (as they are not in dryer places where the heat radiates away from the earth after the sun sets.)

If there is a tree blocking the west-facing roof, then the reason for facing east can be the fact that often east-facing arrays will produce more, because it is colder in the morning and the voltage will be better. There will also be microclimate effects that can be taken into consideration, such as morning fog, evening thunderstorms or evening fog. You can always run a PV Watts simulation to see which direction will be best. In PG&E territory in California when people get solar installed, they can opt for a TOU rate schedule. If people live by the coast, where air conditioning is not common, due to cool sea air, usually TOU is a good option. If people live in a valley, where it is very hot in the afternoons and the air conditioners are working hard, it is usually not beneficial to go with the TOU option.

This is because during the peak times, if you are using more energy than you are making, then you will not benefit from TOU. Another example would be if you put 1kW on a building that could use 20kW to offset the entire bill. In this case, you almost would never export and you could potentially see your bill increase after solar if you switched to TOU. Someday, TOU may be mandatory. Every utility is different, so check what is happening where you live. Often people use software, such as ongrid.net, cleanpowerfinance.com or energyperiscope.com to fine tune the TOU rate schedules, customer energy usage profiles and solar system size to determine what is best for the customer.

Inverter Clipping Question: Inverter clipping is when an inverter has enough PV to make the inverter put out full power at times under ideal conditions. If we put 1.5MW on a 1MW inverter, it would clip around noon hours on sunny cool days. The benefits to clipping include smaller inverter, interconnection and ac components.

If the PV array is large enough to make the inverter work at maximum power at times, it could potentially decrease the lifespan of the inverter. On the other hand, if an inverter is putting out 50% more power on an annual basis due to the large array and it breaks before the warranty expires, it could be a benefit. Even if the inverter lasts longer than the warranty and lasts 14 years instead of 15 years, it still would have made more energy in 14 years than the 15-year-old inverter that never clipped.

I have never heard of an inverter company include anything about clipping power voiding the inverter warranty. I have heard inverter companies recommend clipping.

Clipping electronics analogy: If I use my computer a lot, it will probably effect it’s lifespan, but I will get a lot more work done and be able to afford a modern faster more efficient better computer sooner.

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