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1. Senior Member
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## Some calculations of PV system performance

Seems that the real world situation is far from the understanding of the AHJ. A PV system installed at a tilt of 25°, facing South, will provide more output, about 20% more, at 5pm in the summer than if the tilt was 45°. The higher the tilt the larger the difference. Results of calculations assuming Dallas solar data are attached.

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Originally Posted by BillK-AZ
Seems that the real world situation is far from the understanding of the AHJ. A PV system installed at a tilt of 25°, facing South, will provide more output, about 20% more, at 5pm in the summer than if the tilt was 45°. The higher the tilt the larger the difference. Results of calculations assuming Dallas solar data are attached.
I know that, you know that, and even they know that. Please note that this azimuth and tilt requirement is for approval of their rebate, not for building PV in general; they are only rebating systems which they believe as much as possible match their demand curve in June - September from 3PM to 7PM. Customers are welcome to build systems with other orientations if they forego the rebate, which they will all do since the losses in production they would incur in building PV that fits the rebate requirements would be more than the rebate itself.

They sent me a graphic from NREL that shows what they are driving at.

3. Zee
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Originally Posted by BillK-AZ
Seems that the real world situation is far from the understanding of the AHJ. A PV system installed at a tilt of 25°, facing South, will provide more output, about 20% more, at 5pm in the summer than if the tilt was 45°. The higher the tilt the larger the difference. Results of calculations assuming Dallas solar data are attached.
How bout if its 45 at 270 (and not at 180)? I assume that will produce more than 25 at 180, at the time they want it, which is centered around 5 pm.

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If they set up the criteria that any single system has to match their curve, then possibly the 180 - 80 system does so ... while producing such negligible power as to be of no use meeting the demand. Again, I refer back to the subject line. They have set a goal that is either deliberately or incompetently anti-solar. The goal is definitely not to encourage solar or actually spend the rebate money.

5. ## AHJ acting as Utility Manager?

I am a little confused. You mention AHJ. That generally refers to the electrical inspector who enforces the NEC and building codes for the governmental jurisdiction. The utility may have concerns about the time of day when the generation is optimal based on their daily and annual load profiles, but that is not the jurisdiction of the building code enforcement agency. The utility can use rates, rules and regulations as permitted to give price signals to PV installers but the utility generally cannot set a standard for tilt and orientation. This sounds like the code inspector is enforcing a standard that is not in a adopted code. Am I missing something?

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Originally Posted by beanland
I am a little confused. You mention AHJ. That generally refers to the electrical inspector who enforces the NEC and building codes for the governmental jurisdiction. The utility may have concerns about the time of day when the generation is optimal based on their daily and annual load profiles, but that is not the jurisdiction of the building code enforcement agency. The utility can use rates, rules and regulations as permitted to give price signals to PV installers but the utility generally cannot set a standard for tilt and orientation. This sounds like the code inspector is enforcing a standard that is not in a adopted code. Am I missing something?
Ggunn works in a lot of places in Texas where the AHJ and utility are effectively one and the same. Local public power entities that follow same boundaries as code enforcement. In this case I believe he's talking about a policy that would be the utility's if they were separate.
That said, sometimes a entity offering rebates is ultimately the same as the AHJ, i.e. sometimes it comes from a city or county even when the utility is totally different.

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Originally Posted by jaggedben
Ggunn works in a lot of places in Texas where the AHJ and utility are effectively one and the same. Local public power entities that follow same boundaries as code enforcement. In this case I believe he's talking about a policy that would be the utility's if they were separate.
That said, sometimes a entity offering rebates is ultimately the same as the AHJ, i.e. sometimes it comes from a city or county even when the utility is totally different.
It's simpler that that. The AHJ that is offering the rebate (in this case it's the utility) can set whatever rules they want to regulate that expenditure.

8. Zee
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I guess if the requirements are in fact so strict that extremely few systems qualify, then they will relax them. (assuming they have to spend the rebate cash)

They might, over time, offer a proportionally lower rebate based on orientations and tilts that differ from their ideal.

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Originally Posted by Zee
I guess if the requirements are in fact so strict that extremely few systems qualify, then they will relax them. (assuming they have to spend the rebate cash)

They might, over time, offer a proportionally lower rebate based on orientations and tilts that differ from their ideal.
Actually, I'm pretty sure they are perfectly happy not granting any rebates at all.

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Originally Posted by ggunn
Actually, I'm pretty sure they are perfectly happy not granting any rebates at all.
Lmaooo so true. So here in MA I have run into the scenario with municipalities who run their own power plants. Most of MA is serviced by either National Grid or Eversource, large utility corporations who service many towns across the Commonwealth and also in NH, RI, etc. For customers of these large utilities, the state rebate program is universal and anyone can apply. There are not strict restrictions on azimuth and tilt, and most often the rebate is on a per kWh basis. So it is most beneficial for the south facing, 42 degree tilted arrays who will produce the most amount of energy annually.

However the customers who live in and have a town or city owned municiple electric service, the municipality has it's own incentive program (if any at all). It sounds like this is what is going on for you. For smaller electric providers, the actual time of the day that your solar is feeding into the grid has a much larger impact on the demand curves that the utility has to deal with.

Thus, they will offer a larger incentive, or possible only incentivize, installations which are going to be lowering their demand when it is needed most. Usually this is early AM, but most often late afternoon / early evening when customers return from work and start firing up AC in summer, lights in winter, heat pump(s), etc.

For the utility, they factor in not only lowering instantaneous demand, but the entire demand curve. What is happening in California and states with massive penetration of solar PV is that most of this energy is being produced from 10 AM - 3 PM, when electrical demand is actually lowest (except nighttime). Once customers return home and start consuming more electricity, the East facing solar has pretty much shut off and the south facing solar is starting to drop in production. What the utility faces is a MASSIVE ramp in the electrical demand, and has to fire up expensive peaker plants (Natural Gas usually), to be able to match the ramp required from the electrical demand going up and the solar production dropping for the day.

This phenomenon is why municipalities want to incentivize West facing solar arrays at a higher tilt, as people have alluded to, as it helps the most with the utilities demand response. Late afternoon solar production is the most valuable, and the smaller the utility provider, the larger impact your residential installation will have on their demand curve.

For more reference, google search "Solar Duck Curve, Nessie Curve, utility Demand Response"

Finally, this is also why large scale battery storage of solar and wind energy is going to be EXTREMELY valuable in the coming years as more and more solar and wind gets deployed. We can somewhat predict when the sun will shine and what orientations are most valuable, but no one can predict the wind at a given time. Having methods to store and instantaneously release this energy will allow us to get to 100% renewable energy in our lifetime, despite what the narrow minded Republicans and Drumps have to say about clean coal and fossil fuel energy.

To the future!

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