Is PVWatts accurate(ish) for modeling AC/DC ratios and clipping?

[so_cal_burbs]

I am using PVWatts to model upsizing my existing solar system and the results PVWatts is giving me is surprising but I can't find my error. The inverter is a SolarEdge 7600H which supports up to 11.8 of DC kW.

In PVWatts if I put in my proposed new system in zip 92131 size of 11.36kW, Premium modules, system losses at 10. tilt 20 and azimuth of 142 and in Advanced put DC to AC Size Ratio to 1 I get a result of 19,455kWh/Year.

If I take 11,360/7600 I get a ratio just shy of 1.5. When I put that into PVWatts as the AC/DC ratio and recalculate it says I will get 18,565 from the system. So 18,565/19,455 is .954 or 95.4%.

So even with a 1.5x oversized system the result per PVWatts is I will get about 95% of the output even with a 1:1 inverter. This seems like too much production. I understand that PVWatts is a model and perhaps I'm using it wrong. Did I miss a step or foul my math? If not I'll stick with my current inverter.

[J.P.M.]

What kind of clipping (in max. output in kWh for how many hours) do you get using the hourly output option for each scenario ?

[Ampster]

I have seen the same result when looking at DC to AC ratios. One explanation which I have heard, is that with high DC to AC ratios, the panels ramp earlier and produce longer than a smaller system. This is harder to see visually than the flat top of clipped production. Lately I have had systems with ratios of about 1.3 to one simply because in the last two systems that is where the panel and micro specs worked out economically. I did have a system designed by and installer that was 1.50 to one and it for several years that I owned it, it followed fairly closely the estimate from PVWatts. I wish I had a more scientific or mathematical explanation so consider this anecdotal.

[davidcheok]

Think the way it works is what with more power on possible draw, you get more production during the earlier hours and later hours. You also have available more power to convert when the panels get hotter and produce less yet are able to supply the inverters to max their output. My own system is specced around 1.38 to 1 in order to ensure the inverters are able to push out high output when the panels get very very hot (in fact they are calculated to match inverter output when at max operating temps). Over time as the panels degrade, the other advantage is that the max output of the system can be attained over a longer timespan as the 80+% capacity over 25 yrs will still be higher than the inverter output assuming they are still working.

[Ampster]

Yes, I forgot the long term advantage as the panels degrade the production has a chance to still take advantage of the optimum inverter capacity. I view solar is a long term expenditure of funds to hedge the cost of electricity. Optimizing that expenditure has its benefits which may not be obvious if one only looks at a flat top production curve during the middle of the day and does not consider the longer term kWhs over the life of the system.

[davidcheok]

Yes, I forgot the long term advantage as the panels degrade the production has a chance to still take advantage of the optimum inverter capacity. I view solar is a long term expenditure of funds to hedge the cost of electricity. Optimizing that expenditure has its benefits which may not be obvious if one only looks at a flat top production curve during the middle of the day and does not consider the longer term kWhs over the life of the system.

I look at solar systems as inverter-centric which means maximising output as much as possible throughout the day. I designed mine with the view that the longer i can sustain a flat top, the more energy i can produce at a given max output so i specified the panels around that and that includes working out the lowest output from the panels in the worst possible scenario (highest heat). This has the added advantage of higher output at lower irradiance but at cost of increased panel cost. Price difference between a 500w and 600w panel in my area isnt big and given that I do not want to spend extra later down the line replacing them (cost of bringing panels up and down with scaffolding etc).

[Ampster]

.......Price difference between a 500w and 600w panel in my area isnt big and given that I do not want to spend extra later down the line replacing them (cost of bringing panels up and down with scaffolding etc).

Yes the price difference between more kW in panels is small compared to the cost of racking and getting the panels on the roof. Not to mention the fixed costs of permits and NEM applications. Sometimes a bigger inverter may mean an entire main service panel upgrade so that has to be factored into the cost benefit equation. The decision to go with a higher DC to AC ratio may depend on where you are standing.

[OCJ]

A lot of people overestimate clipping. If you calculate the integral of a sine wave you can put some numbers to it that are pretty close. The other thing to remember is as you approach summer solstice, late June, the days are getting longer but the temperature is rising, which reduces your panel output. After summer solstice the days start getting shorter again as well as the temperature continuing to rise. Of course this is the Northern hemisphere. The end result is that there are only a subset of months where you will clip.

Here are some examples of integrating a sine wave, this would be full output with no clipping at 9.3kW AC producing 74.4kWh, the line is 7.6kW.
solar-integral-full.png

And here is the amount that's clipped, 5.53kWh.
solar-integral-clipped.png

So that's 5.53kWh out of 74.4kWh, or 7.4%, at the absolute peak. Most months out of the year there will be 0 clipping happening. So over the whole year you're looking at a few percent total.

I know there are other efficiencies regarding inverters, but this is just addressing the actual area in a perfect sine wave.

[davidcheok]

A lot of people overestimate clipping...

I know there are other efficiencies regarding inverters, but this is just addressing the actual area in a perfect sine wave.

Such a beautiful curve hahaha

but in real life.. This is a 1.38 ratio. As you can see, due to the oversize, the left and right of the curves bulge up. The slope at the top is the system throttling due to heat and is clipping.

Screenshot 2023-02-13 at 12.59.35 PM.png

[J.P.M.]

A lot of people overestimate clipping. If you calculate the integral of a sine wave you can put some numbers to it that are pretty close. The other thing to remember is as you approach summer solstice, late June, the days are getting longer but the temperature is rising, which reduces your panel output. After summer solstice the days start getting shorter again as well as the temperature continuing to rise. Of course this is the Northern hemisphere. The end result is that there are only a subset of months where you will clip.

Here are some examples of integrating a sine wave, this would be full output with no clipping at 9.3kW AC producing 74.4kWh, the line is 7.6kW.
solar-integral-full.png

And here is the amount that's clipped, 5.53kWh.
solar-integral-clipped.png

So that's 5.53kWh out of 74.4kWh, or 7.4%, at the absolute peak. Most months out of the year there will be 0 clipping happening. So over the whole year you're looking at a few percent total.

I know there are other efficiencies regarding inverters, but this is just addressing the actual area in a perfect sine wave.

Closed form solutions have their place, but numerical integration from a model might be more appropriate, understandable and useful. This is the real world here.

[OCJ]

Well this is my real world example from last June. As you can see, there is clipping, but since my curve isn't perfect because I have 4 difference azimuths, I'm even less worried about it. There is probably not even 1 kWh up there.

So in regards to the OPs question:

So even with a 1.5x oversized system the result per PVWatts is I will get about 95% of the output even with a 1:1 inverter. This seems like too much production. I understand that PVWatts is a model and perhaps I'm using it wrong. Did I miss a step or foul my math? If not I'll stick with my current inverter.

This is why you only lose 5% over the whole year. It looks bad, but the amount up there at that peak isn't that much.

solar-clipping.png

[OCJ]

Such a beautiful curve hahaha

but in real life.. This is a 1.38 ratio. As you can see, due to the oversize, the left and right of the curves bulge up. The slope at the top is the system throttling due to heat and is clipping.

Screenshot 2023-02-13 at 12.59.35 PM.png

Do you have a less cloudy day to look at? When my system clips it's basically just a flat top.

[davidcheok]

Do you have a less cloudy day to look at? When my system clips it's basically just a flat top.

Believe it or not, no. Our area is full of clouds. That chart is actually a very good day with just sparsely scattered clouds. Its usually a lot worst. We have very high irradiance here up to 1200w/m2 and my inverters start to max out at around 800w/m2. Below is the radiation chart for that day.


Screenshot 2023-02-14 at 5.52.03 AM.png

[Ampster]

Such a beautiful curve hahaha

but in real life.. This is a 1.38 ratio. As you can see, due to the oversize, the left and right of the curves bulge up. The slope at the top is the system throttling due to heat and is clipping.

To me it is important to understand the difference between reduction of panel output because of heat and inverter clipping. Reduction of panel output because of heat will occur with any DC to AC ratio. Clipping typically happens with higher DC to AC ratios, and is an inverter phenomena. Some of the arguments for a higher DC to AC ratios are to compensate for degradation of panel output over time or in the short term because of heat.

[davidcheok]

To me it is important to understand the difference between reduction of panel output because of heat and inverter clipping. Reduction of panel output because of heat will occur with any DC to AC ratio. Clipping typically happens with higher DC to AC ratios, and is an inverter phenomena. Some of the arguments for a higher DC to AC ratios are to compensate for degradation of panel output over time or in the short term because of heat.

https://global.apsystems.com/wp-cont...generation.pdf - have a read