confused by PVWatts open rack worse than roof-mount

[nerdralph]

When I simulate a 10kW system with premium modules & 45-degree tilt and open rack the estimate is 11,760 kWh. For roof-mount it is 12,263 kWh. I'm wondering if this isn't a bug and the two results are reversed.

[J.P.M.]

What leads you to think it might be a bug ?

[nerdralph]

What leads you to think it might be a bug ?

Open rack mounting allows more back-side cooling. Lower panel temperatures means higher production, since Si panels have a negative Tc for Pmax.

[J.P.M.]

If all the other inputs are the same, the model's NOCT of the rack mount is 4 C. lower than the roof mount option meaning its usual operating temp. will most likely be lower for the rack mount (provided all other inputs are the same).

However, the other inputs may not be the same. Assuming the irradiance inputs the model uses are correct or not fouled up in some way, other possible knock on/secondary effects might be in the way the Perez model treats irradiance as f(array location, orientation) particularly with respect to array tilt and how all that affects the POA irradiance, particularly tith respect to albedo and view factors for diffuse irradiance and horizon brightening.
See the PVWatts manual and/or references for the Perez model (Duffie and Beckman have a particularly good and thorough but still readable description of the Perez model).

Another possible way to see if the irradiance input data itself has an error would be to use the old TMY input data instead of the Solar Anywhere data and see if that change produces the same relatively large difference from rack to roof mount open mount outputs.

[nerdralph]

It looks like this won't be easy to figure out. The pvwatts site links to the v5 manual, even though it's using v8. Searches for the v8 manual have come up with nothing.
I don't use pvwatts for customer estimates, just for quick comparisons. SAM is what I use for PV designs. Until I figure out what's up with PVWatts v8, I'll stick with SAM for everything...

[J.P.M.]

It looks like this won't be easy to figure out. The pvwatts site links to the v5 manual, even though it's using v8. Searches for the v8 manual have come up with nothing.
I don't use pvwatts for customer estimates, just for quick comparisons. SAM is what I use for PV designs. Until I figure out what's up with PVWatts v8, I'll stick with SAM for everything...

I use SAM when I want serious results for things that PVWatts doesn't do, or for cases/designs I've already done in SAM.
One way I've looked at it is that SAM is like PVWatts on steroids.
The two have a lot of the same programming, particularly when it comes to the resource and particularly when the thermal model is used.
If you're a regular (registered) SAM user you can send an inquiry to the NREL site and my guess is you'll find an answer to your question. I've found them quite helpful in the past.

[Calsun]

A rule of thumb is that best annual output is with panels at a tilt angle that corresponds to the degrees of latitude for the location. In southern locations a roof pitch is going to be less than optimal. The best angle also depends on whether you want to maximize annual output or maximize output during the summer months when AC demand is highest.

There are other calculators that include inputs for the type of panel, number of panels, type of inverter(s) used and other factors. I am not allowed to post the URLs but you can find them easily enough. Shading of one array by another array is more of a concern with rack mounted panels and needs to be considered in the design.

A separate by big concern is the demise of multi MPPT inverters in recent years. One is much more likely to be forced to go with micro inverters as a result. At least with micro inverters one can keep several spares so when one fails it can be quickly replaced if the racking or roof attachment method permits.

My SunPower 2 MPPT inverted failed in September and is under warranty, but it will be March 2023 before they can provide a replacement inverter. I looked for other 2 MPPT inverters that I could buy but I found nothing. A warranty only has value if the manufacturer is willing an able to replace a failed unit.

[nerdralph]

A separate by big concern is the demise of multi MPPT inverters in recent years.

If you read this forum, you'll find posts from users buying SMA & Fronius string inverters. I've bought Fronius, Growatt, & Solis string inverters with 2-4 MPPT inputs.

[Calsun]

Buying them now or in prior years? The current situation is vastly different with IC supply problems.

[nerdralph]

Buying them now or in prior years? The current situation is vastly different with IC supply problems.

I've bought several this year. Some models have been hard to find in stock, but low inventory is not, "the demise of multi MPPT inverters". In fact, through much of 2022, it was even harder to find distributors with stock of Enphase or APSystems micro-inverters.

As of today, BayWa Canada lists 301 SE6000H-US inverters in stock. They only have a few Fronius inverters in stock, and currently no SMA inverters.
Another distributor had over 100 Solis inverters in stock the last time I checked with them. The same distributor told me they will be carrying GoodWe inverters in 2023.