Underperforming solar? Need advice please.

[Keoni8888]

Hi all,

I need some advice on whether the output of my systems seems to be performing up to par. I would seem to me that it is not, but I have very little experience in this area and am looking for guidance / help. I recently took the plunge into a Tesla Solar Glass roof (v3) and 2 Powerwalls. In general, I love it. Beautiful roof and keeping me mostly off grid. We maxed out the roof and I was told that the system would rate at 12.98kW. This equates to 222 PV roof tiles. Here are the specs for the tiles:

At 25 degrees C:

Voc (v): 13.34
Vmp (v): 10.99
Isc (A): 5.65
Imp (A): 5.32
Pmax (W): 58.47

Temp coefficient:
Voltage at open circuit: -0.299
Max power: -0.395
Short circuit current: 0.047

Using the Tesla app I observe the following. I live in S. Calif. and summer is kicking in. My roof is not shaded. Tiles are installed on both North and South facing roof angles. At mid-day, and to a couple hours on either side, the sun hits both N and S surfaces and is unobstructed.

On hot days (outside temperature of 90+F) I have observed the system producing a maximum instantaneous output of 8.2kW. Hotter days I have seen less. This is at 12:30PM

On cool days with clouds that part momentarily to let through the sun, I have observed up to 12kW instantaneous output. But in general on cool days I observe somewhere in the 9.8 to 10.5 kW sustained instantaneous output.

This seems to be a really huge swing. It seems that the temperature coefficient should account for only about a 20% drop. Factor in some general loss in the system itself, and I would think that it should be producing about 9.75kW sustained on a really hot day, at a minimum. But I may be looking at this wrong. That is where I am looking for advice for you.

I cannot be 100% certain if they actually put up 222 PV tiles as the roof seems to be too slippery for me to go up and count them .

Any thoughts, advice, direction would be hugely appreciated!!

John

[Mike90250]

Welcome to the first hazard of data collection - Is the data good enough to make decisions on.

Solar shingles (If you have 220 tiles, they are likely flat on the roof, not on 6" standoff rails ? ) get pretty hot, and loose power in the heat of the day. I see a large effect on my pole mount arrays on hot/cold days with production swings.

Then you have another issue, sometimes a complete panel (tile) can flake out from heat and recover when it's cooler. You need good instruments to detect that, and with 220 tiles, I'd bet you have at least one dud

Having different array orientations makes it harder to do a A : B comparison.

Sorry I don't have a magic wand solution

[peakbagger]

The Tesla solar roof is new technology and they publish almost no technical information on the configuration, that makes it hard to diagnose remotely. My guess is you have one or two string inverters fed from multiple paralleled strings. As Mike noted, the little information published is that these are directly mounted to the roof sheathing with no space underneath for cooling. That means they heat up a lot on warm days. Solar panels are rated at STC (Standard Test Conditions) and the STC temp is 77 deg F. Typical panels have published rating sheets that have temperature coefficients, the performance of the panels reduce as the temps go up. The TCs are based on the surface temp not the ambient temp which can vary widely. A roof top mounted array may exceed 140 F so the 12.5 KW nameplate roof may be lot less on a hot day. (conversely folks like me were it gets down to -30F on occasion can see much higher output well over nameplate). The other odd part of your installation is having north south tiles. This generally means that one set of those tiles are not at an optimum angle. Rarely do north facing panels in most locations in the US pencil out economically. For much of the year they are facing the wrong direction and the sun is lighting them up at very steep angle.

So you have several issues that may be contributing to lower than design output. Someone with access to modeling software and information about your home can model the performance but even they cant control the weather and irradiation. 12.5 KW is a large system, and when I hear north facing panels in California my first response is that some salesman got a big bonus by installing more panels than they should have. The Tesla system is expensive, yes they make dummy tiles but expect the cost is still steep. Might as well install PV and get the fed to subsidize 30% (less this year) and any other local incentives to install PV tiles even though they are poorly oriented.

The wonders of being an early adopter

[J.P.M.]

This situation could be a poster session for what happens to those who do not investigate before they invest.

Keoni: Your system is probably performing about as good as can be expected given the whole concept of roof tiles as PV panels in the first place as well as with about half of them being in a way less than optimal orientation.

Looks to me like you drank the peddler's Kool-Aid without knowing what you were doing or getting into.

All opinion so take it FWIW: For starters, Tesla tiles are way overpriced. A decent and more conventional turnkey system from a reputable and professional installer can be had in/around so. CA for maybe ~ $2.75 - $3.25/STC W or so before solar tax credit for annual output at least as great per installed STC W (or $ invested) as what you'll see from what's on your roof. What did you pay per STC W ?
Besides that:

1.) It's a product that's new and unproven, and so without a track record. Tesla fans and those with more $$ than the necessary basic solar knowledge will probably love them.
2.) By virtue of the idea they're tile replacements, things get complicated. That makes for problems both for errors during assembly and down the line when maint. is required. The KISS principle applies.
Good luck on being able to service the array.
3.) Facing panels away from the equator is somewhat similar to placing them in some partial shade with respect to annual output. Not something usually done and only then with (hopefully) some or a lot of consideration(s) for the economic consequences. Looks to me that some peddler got you there as well.
4.) Expect a panel temp. about what Peakbagger notes, may be a bit more on the south facing panels. Figure about 30 C to maybe 40 C or so above the ambient air temp. on the roof. Being flush mounted means having only half the cooling surface as a conventional panel. Less available cooling surface means your stuff runs hotter than a conventional panel setup and that hurts efficiency as well as lowers the probability of a longer service life.

As for counting the panels, I'd do that from the ground with binoculars.
If you want to learn about what's being talked/written about in this thread, buy a new or download a free but slightly dated version of " Solar Power Your Home for Dummies".

A read will help you understand what's being written here and why your system may well be doing about as good as can be expected, but that you probably overspent for what's likely to be a system that produces less than a more proven, more efficient, and more serviceable design.

Welcome to the neighborhood and the forum of fewer illusions.

[Ampster]

@Keoni8888
Did Tesla give you any solar output estimate?
When my system was new I looked at daily output a lot. After a while I began focusing on the monthly trends and daily variances seemed less significant. I did have the advantages of both an estimate by the installer and one that I got from PVWatts.

[SunEagle]

Based on history DOW solar shingles never produced what they were supposed to and due to heat had a very short life. Tesla was supposed to fix that problem but maybe not based on the OP's system.