is this low production?

[ajdadja]

The sun has set on my first day of production.

I figure that today was probably a better-than-average day. No clouds, slightly closer to the summer solstice than the winter solstice. When I look at the solar chart in the app, it looks like a reasonable bell curve.

I have 39 T400H panels, 2 powerwall+ and 1 powerwall2. They seem to have wired 2 strings on each powerwall+.

The tesla app shows 53.8 kWh generated. If I calculate 53.8 kWh * 365 days, I get 19,637 kWh per year. But my original system design claimed 21,887 kWh per year.

Am I missing something?

Thanks!

 

[jjrandorin]

Production is measured against previous months, you can not extrapolate anything from a single day (or a single month). You can go model your estimated production at a site like pvwatts but one day of production, is not going to give you any real data (nor will your first week of production).

To answer "am I missing something?" the short answer to that is "yes, you absolutely, positively, 10000% can not model a days production as what is going to happen each day, because the angle of the sun changes daily (not accounting for weather).

Starting in March, you will see, in general, production scale up higher and higher till you get to the longest day of the year, (which has passed) and then production will grow progressively shorter, daily. We are in September, and you will see production get progressively shorter as the days get shorter, with Dec and January being about 1/2 of June / July's production.

 

[ajdadja]

Seems that my initial post was not very clear. Sorry for that.

The point I was trying to make is that today would have been a better-than-average day. So, if anything, projecting today to an entire year is overly optimistic.

In other words, the 19,637 kWh figure is a conservative upper bound on annual production.

 

[wwu123]

I further what jjrandorin said, and while this type of query is common here, this is the first time I've seen it with a single day of production....

But to give some reasoning to it, 6 of the past 7 days have been "clear" cloudless days with typical smooth curves, yesterday was by far the lowest of those 6, and 10% lower than the best of those days. All "clear" days are not the same, because there are differing levels of haze and particles in the atmosphere that are fairly undetectable to the naked eye, that affect the insolation that actually reaches the ground. Temperature, wind, other factors also have an effect that can vary (production is affected by panel temps).

You're right that Sept 14th would be somewhat above average across the year, if all days were equally "clear" - but perhaps this Sept 14th was a below-average Sept 14th across most years....

 

[taphil]

The point I was trying to make is that today would have been a better-than-average day. So, if anything, projecting today to an entire year is overly optimistic.

No, your math or modeling is wrong. You cannot pick a good day's production and multiply that value to estimate a year's production. Just like you can't pick an hour's production, let's say at high noon in May, and multiply that by 10h/d to estimate daily production.

In May, you might get 70khw per day. If you multipily that by 365, you get 25,550 which would be a gross overestimate.

Solar production follows a curve throughout the day as the sun moves across the sky, and the daily production follows a curve as the earth goes around the sun and the path of the sun changes and daylight hours changes.

 

[Matt-FL]

Mine varies greatly. My largest producing months are actually spring, where I'm getting 130kWh+ on clear days. Peak summer, when I thought I'd be making a lot more, I was actually only making just over 100kWh a day. It is now September, and we've had lots of clouds, so I'm averaging about 80-90kWh. I think you will find over time it should average out to what they expected, or higher. I was estimated at 24MWh per year, but I produced 29.8MWh the first year (which I just hit Aug 30, 2023).

 

[GregBallantyne]

Seems that my initial post was not very clear. Sorry for that.

The point I was trying to make is that today would have been a better-than-average day. So, if anything, projecting today to an entire year is overly optimistic.

In other words, the 19,637 kWh figure is a conservative upper bound on annual production.

Today may well have been a better than average day for the time of year approaching the equinox. I'll say that is was better than my day today, and my optimum generation capacity is higher than yours, although the inverter capacity is the same.
This close to the equinox, we are close to the spot where half the year will be better and half the year will be lower - for an exact replication of conditions other than sunrise and sunset hours. Sun angle etc... is all dependent more on your latitude. Number of cloudy days is based on far less predictable processes.
Bottom line, despite any data that makes any day seem quite representative, a calculation on annual yield based on that day will fail. Far too many variables, in a chaotic system. The only value in looking at a figure derived by that basis is to occupy your time while the actual year goes by, on the way to knowing a number that has some meaning. If you're like me, and watch daily production closely, you are on the path toward far greater detailed sun movement and weather observations than you thought you would ever have. It's fascinating, and a lot of fun. And easy to draw erroneous conclusions, especially early on.
I talk like I know so much, but only have been operating my system since June 10 of this year!
BTW, I wouldn't at all call that low production.

 

[ajdadja]

About 5 months later, I think I'm still underproducing.

My project advisor at Tesla broke down my 21,887 kWh expected annual production by month and emailed me the result. They broke it down as:
Jan. 32kwh, Feb. 44kwh, Mar. 57kwh, April 73kwh, May 78kwh, June 85kwh, July 82kwh, Aug 77kwh, Sep. 68kwh, Oct. 49kwh, Nov. 39kwh, Dec. 29kwh.

That adds up about right. 32*31 + 44*28.25 + 57*31 + 73*30 + 78*31 + 85*30 + 82*31 + 77*31 + 68*30 + 49*31 + 39*30 + 29*31 = 21717 kWh, which is within rounding errors of the original 21,887 kWh number. Great! That should let me figure out how I'm doing without waiting a full 12 months.


In September, I was only live for 17 full days. So, I should have generated 68 kWh * 17 days = 1156 kWh. According to the Tesla app, I only generated 69.2% of that (800.4 kWh).
In October, I should have generated 49kWh * 31 days = 1519 kWh. According to the Tesla app, I only generated 73.5% of that (1117.3 kWh).
In November, I should have generated 39kWh * 30 days = 1170 kWh. According to the Tesla app, I only generated 59.5% of that (696.7 kWh).
In December, I should have generated 29kWh * 31 days = 899 kWh.. According to the Tesla app, I only generated 53.1% of that (477.0 kWh).
In January, I should have generated 32kWh * 31 days = 992 kWh.. According to the Tesla app, I only generated 54.0% of that (535.9 kWh).

When I asked Tesla, they said that the system will underproduce before PTO. But that doesn't make sense to me because I have plenty of powerwall capacity. My 3 powerwalls have never gotten to 100%, so they should be sucking up extra power even if I can't export to the grid.

Do you agree that I have low production? Or is there something that I'm still missing?

At the very least, I figure it's worth sharing with the forum that the project advisers have access to estimated monthly production numbers. Could be useful to other folks.

 

[offandonagain]

You production does seem low enough to warrant more investigation.

1. Do your grid imports (as reported by your utility) match the data in Tesla's app? Your solar production could be measured incorrectly.
2. Can you post some solar production graphs on clear days, e.g. your best Sep and Jan days?
3. The Tesla One app will allow you to connect to your equipment and look at production per inverter string. If you do that during solar noon on a clear day, it might pinpoint issues with specific strings or inverters.

For estimated monthly production values, you can also use PVWatts: PVWatts

 

[Grayson73]

I put in my panel size in PVWatts and left everything else at default and it seems to be way off.

January says 1,238 kWh and I only generated 259 kWh.

 

[offandonagain]

I put in my panel size in PVWatts and left everything else at default and it seems to be way off.

I responded on your other PVWatts thread.