What is a "normal" drop off for solar production in the Winter?

[dhrivnak]

I live in the San Francisco Bay Area and had my solar system installed by Tesla back in April. I'm noticing a dramatic drop off in production for the last several months and I'm hoping someone can help me understand if this is normal or if there may be an issue that I need to escalate. I have an 8.16kw Solar Panel system with 2 Powerwalls. Here is some data from my app to help:

Month	Low (daily)	Avg (daily)	High (daily)
July	34.7 kwh	46.2 kwh	50.9 kwh
August	26.0	36.7	45.3
September	17.8	25.2	33.5
October	6.4	12.7	17.8
November	3.9	6.7	11.9
December	3.7	5.3	8.0
January	1.7	5.6	9.5

I totally understand that the angle of the sun, cloud cover, rain, etc. all will impact production and that I should expect lower production in the Fall/Winter, however this drop off seems far more extreme than what I read about on the web. I was expecting maybe a 50% drop off and not something in the 80-90% range.

I did ask Tesla to look into this and they escalated to Tier 2 support. The response I got back was that the system is "...electrically and mechanically sound. There are no indications of an electrical malfunction and all wires that combine the solar panels are operational. This system is performing optimally at 99% of its bid estimates." Before I drop the issue, I was hoping to hear from others that either my experience is normal or that something doesn't smell right and I should push harder on Tesla to investigate.

Thanks!

Not uncommon, but go to PVWatts for your location and orientation and compare against what the model predicts, I find it to be quite accurate overall.

 

[dareed1]

I'm in Belmont, so only a few miles from the OP. In July we averaged 46.4kWh; in December 10.0kWh. We do lose some production from shading in morning/evening, but probably not more than 1-2kWh. Our roof is pitched at 27 degrees.

 

[woof]

The moment daylight saving time ends you lose an extra hour of sunlight. It makes a big difference in the daily power output during the winter as compared to the summer.

But it'll be back next spring.




(Yes, this is sarcasm, for those both humor and astronomically impaired)

 

[FurryOne]

I don't know whether to laugh at or cry with you people. I've got an 8K system - 4K East, & 4K West, and my system registered 0.9KW generated today. The East panels are covered with snow, and the West panels are covered with ice, and most days I don't even see the Sun in the Winter! So take solace in the fact that you're doing as good as you are.

 

[jboy210]

I don't know whether to laugh at or cry with you people. I've got an 8K system - 4K East, & 4K West, and my system registered 0.9KW generated today. The East panels are covered with snow, and the West panels are covered with ice, and most days I don't even see the Sun in the Winter! So take solace in the fact that you're doing as good as you are.

Move?

 

[SilverSp33d3r]

My best of a 30Kw system has been 185kwh, worst now has been 4kwh

Wow I thought I was bad. Your system is 2.5 times larger than mine And my lowest production day was 3.5 kWh.
December was a terrible month for solar in SoCal, in January I have produced ~400 kWh in 12 days. December’s whole month was 473 kWh generated on my 12.24 system.

 

[SSonnentag]

I live in the San Francisco Bay Area and had my solar system installed by Tesla back in April. I'm noticing a dramatic drop off in production for the last several months and I'm hoping someone can help me understand if this is normal or if there may be an issue that I need to escalate. I have an 8.16kw Solar Panel system with 2 Powerwalls. Here is some data from my app to help:

Month	Low (daily)	Avg (daily)	High (daily)
July	34.7 kwh	46.2 kwh	50.9 kwh
August	26.0	36.7	45.3
September	17.8	25.2	33.5
October	6.4	12.7	17.8
November	3.9	6.7	11.9
December	3.7	5.3	8.0
January	1.7	5.6	9.5

I totally understand that the angle of the sun, cloud cover, rain, etc. all will impact production and that I should expect lower production in the Fall/Winter, however this drop off seems far more extreme than what I read about on the web. I was expecting maybe a 50% drop off and not something in the 80-90% range.

I did ask Tesla to look into this and they escalated to Tier 2 support. The response I got back was that the system is "...electrically and mechanically sound. There are no indications of an electrical malfunction and all wires that combine the solar panels are operational. This system is performing optimally at 99% of its bid estimates." Before I drop the issue, I was hoping to hear from others that either my experience is normal or that something doesn't smell right and I should push harder on Tesla to investigate.

Thanks!

Looks like our 12.5 kW system here in Arizona drops off up to 60% when comparing the highest and lowest months.

 

[ucmndd]

Your experience is very normal. I’m at a similar latitude but inland - I have some substantial shading issues in the winter but less cloud cover / marine layer - my Dec production is about 20% of what I see in June.

 

[cfbackrow]

I don't know whether to laugh at or cry with you people. I've got an 8K system - 4K East, & 4K West, and my system registered 0.9KW generated today. The East panels are covered with snow, and the West panels are covered with ice, and most days I don't even see the Sun in the Winter! So take solace in the fact that you're doing as good as you are.

I'm with you...I'm south of Boston and I've had several days of under 1kWh production

 

[MorrisonHiker]

Our 20.5 kW system peaks at about 150 kWh in June but it drops to around 50 kWh in late December.

 

[wwu123]

This has been helpful. Thank you!

In looking at PVWatts and the other calculator provided by @k808a, I should expect somewhere between 2.093 and 2.95 kWh/M^2/day. For an 8.16kw system, that would equate to 17-24 kwh per day in expected generation for January. I'm clocking in at 5.6 which seems pretty low. I plugged in July as a comparison and the expected is 48.5-63 kwh per day which is somewhat in line to what I actually observed in the app for July. There is an annoying tree from a neighbor that shades my roof a bit and affects my south facing panels more in the winter. The other plans in my design are unaffected by shade. So my production does seem lower than I would have expected, but maybe not so low that would indicate a mechanical issue. I guess I will need to impatiently await the Spring to improve production.

I think you partially answered your question in two ways here. One, you have non-south-facing roof planes, they're just going to have a more dramatic drop-off between summer to winter - if north-facing the worst of it. Two, you have (worse) shading in winter on your (best) south panels. Shading effects can be non-intuitive - even a bit of partial shading on one or two of the south panels could whack the production of the entire string (exact amount often debated here) quite dramatically.

Good news is, if you're going to have seasonal shading, better to whack 50% of the winter production, which is already low due to sun angle, marine layer, winter storms, etc., than 50% of your higher summer production, which gives you lots of NEM credits you can apply across the whole year. Solar installers can model your shading the whole year, and may have factored it into any annual or seasonal production estimates they gave you.

Bad news is, since you have Powerwalls, low winter production gives little opportunity for TOU load-shifting within each day - but unless on EV2-A most of the other rate plans are pretty flat and not worth load-shifting in winter anyways.

Lastly, I'm only a bit south of you, this summer was cooler than most, cooler panels led to better solar productivity in July/Aug, and this winter has been rainiest Dec/Jan in years, both only maybe +/-10% of typical years, but can further dramatize the seasonal differences.

 

[jboy210]

...

Bad news is, since you have Powerwalls, low winter production gives little opportunity for TOU load-shifting within each day - but unless on EV2-A most of the other rate plans are pretty flat and not worth load-shifting in winter anyways.

This...

A few weeks ago the app was displaying a message that there was not enough difference between peak and off-peak to switch to using Powerwall during the peak.
This time of year my TOU plans rates for peak and off-peak only differ by $0.019 per kWh. In summer they are $0.20 different.

 

[roblab]

I find it so much more relaxing to just let the solar panels do their thing and not worry about it. Wintertime sun is lower in the sky and weaker, even though cold weather is better for the solar panel's generation, but it makes up for it in the summer. I tried to figure on how much sun I NEEDED in the winter, and then I sell back to PG&E in the summer. Works for me, though I can't say I've broken even. Panels are expensive, and I have 66. I doubt that I've paid for them yet in power generated, and it's been years.

 

[bob_p]

We have 15.4KW of solar panels and 4 PowerWalls.

Our peak solar power months were June (1711KWh), July (1988 KWh) and August (1854 KWh) last year.

Our lowest months were February (876), November (986) and December (673). Evidently we had a lot of sunny days in January, since we had 1027.

 

[astrorob]

I find it so much more relaxing to just let the solar panels do their thing and not worry about it. Wintertime sun is lower in the sky and weaker, even though cold weather is better for the solar panel's generation, but it makes up for it in the summer. I tried to figure on how much sun I NEEDED in the winter, and then I sell back to PG&E in the summer. Works for me, though I can't say I've broken even. Panels are expensive, and I have 66. I doubt that I've paid for them yet in power generated, and it's been years.

66 panels! you're going to be a big fan of the proposed NEM3 rules then. you'll be paying ~$120 a month just for the privilege of having those panels if you're past the grandfathering period.

i think all solar owners go thru this when they are new - eventually you learn what is normal and roll with it but the low generation in the wintertime does come as a shock to most.

anyway, i'm glad the solstice has passed and generation is inching up again. my 5kw system is making 13kwh on sunny days now, whoohoo! get at best about 40kwh in the middle of the summer here.

 

[tfan2018]

For PV watts, how do we provide the inputs to account for some panels being south facing and some being west facing?

 

[zƬesla]

For PV watts, how do we provide the inputs to account for some panels being south facing and some being west facing?

Run them separately, then add

 

[Sayo]

I'm in same boat as OP here in the east bay. 8.25 system and going from avg 30-40 daily in June to 1-4 daily now.

 

[astrorob]

i have this stupid cedar tree which shades my panels about 5 months per year. there's a tree-shaped dent in the curve which grows from equinox solstice and then shrinks back from solstice to equinox.

lol this windstorm just took out my tree. luckily it seems like it did not do too much damage. 50-foot cedar tree.

 

[jrweiss98020]

Max continuous inverter output on my system is 10150 VA. Peak output I've seen is 10.26 kW in April and 5.05 kW in November, so right at 50% in the dead of winter. That's at N47°48'. Production varies with cloud cover, with a max of 2116 kWh in July and a min of 192 kWh in December.