Three Consecutive Perfect Sunny Days - Variation in Output

This is more of a dissertation than a question.

I had three perfect, consecutive sunny days, April 13th through April 15th.

I know temperature affects solar output, but what about humidity and barometric pressure? The production variations are not very big, less than 5%, but I expected them to gradually increase as the days get longer each day, but was looking for the magic number that temperature begins adversely affecting. Temperature wasn't a large increase, but humidity and barometric pressure were.

If nothing else, I figured it would spark some conversation

My stats:
April 13th - 125.2 kWh, temp 77.2 F, humidity 77%, barometer 29.87
April 14th - 122.2 kWh, temp 81.9 F, humidity 84%, barometer 30.12
April 15th - 119.7 kWh, temp 81.8 F, humidity 86%, barometer 30.28

 

[ewoodrick]

I saw some clouds pass by.

 

[Matt-FL]

I saw some clouds pass by.

Yes, there were some clouds at the tail end of April 14th, around 6:45pm. Around then I am typically down to 2.1kW production. It probably impacted slightly. I actually seem to have had more high clouds on the highest production day, April 13th, throughout most of the afternoon.

 

[ewoodrick]

Yes, there were some clouds at the tail end of April 14th, around 6:45pm. Around then I am typically down to 2.1kW production. It probably impacted slightly. I actually seem to have had more high clouds on the highest production day, April 13th, throughout most of the afternoon.

You are talking about Mother Nature, she never does the same day twice. Too many variables between you and the Sun.

BTW, the ones I saw were high Cirrus earlier in the morning on the first video.

 

[yblaser]

Even with no clouds the amount of PM in the atmosphere will change how much of the light is scattered. It might be an artifact of the camera's autogain, but the last day looks like there might be more scatter as the sun looks "bigger." A breeze will also change the temperature of your panels since they will be much hotter than the air temperature.

 

[Unclepaul]

Also, Solar output metering is not of labortory quaility. Imagine that will induce some variance as well.

Most interesting thing I saw on my app, was how you could get a visualiation to the recent Solar eclipse. Obvious dip in production in our area of partial eclipse.

 

[BGbreeder]

Any given panel has a decrease in output per degree of temperature rise at the panel. Typically, it is in the range of about 0.5%/deg C.

There is every reason to suspect that the panel temperature is not linear with respect to outside air temperature, and small changes of wind direction and magnitude will have significant effects on the actual panel temperature.

Using the ballpark 0.5%/deg C, I would have expected your panels to be down by at least 1.4%, and yours appear to be down by slightly more, again, that is to expected with non-linear temperature effects, both within the panels, and in the cooling thereof.

Humidity and barometric pressure do affect the heat capacity of the air, but the effect is quite small compared to forced air cooling, aka breezes.

All the best,

BG

 

[GregBallantyne]

But those 3 almost perfect days are great. I just had 3 out of 4, with the 4th damn good, or nearly almost perfect. Last of those was today. Last day of my power billing period was today, and those 4 days put me on the good side of the self powering line for the billing period.

 

[Matt-FL]

Adding more data. Not trying to decipher specific reasons, just think it is an interesting exercise in looking at variables.

Wind speed was pretty close on each day.

Wind direction was slightly different... I bet that is a key factor.

 

[ewoodrick]

The wind speed is a great example. How consistent is it? It's all over the place.


Mother Nature!

 

[BGbreeder]

@Matt-FL Thanks for sharing the wind direction and speed data. People have done (small) studies with forced ventilation under solar panels. In every case that I have read or seen, the energy gain is much larger than the energy used in the cooling fans. It is one reason why ground mounts are slightly more efficient, the other being more reflected light. However, I am not sure that adding fans and ducting would have much of a return on investment, as the energy gain is slight. Plus, I believe some jurisdictions have rules regarding skirting solar panels.

All the best,

BG

 

[W7PP]

Temp changes, wind speed and direction, clouds, humidity, way too many things too effect change. Unless you see a 5% or greater change, don't worry, be happy!

 

[Visvism]

Ooo a fellow Tempest user!

 

[EatsShoots]

How big is your system? I have an 11.4 kWh system (not Tesla) installed a year back. I've seen it hit 10.4 kWh, but most production in a day has been around 72 kWh. I'm in New York with a pretty nicely angled southern roof. Biggest month for me last year was May. Although the days are longer later on it seems that the cooler temps more than make up for it.

 

[Matt-FL]

How big is your system? I have an 11.4 kWh system (not Tesla) installed a year back. I've seen it hit 10.4 kWh, but most production in a day has been around 72 kWh. I'm in New York with a pretty nicely angled southern roof. Biggest month for me last year was May. Although the days are longer later on it seems that the cooler temps more than make up for it.

I have a 20.4kW Tesla system, 51 T400 panels with two 7.6kW inverters. My max daily production (to date) was 137kWh.

 

[EatsShoots]

I have a 20.4kW Tesla system, 51 T400 panels with two 7.6kW inverters. My max daily production (to date) was 137kWh.

Very similar numbers then to mine in terms of system size to daily output