Need help with a Solar Calculation

I am going to guess, you are using micro inverters, one per panel. To start
with, if your AC output is 267W and your inverter is 95% efficient, the panel
DC output would be 281W. Check the current angle (from perpendicular)
of the sun to panels, dividing output power by the cosine of that angle will
approximate the output under similar sun when/if the error angle is zero.
Bruce Roe

Thanks bcroe, seems like that's all knowable stuff with my lat/lon. pvwatts just gives me total power generated and I want peak watts. Is there a calculator that will figure that out? or tell me what the sun angle is based on the day of the month for a given zip (and my tilt and heading of the panels). Seems like the calculation would be a little more involved but definitely deterministic.

You also need to clarify if these have micro inverters or are connected to a string inverter, and the type /. capacity of the inverter / micro inverter as if you have clipping, they may never be able to produce their max..

Finally panels may no produce their max when the sun is at its peak for the year. Other factors like temperature, cloud refraction etc... can make panel produce more even if sun is not at peak angle.

I have 320W panels on my roof that peaked at 324 Watts a few days ago during a hazy day with thin white clouds

They are on solar edge optimizers. I’m trying to figure out the max number of panels I can put on an inverter without getting any clipping (not accounting for odd spikes).

my thinking is that all the losses don’t matter as I’m looking for a relative number. My 267 watts now (April 10) should lead to an another number some percentage higher on the day the sun is most direct to the panels. I would think that it would be June 21.

What is the reason for not wanting any clipping?
This is not necessarily a bad thing. You may loose a tiny bit on a few days, but win overall over the year

I’m trying to quantify “a tiny bit” to make an educated decision on how many more panels to buy.

You should buy as many as you can afford to achieve your production and ROI goals and not worry about the clipping

Hmm. That seems like advice not to be concerned about the inverter size which has a direct relationship to total production.

Inverters are designed to work with more solar input potential than they can produce (up to a certain point), it is the Inverter Load Ratio. Clipping is a natural and planned mode of operation (again within limits).

So all I am saying is that if you already have the inverter. The maximum number of panels you can put on it is defined in the inverter specs, and would lead to some clipping during optimal production days.

Whether you decide to buy the max number of panels the inverter can handle should depend on what is your production goal (how much yearly energy you need to produce) and your return on investment goal (if any).

But certainly not whether there will be some clipping or not.
What is the Inverter model ?

I just Googled sun elevation and came up with the NOAA solar position calculator. June 21st, sun elevation is 80.45 deg for your location; current sun elevation is 65.38 deg. Your panel tilt angle and heading are constant.

Sounds (reads) like you're still in over your head. Have you read "Solar Power Tour Home for Dummies" yet ?

The peak power a panel will produce at site conditions will vary as the irradiance/other weather variable combinations change.

Because the weather and irradiance variables are never the same or in the same combinations, It's not possible to give a closed form answer to your question like is possible for STC measurements.

I had a model for calcing array output under clear sky irradiance and using TMY meterological values for 92004 that I used when designing my solar magnum opus out B.S. Rd., but that model seems to have been misplaced/lost in a computer changeout about 8 yrs. ago. Pissed me off some when I discovered the loss.

You can get close to what the max. modeled DC output of a 320 STC W panel is by using PVWatts hourly output option for a 320 STC watt "array" and looking for the max. hourly DC output value among the 8,760 hourly values. The max. modeled AC output will be the max. DC output multiplied by the inverter efficiency up to the point where clipping occurs. My guess is the optimizers or micros will cause some clipping but not a whole lot over the course of a year.

You can estimate how much clipping will occur with info you'll find in the Dummies book, which is also avail. as a free PDF download for a dated but still useful version.

My guess is both the AC and DC outputs will max out sometime in the spring around this time of year between maybe 1100 and 1300 hours P.S.T. Todays' max. amb. air temp. near your home was ~ 96 F. A ~ 1400 hrs. P.D.T.0Your max. roof ambient was probably about 110 F. or so before the wind came up ~ 1400 hrs. P.D.T. At the time your original post to this thread was written, the GHI at your array was probably ~ 700 W/m^2

J.P.M. , I could very well be over my head. I just read the relevant part of the pdf you reference and I'm still no smarter. I do have a masters in mechanical engineering, 12 semesters of math, and several courses on different modes of energy transfer but maybe not enough. My assumption is that, the primary driver of power output from my panels is the projected surface area of the panel towards the direction of the sun, assuming the sun acts like a point light source. I'd assumed that significant light scattering from things like water in the air would not be that significant, and also, that temperatures of the panel over time would average out to not make a lot of difference.

You definitely have the bandwidth. Projected surface area = good assumption. It is amazing how many times days I go out and wonder why solar production is slightly off today only to realize the hardly noticeable cirrus cloud in the sky. Solar panels have a -%/deg C temperature coefficient for power. My panels have a temperature coefficient of Pmpp -0.3%/deg C. Temperature definitely makes a difference in my power output.

Well, on 03/11 you wrote you were in over your head. Just goes to show 'ya the limited value of formal classroom training that some call education.
FWIW, most people report learning something from reading the Dummies book. I sure did.
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OK. Your assumption is wrong. The primary driver of PV power output is solar irradiance.
Maximizing power output for a given irradiance level usually involves, among other things, maximizing cos(incidence angle) and so projected area.
Maximizing energy output involves maximizing the time integrated values of cos(incidence angle).
Your assumptions as you've stated them will lead you astray.
If you hold an MSME degree, get ahold of a copy of Duffie and Beckman's "Solar Engineering of Thermal Processes", ISBN # 0-471-51056-4. Chaps. 1 & 2 cover resource availability and address why your geometric assumptions will lead to errors, particularly in the treatment of diffuse irradiance and albedo.
Most of the rest of the volume deals with solar thermal processes except for Chap. 23 :"Design of Photovoltaic Systems", which is probably the best short primer I've seen for engineers who want to get up to speed quickly on PV. You may find it more to your liking than the dummies book. It's full of the basic science and practical knowledge of PV. That's were you'll learn the story behind how and why panel temp. changes can lead to measurable changes in cell and so panel output, and why using time averaged cell temps. will lead to errors, and why and how atmospheric constituents (including clouds) scatter and reflect a significant portion of the irradiance that hits the top of the atmosphere on its way to the ground.