How to calculate for shade hours?

[Feisal]

How to calculate for shade hours? I mean do you just
cut down the insolation hours? IDK. Shade is not like total darkness.
Thanks in advance.

[J.P.M.]

How to calculate for shade hours? I mean do you just
cut down the insolation hours? IDK. Shade is not like total darkness.
Thanks in advance.

It can get a bit complicated. If you are referring to "sun hours", you are highlighting one of the disadvantages of using that method of describing insolation availability.

Short answer: While shade is not total darkness, the diffuse irradiance that is present under shade (or under full cloud cover) usually amounts to something like 10% or less of the total irradiance of a clear sky condition.

Under complete cloud cover, the diffuse portion is 100% of the irradiance and still about 10 % or so as intense as the irradiance under a clear sky.

[Sunking]

Real simple. Determine the Sun Hours, Say 4 Sun Hours. During the day your panels are in the shade 25% of the day. Adjusted Sun Hours = 3. That will get you close enough for DIY work

[bvmm]

The amount of cloud/rain/snow shade is highly dependent on the location, and also dependent on the location. IMO PVWatt does a pretty good job of including this. I did some analysis of the variation between the modeled and actual production for my system. It works out that the production is about 60% of the theoretical production without shade. Most of that is due to weather. Over something like 2 years, the picture below gives a sense of proportion. The most typical day had a bit of shade, although some days had a great deal of shade (likely snow or a day of rain). A few days produced more than the model: generally due to snow reflection or very-cold-but-sunny days.

[Sunking]

The amount of cloud/rain/snow shade is highly dependent on the location, and also dependent on the location. IMO PVWatt does a pretty good job of including this. I did some analysis of the variation between the modeled and actual production for my system. It works out that the production is about 60% of the theoretical production without shade. Most of that is due to weather. Over something like 2 years, the picture below gives a sense of proportion. The most typical day had a bit of shade, although some days had a great deal of shade (likely snow or a day of rain). A few days produced more than the model: generally due to snow reflection or very-cold-but-sunny days.

While PV Watts is accurate, it assume you have clear veiw of the Horizon from East > South > West with panels at due south orientation and proper tilt angles for your location. If you have any shade, orientation, or tilt issues makes PV Watts useless. Not many folks have clear view of the Horizon.

[J.P.M.]

While PV Watts is accurate, it assume you have clear veiw of the Horizon from East > South > West with panels at due south orientation and proper tilt angles for your location. If you have any shade, orientation, or tilt issues makes PV Watts useless. Not many folks have clear view of the Horizon.

SK: Respectfully, That's not entirely correct. While PVWatts does assume an unshaded and unobstructed site, including to the north BTW, and therefore has no way to account for shading by terrain, buildings, trees, etc,, it will handle any tilt < 90 deg., and any azimuth between 0 and 359 deg.

Also, to call PVWatts, or any model "accurate", it may be necessary, or at least helpful to define the context of how the word "accurate' is being used. Within the context of how close a model gets to meeting its stated goals "accurate" might not be the best word to use, as it may imply some predictive aspect for less experienced users. For example, if a model has a stated 90% probability of producing >95% of estimated yearly output, or, say, a 10% probability of producing > 100% of estimated output, is that "accurate" ?

In any case, I believe the OP's original question, while certainly using the word "shade", seemed more about the nature of diffuse radiation that "washes" into shaded areas and how to determine both the intensity of the irradiation as well as the times and duration of shading of a solar device. IMO, the question, while most certainly a valid one, needs a fair amount of text for a proper answer, which is why my original response to the OP stated "It can get a bit complicated".

As stated in the PVWatts intro/help screens, The model has no way to account for shading from nearby objects, or self shading from adjacent rows of collectors. To say something is "useless" because it does not do something which it was never intended to do seems to dismiss the model and its usefulness in a way that may lead the less informed ( such a the OP, apparently) to avoid using something that may be quite useful when used within the stated limitations.

If the OP has an inclination to learn a bit about diffuse radiation and can take a stab at estimating shading, and/or use other models that claim to account for shading, the results can be worth the investment in time and effort.

I'd suggest that in geographical areas where PVWatts can be used, once aware of the limitations, its a hell of a lot better, more useful, and more flexible than the use of "sun hours", which seems to have an anachronistic aspect to it not unlike the old monthly ASHRAE insolation tables we all used before PC's and the internet came along. Time and technology marches on.

Finally, I don't know for sure, but I don't think the "sun hours" data or method has a way to account for shading from terrain or other blocking features either, making it as useless as any other modeling method that does not account for shading, including PVWatts.

[Sunking]

SK: Respectfully, That's not entirely correct. While PVWatts does assume an unshaded and unobstructed site, including to the north BTW, and therefore has no way to account for shading by terrain, buildings, trees, etc,, it will handle any tilt < 90 deg., and any azimuth between 0 and 359 deg.

Agreed. But from a laymen POV is accurate. PV Watts assumes perfect conditions. Yes you can change the orientation and tilt from optimum and it will calculate for those limitations, but it does not account for hill, terrain, or tree shade which is my point. If you rpanels have tree shade from sunrise to solar noon, well 50% is about right for a DIY. Otherwise use a pyranometer or a test panel.

[J.P.M.]

Agreed. But from a laymen POV is accurate. PV Watts assumes perfect conditions. Yes you can change the orientation and tilt from optimum and it will calculate for those limitations, but it does not account for hill, terrain, or tree shade which is my point. If you rpanels have tree shade from sunrise to solar noon, well 50% is about right for a DIY. Otherwise use a pyranometer or a test panel.

I believe I understand your point. However, I do not agree PVWatts assumes perfect conditions. Nor does it assume an unshaded sky - if that's the definition of perfect conditions being used here - it so states, uncategorically. No assumptions about it. Read the info screens for confirmation.

The sun hours model, to the best of my knowledge and like most simple models, also assumes an unshaded field of view.

Therefore, seems to me that any assumptions or methods used with respect to shading estimates will apply equally to either model along with all of the associated limitations and shortcomings in the shading method used. To the degree that's true and correct, I see no advantage, and some real, practical limitations to the sun hours method, while seeing no advantages, provided the job site is in North America. Help me out here.

The common insolation estimate that PVWatts and other models use, typically TMY2 or TMY3, while not perfect, give more and better information about the type and time distribution of insolation data that, while still an estimate, seems to have stood the (short ?) test of time as to giving results that make sense and produce results that are useful in more ways than the sun hours method.

[Feisal]

It can get a bit complicated. If you are referring to &quot;sun hours&quot;, you are highlighting one of the disadvantages of using that method of describing insolation availability.

Short answer: While shade is not total darkness, the diffuse irradiance that is present under shade (or under full cloud cover) usually amounts to something like 10% or less of the total irradiance of a clear sky condition.

Under complete cloud cover, the diffuse portion is 100% of the irradiance and still about 10 % or so as intense as the irradiance under a clear sky.

Thanks a lot J.P.M.

[Feisal]

Real simple. Determine the Sun Hours, Say 4 Sun Hours. During the day your panels are in the shade 25% of the day. Adjusted Sun Hours = 3. That will get you close enough for DIY work

Than you Sunking.

[J.P.M.]

Thanks a lot J.P.M.

FWIW, you're welcome.

[Scott Sousa]

PV watts defaults to 3% shading..
You can adjust this on the system info page under system losses, click on the loss calculator.
Without a suneye or pathfinder it is difficult to come up with an accurate number.

[Feisal]

PV watts defaults to 3% shading..
You can adjust this on the system info page under system losses, click on the loss calculator.
Without a suneye or pathfinder it is difficult to come up with an accurate number.

Thanks Scott. Where can I find the system info page so as to click on the loss calculator? You mean on this site? What is a suneye or pathfinder? I'm lost here, excuse my ignorance on this matter.

[Scott Sousa]

http://pvwatts.nrel.gov/

I am just saying that PV watts plugs in a 3% shade as default- without proper tools it is difficult calculate shade accurately.
System info is the third page after you type in the address. there is a little picture of a calculator about the middle of that page- next to the line "system losses". click on the calculator and a new box will open. this is where you can adjust the shading amongst other things.

[Mike90250]

forget the shade hours, you've easily wasted more electrons talking about them, Just total up the good sun hours and you will be close enough.
I've had a couple days this winter, where the clouds were so heavy, I got only a couple watt hours of harvest. Shaded panels produce no usable power in my experience.
(522 wh for the whole day, off a 5Kw array).