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Old 12-08-2017, 13:29   #1
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Solar insolation reference info from " Wunderground.com ", the "Weather Underground"

OK, every "solar" thread always mentions one way or another the amount of solar available to your PV panels, and/or the power available, the actual "solar insolation" value which generates the 'juice'.
Here's a handy means of determining just how much solar insolation (raw solar power, watts per meter-squared) is available to you, locally, practically anywhere, close by.
And before the nay-sayers get going, no, these readings won't be coming right from your own deck (unless, see below), with every little cumulus cloud passing by reckoned for.
But they'll be close enough for most purposes, on average. And, you can make some solid estimates of the potential solar power available right on your own PV panels, right where you are. Further, you could track your PV panels' overall performance to some useful degree, vs. local available insolation on given days or longer periods, IMO.

Yes, there are many non-USA PWS WX site readings available too, worldwide.

There have been some handy general solar 'maps' and links on various threads here and elsewhere, but they are just that, pretty general guidelines for large geographic areas. This is a very 'local' insolation resource, accurate, and provided by dedicated amateur WX peeps.

This will give you near real-time (and historical) insolation readings that you can correlate to your own local PV panel installations, usually within a few miles, and fairly accurate.

Simply go to " http://Wunderground.com ".
There are some other such online sites, but WU has perhaps the largest easily accessible database, outside of the difficult to access NOAA-MADIS, etc., sites. Founded by Jeff McMasters.
Look around there for some "Personal Weather Stations" located near you.

https://en.wikipedia.org/wiki/Weathe...ather_service)

Find one nearby that is using an insolation measuring instrument close to you, it will have a reading for insolation measured in watts per meter squared, w/m2 (the international "SI" standard measurement value). A somewhat lower percentage of all the PWS's available have this, but they're out there almost everywhere. Many of the higher-end WS's now have this instrument in the instrumentation package, along with a UV one, Davis for example (yeah, 'that' Davis).

Latch on to that sucker, scroll down to where the "graph/table" options are, and configure a suitable time period; I suggest 6-7 days to start ("Custom" or "Weekly" on the dropdown menu). Hit the "View" button to update the graph/table.
If you place your cursor near the top or bottom of the graph, you will see a movable vertical "time cursor line" for 'marking' (doesn't actually fix a point) time points, EG: you see >500 w/m2 insolation readings from 0900h to 1730h.
Jot down and run some numbers, and you have a pretty good idea of how much total incoming solar power is available to you right close, and for which hours of the day; both on local average, and the current minute-to-minute readings (from the 'current readings' area at the top of the given WU PWS page).
Extracting those insolation values from the "table" display would yield some very accurate numbers too, that could be plugged into a spreadsheet or whatever.
Leave this WU PWS display up on one of your browser tabs 'permanently' and the 'current data' fields will usually update themselves (your graph/table area needs a manual refreshing/updating).
Almost as good as having your own solar insolation instrument ($$$), but free.

You can also specify any time periods you might like to see, monthly, yearly, quarterly, seasonly, whatever; as long as that station was sending upload data to WU, it's available to you to graph out and use.

Take some screen caps and peruse at your leisure for reference calculations.
You'll see a graph (or table, if you prefer) like the below screen capture I did.
This graph will give a very good display of actual solar insolation conditions very near to your own PV panels.
The only better way would be to get your own insolation measuring instrument right next to your panels, but they're kind of pricey...

Hope y'all find it useful, never saw any ref to it before, and it's a very useful resource, even far beyond just solar readings.
You can see WX conditions all around your locale, and those PWS pages have a very good zoom-and-pannable radar display too, all the better if there's one nearby to you.

I'm a weathernut too, with a PWS there on WU also, uploading nearly 24/7 to WU, CWOPS, and NOAA-MADIS, a PITA sometimes to keep it all going.

[I haven't seen anyone else reference this great resource here on CF yet, I posted this info pretty much as-is on another CF thread on this forum (have edited it a little), john61ct advised me to put this up as a separate thread.]

Typical solar insolation graph display from a S. TX PWS, hosted on " Wunderground.com " :
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Old 12-08-2017, 13:36   #2
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Re: Solar insolation reference info from " Wunderground.com ", the "Weather Undergrou

Excellent contribution, thanks!

Many people cruising near the equator, or deep south, dry southwest this time of year, say their panels put out very close to full STC maximum power Amps (lmp) rating for many hours per day in real life.

I wonder how this insolation data can help us come up with ballpark predictability for other regions and seasons?

Obviously tilting makes a huge difference much of the time, but I think fair to say most of us don't go to that level of trouble, but maybe we would if the advantage were quantified.
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Old 12-08-2017, 14:06   #3
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Re: Solar insolation reference info from " Wunderground.com ", "WeatherUnderground

Quote:
Originally Posted by john61ct View Post
Excellent contribution, thanks!

Many people cruising near the equator, or deep south, dry southwest this time of year, say their panels put out very close to full STC maximum power Amps (lmp) rating for many hours per day in real life.

I wonder how this insolation data can help us come up with ballpark predictability for other regions and seasons?

Obviously tilting makes a huge difference much of the time, but I think fair to say most of us don't go to that level of trouble, but maybe we would if the advantage were quantified.
(gwaaarssh, )

The data is all in there on the WU pages, just waiting to be mined out.
Check out the "Table" sub-area display on such a PWS page, it will have a column for "w/m2" (I think). That field can be extracted and smoothed out (averaged/median/per hour or minute, etc). Then compare to your panel readings or whatever.
Brings some useful, fairly hard data, to a formerly contentious area around here.

Buying a standalone solar insolation instrument might not be an ideal option yet, but then I haven't looked around for one yet. They may actually be out there, for very inexpensive prices (like those nice little Chinese electrical volt/amp/etc meters for $5-25, that used to cost $$$).

I know that the 'solar insol meter' option on a top end Davis weather station suite is $100+ (~$1200+ for the whole package). Which is why I just have a fairly cheap, but every bit as accurate (ruh-oh, incoming...), PWS system. Actually two of them now, just bought a whole 'refurb' system to use as a parts backup, was cheaper than just buying two components separately. And those sensors do seem to fail periodically.
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Old 12-08-2017, 21:25   #4
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Re: Solar insolation reference info from " Wunderground.com ", the "Weather Undergrou

I was curious as to what the Davis weather station insolation sensor actually measures, so I did a bit of research.

All of their documentation talks about a "diffuser" with "excellent cosine response".

That suggests to me that the instrument records insolation in W/mē at 90° to the sun's rays. i.e what a solar panel would see it it is pointed directly at the sun.

That means that a horizontal panel or even a fixed, angled panel will receive considerably less "effective insolation" than that shown by a weather station almost all of the time.
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Old 13-08-2017, 09:49   #5
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Re: Solar insolation reference info from " Wunderground.com ", the "Weather Undergrou

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Originally Posted by StuM View Post
I was curious as to what the Davis weather station insolation sensor actually measures, so I did a bit of research.

All of their documentation talks about a "diffuser" with "excellent cosine response".

That suggests to me that the instrument records insolation in W/mē at 90° to the sun's rays. i.e what a solar panel would see it it is pointed directly at the sun.

That means that a horizontal panel or even a fixed, angled panel will receive considerably less "effective insolation" than that shown by a weather station almost all of the time.
Those of us with grade 11 math (or a scientific calculator) can usually handle these calculations, part of which rely on the angle of the panel in certain types of panels. Yes, is there a drop-off? Sure. Is it still worth having solar? Sure, and with a shunt and some means to monitor output on cloudless days of full sun where you can know the date and your latitude (and thus determine angle of insolation, assuming your panels are perpendicular to the sky), you can make up "deposit" graphs similar to the "withdrawal" graphs people tend to devise to estimate daily drawdowns plus a buffer.

After all, ballpark works here, as days at anchor tend to reduce, say, radar usage, just as clouds at lunch reduce input. All you have to be is a few Ah on the plus side after losses are factored in, and you can make ice cubes while the sun shines.
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Old 13-08-2017, 13:46   #6
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Re: Solar insolation reference info from " Wunderground.com ", the "Weather Undergrou

Mountain out of a molehill. You don't need "local" stations. You don't need maths and averaging and pulling out numbers.

There are plenty of long-term sources like Sandia National Labs who have had charts posted for ages. The amount of sunlight that you will get depends very simply on latitude and calendar (time of year, as the earth shifts on its axis) and that's it.

So you look up latitude, NOT location. And then you look at the calendar. And there's a chart line for every latitude showing how much sunlight you're going to get.

There are also common references for how much you're going to loose by not orienting your panels to the sun. Roughly, a 10% power loss for every 15 degrees that your panels are misaligned. And that means either you tilt them every hour, because the sun conveniently moves some 15d per hour, or you accept that flat panels will only manage to get "5 hours of noon strength" sunlight per day, more or less.

Fancy local calculations? Oh, right, the sun IS brighter high up in the the Rockies...but that's rarely an issue for boaters.(G)
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Old 13-08-2017, 18:36   #7
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Re: Solar insolation reference info from " Wunderground.com ", the "Weather Undergrou

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Mountain out of a molehill. You don't need "local" stations. You don't need maths and averaging and pulling out numbers.

There are plenty of long-term sources like Sandia National Labs who have had charts posted for ages. The amount of sunlight that you will get depends very simply on latitude and calendar (time of year, as the earth shifts on its axis) and that's it.
So you look up latitude, NOT location. And then you look at the calendar. And there's a chart line for every latitude showing how much sunlight you're going to get.
It's not that simple;

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Old 13-08-2017, 20:04   #8
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Re: Solar insolation reference info from " Wunderground.com ", the "Weather Undergrou

In the real world, nobody that I know goes through an energy analysis prior to installing solar. Most just load all available space and capture what they can and usually, it is never enough.
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Old 13-08-2017, 20:16   #9
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Re: Solar insolation reference info from " Wunderground.com ", the "Weather Undergrou

Good point, Stu. No doubt the unmentioned differences in the Aussie map reflect something like local cloud cover and local weather conditions, so someplace like Alice Springs may get 50% more sunlight than somewhere on the east coast, because it gets less cloud cover and rain?

Still, latitude alone is good enough to get you in the ballpark. Things like weather...Yeah, folks don't realize that Fort Lauderdale gets more annual rainfall then Seattle. More rainfall, and yet, still more "death to all living things!" scorching sunlight. I'm not sure you need extensive local measurements to guess at that, though.(G)
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