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Calculating Insolation - All methods produce different results.

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12-02-2008 07:31 PM
I have found three methods to obtain insolation values for an object (in this case a 6ftx4ft window). There are probably more that I havent yet discovered.
Each method produces different values, if they were within 10% to 20% I would not be concerned but these are off by factors of at least 2.
Am I doing wrong, if anything?
Are the different methods using different assumptions (sky conditions, shading masks, etc)?
How can I get these to agree (more or less) with one another?

All calculations are 24/7 for an entire year.

Here are the methods and results:
1. Calculate > Solar Access Analysis... , incident radiation, cumulative values.
This I believe results in a single value for the center of the window which is 1.86x10^6 Wh/m2 (Direct radiation), which translates to 590,000 BTU/ft2 then for a 24ft2 window (6x4) becomes *** 14x10^6 BTU ***
Note: I came up with 1wh/m2 = .317 btu/ft2

2. A 24x24 grid on the window produces the avg value of *** 2.1x10^6 BTU ***
Is this the BTU thru just one grid square? that doesnt make sense, so I think this may be the insolation value for the entire window if all grid values were the avg value, can anyone clarify what the BTU values for a grid represents.

3. Analysis tab, Solar Exposure, Total Monthly calulation for the window object with shading mask, the table then shows Incident total BTU's for each month and then the sum (annual total) of *** 5x10^6 BTU *** (Incident)

I know that this has probably been discussed at some length on the squ1 forums but it is difficult to search the database.
If anyone can shed any light on any/all of these methods I would be most grateful.

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