Efficiency of a horizontally mounted panel?

[Stevet47]

I have a unique application that may lend it self to use panels in a flat orientation. I have looked at all of the tilt angle calculators, etc that I can find, but have not been able to locate anything that will tell me my performance loss at a desired angle.

For example, I know I need a single 180W panel mounted at 37 degrees true south for my system. If I mount the panel at 0 degrees, obviously I will loose significant efficiency, but just how much? I need to determine the required wattage of an array at 0 degrees to equal 180W at 37 degrees.

How do I go about doing this?

[Naptown]

At 0 deg do you mean horizontal or vertical?

mounting horizontal you will be cleaning the panel a lot. The output will be very high in summer and very low in winter. Vertically the opposite will happen.
I don't like to lay the panels at less than 10 degrees.

[Wy_White_Wolf]

Use PVwatts.

http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/

First run it with the panel set for ideal angle no DC to AC derate. Then run again with angle set at 0. Divide the former into the latter for %.

WWW

[Stevet47]

Naptown, I mean horizontal.

Wy_White_Wolf, thanks for the link. I am not quite sure I understand what that is doing, but I think I got the results I need. Does this make sense to you:

I entered a DC Rating of .18Kw because I have a single 180w panel.
AC to DC Derate: 0
Array Type: Fixed Tilt
Array Tilt: 37 degrees (based on http://www.macslab.com/optsolar.html at fixed tilt)
Array Azimuth: 180 degrees (south)
And came up with: 4.26 yearly solar radiation.

Then I repeated for horizontal panel:
The only thing I changed was Array Tilt: 0 degrees
And came up with:3.78 yearly solar radiation.

4.26 / 3.78 = 1.12 or 112%. So that means I need 112% more wattage, or 201.6 watts?

Does that math seem correct?

Thanks!

[Naptown]

What is the intended season or seasons you plan to use this what you got was the average annual insolation.

[Stevet47]

It will be used at a cabin only occupied June - August.

Insolation is:
Jan: 1.45
Feb: 2.36
March: 3.32
April: 4.29
May: 5.03
June: 5.60
July: 5.61
August: 4.88
Sept: 3.68
Oct: 2.37
Nov: 1.48
Dec: 1.21

[Wy_White_Wolf]

Naptown, I mean horizontal.

Wy_White_Wolf, thanks for the link. I am not quite sure I understand what that is doing, but I think I got the results I need. Does this make sense to you:

I entered a DC Rating of .18Kw because I have a single 180w panel.
AC to DC Derate: 0
Array Type: Fixed Tilt
Array Tilt: 37 degrees (based on http://www.macslab.com/optsolar.html at fixed tilt)
Array Azimuth: 180 degrees (south)
And came up with: 4.26 yearly solar radiation.

Then I repeated for horizontal panel:
The only thing I changed was Array Tilt: 0 degrees
And came up with:3.78 yearly solar radiation.

4.26 / 3.78 = 1.12 or 112%. So that means I need 112% more wattage, or 201.6 watts?

Does that math seem correct?

Thanks!

Run that for the months you will be there (June, July, Aug) and use the worse case.

WWW