Solar heat gain calculator

[Lilly_Pines]

In the spirit of @Jeremy Harris's heat loss spreadsheet, I made a spreadsheet to estimate solar heat gain through windows. It's a bit more complicated topic than straightforward heat loss, but I tried to make it as easy as feasible.

 

To use the spreadsheet, divide the windows and rooflights you want to calculate into groups, where windows in the same group face the same direction and have the same g-value. The groups can be named whatever you want; the names will be used as labels later in the sheet. For each group, input the direction the windows face (in degrees, north is 0, south is 180) and their slope (90 is vertical, 0 is horizontal). Sum the areas of all windows in each group.

 

Once the window groups have been defined and the relevant values specified, the sheet will calculate the inputs you need to use in PVGIS. Windows facing between east and west are fully supported, and for these the inputs to PVGIS are exactly the same as used to specify the window groups, except that the direction (azimuth) is -90 at east, 0 at south, and 90 at west. For windows facing slightly northeast or northwest, the PVGIS values for windows facing east or west are used, and the spreadsheet calculates two different estimates of correction factors in an attempt to give an upper and lower bound for the heat gain. Windows facing significantly northeast or northwest should be ignored, as the error in the estimates will be too high for them to be useful. For rooflights only directions between 90-270 (east to west) are supported as I didn't feel like figuring out a correction method for them.

 

For each window group, input the given azimuth and slope in PVGIS, and go to the "Monthly in-plane irradiation for fixed angle" in the result visualisation. For each month, input these values in the correct field. This is a bit tedious, but making the data collection automatic would be a lot more complex.

 

You can add a shading value (as a percentage between 0-100) to refine the results if you want, but it is not required. The shading value can be defined monthly for each window group, to better account for factors like overhang, deciduous trees, or external blinds. Getting useful numbers is left as an exercise for the user, so just leave the fields empty if in doubt.

 

Once all inputs are given, the spreadsheet calculates two different estimates of solar heat gains in each month. If there is a difference between the estimates, the high estimate is extremely likely to be an overestimate, and can be used to approximate an upper bound of cooling needs. The low estimate should be more accurate for winter heating needs, but should still be taken with a grain of salt. If the estimates agree, you don't need to worry about any of this.

 

Credit to @Adam2 for the original idea of using PVGIS. Any errors and mistakes are mine. Feedback appreciated.

 

Solar heat gain calculator.ods

[Radian]

Great share thanks! Estimating g-value is proving tricky for some glazing as the shading due to certain overhangs depends on time of year. How have you accounted for that in your design (which I believe makes use of overhangs)?

[Lilly_Pines]

2 minutes ago, Radian said:

Great share thanks! Estimating g-value is proving tricky for some glazing as the shading due to certain overhangs depends on time of year. How have you accounted for that in your design (which I believe makes use of overhangs)?

Overhangs are modeled by variable shading; you'd increase the shading percentage in the summer months when the sun is higher so the overhang covers more. It's a bit fiddly and I'm guessing prone to errors but I'm not sure how this could be represented better. The g-value itself is just the g-value of the glass.