title: How to calculate solar radiation on vertical (house) walls?
body: <p>I want to calculate the solar radiation on walls which are oriented towards north, south ect. I have the solar radiation for the location and I have tried the following equation:</p>
  
  <pre><code>S_module = S_incident * (math.cos(alpha)*math.sin(beta)*math.cos(psi-theta) + math.sin(alpha)*math.cos(beta))
  </code></pre>
  
  <p>with</p>
  
  <pre><code>alpha = 24.86 #sun elevation angle 
  theta = 81.58 #sun azimuth angle
  beta = 90 #wall tilt angle (vertical module beta=90)
  psi = 135 #surface orientation: psi=0 == facing north, psi=180 == facing south; is the azimuth angle that the module faces 
  S_incident = 663 [W]
  </code></pre>
  
  <p>this is for 2019-6-21 6h timezone=utc</p>
  
  <p>I tried the python package pysolar and the equation above was found here: <a href="https://www.pveducation.org/pvcdrom/properties-of-sunlight/arbitrary-orientation-and-tilt" rel="nofollow noreferrer">https://www.pveducation.org/pvcdrom/properties-of-sunlight/arbitrary-orientation-and-tilt</a></p>
  
  <p>My problem is that I did not get correct results, e.g. for the above properties the result is -495 [W]. This could not be true because in the morning there must be at least a small positive value for the radiation.</p>
  
  <p>Thanks for any hints!</p>