HOMER Energy Support

Solar time

Last Updated: Jul 25, 2016 02:17PM MDT

If I change longitude 80° to  86° and keep the same time zone GTM-5, is it valid?


The latitude matters a lot in HOMER’s solar calculations.  You can see it in many of the PV-related equations that appear in the help file, for example in the article labeled “How HOMER Calculates the Radiation Incident on the PV Array”.  But the longitude and the time zone appear in only one equation, namely the equation for solar time.  So they affect nothing but the timing of the solar radiation.  In a case like yours, where we want to shift the timing of the solar radiation by several minutes, we can do so by adjusting either the time zone or the longitude or both.

So no, changing the time zone will not cause a problem, and yes, you can adjust the longitude instead of changing the time zone.  15° of longitude is equal to a one-hour difference in time zone, because the sun moves 360° in 24 hours, or 15°/hr.

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