Solarmax definition?

[WS Heinenoord]

In MB Wiki there's a system variable called "solarmax" and described as "maximum possible solar radiation on that day at that point of the earth. Needs latitude and longitude set and pressure data available."
Would it be possible to get more details like how it's calculated and what unit is used?
For my station I got today a value of "190" for the MB variable [mbsystem-solarmax:---], while my Davis solar sensor reports around noon a 269 W/m2 as maximum for this december 1st. Other sources indicate a theoretical max for today around 280 W/m2.

Having a bit more detail on how "solarmax" is defined can help me finding why my measured numbers differ.

[admin]

Code: Select all

double max_solar_radiation (double latitude, time_t t, int press10, int sealevel10)
{
  struct tm *tp;
  int doy;                      // day of year
  double e0 = 1380.0;           // exo-atmos. solar irradiance
  double tau = 0.7;             // atmos transmittance
  int hour_angle = 0;           // solar hour angle
  double p_alt, p_sea;          // altitude pressure, sealevel pressure
  double deg2rad = (2 * 3.1415927) / 360.0;
  double declination, cos_zenith, air_mass;
  double i_direct, i_diffuse;

  tp = localtime (&t);
  doy = tp->tm_yday;
  p_alt = press10 / 10.0;
  p_sea = sealevel10 / 10.0;
  latitude *= deg2rad;
  hour_angle *= deg2rad;

  declination = -23.4 * deg2rad * cos (deg2rad * 360.0 * (doy + 10) / 365.0);
  cos_zenith =  (sin (latitude) * sin (declination)) +
                (cos (latitude) * cos (declination) * cos (hour_angle));
  air_mass = (p_alt/p_sea) / cos_zenith;
  i_direct = e0 * pow (tau, air_mass) * cos_zenith;
  i_diffuse = 0.3 * (1 - pow (tau, air_mass)) * e0 * cos_zenith;

  return i_direct + i_diffuse;
}

[zakos52]

Try this variable [sol0!0rad-dmax.1:--]

[Mattk]

Try this variable [sol0!0rad-dmax.1:--]

That variable is for a physical sensor, mbsystem solarmax is quite different