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Convert Solar Radiation - W/m2/hour to MJ/m2/hour

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  • Convert Solar Radiation - W/m2/hour to MJ/m2/hour


    Can someone give me a confirmation about this conversion?

    100 W/m2/h = ? MJ/m2/h

    Is it simply: 100 / 1.000.000?

    Thanks in advance!

  • #2
    Originally posted by Ostracizado View Post

    Can someone give me a confirmation about this conversion?

    100 W/m2/h = ? MJ/m2/h

    Is it simply: 100 / 1.000.000?

    Thanks in advance!
    A watt is a unit of power, one joule per second, so a watt-hour is 3600 joules because there are 3600 seconds in one hour. A joule is a unit of energy. There is no direct conversion between watts and joules.

    Dividing a watt (or watt per square meter) doesn't seem useful, so I'm not sure what you are trying to convert. Can you provide some more context.


    • #3
      I retrieved some data from Visual Crossing; more specifically Solar Radiation. And they confirmed that the hourly data it's in W/m2/h and the daily one it's in W/m2/d (mean).

      To do some calculations of evapotranspiration I need to convert W/m2/h into MJ/m2/h.

      The usual conversion should be from (W/m2 to MJ/m2/h):
      100 W/m2 = 100 J/m2/s
      100 J/m2/s * 3600 s/h = 36000 Joules/m2/h
      36000 J/m2/h / 1000000 = 0.36 MJ/m2/h​

      But, as they said it was already W/m2/h... That's my doubt.

      Hmm, I found their documentation:

      The total amount of solar radiation energy for a day is found by integrating (i.e. summing) the individual solar radiation values for the day. As measurements are not record for every second, it is generally assumed that the same solar energy was recorded for the whole previous interval. For example, if the observations are hourly, the total solar energy can be found by:

      Energy=power x time

      If our observations are hourly, the time is 3600 seconds. Therefore if our observations are hourly, we estimate the total energy for a single hour by multiplying by 3600

      Hourly Energy (Joules, J) = observed instantaneous power x 3600
      For a hourly measurement of 150W/square meter:
      Energy (J) = 150 x 3600 = 540000J

      Energy is typically expressed in megajoules per square meter (0.54 MJ/m2) or kilowatt hours (kWh/ m2 ) . To find the kilowatt hour value from megajoules, divide by 3.6. The API and Query Builder returns values in MJ/m2 for all unit group selections.​

      So I guess the values are, in fact, W/m2.
      Last edited by Ostracizado; 04-20-2023, 03:17 AM.


      • #4
        Yes, and the power flux (W/m) will vary throughout the day as a function of sun angle, cloud cover, shade from nearby obstructions, etc. So the readings throughout the day have to be properly totaled or integrated. As electricity is normally billed in kilowatt-hours, if you are looking at the cost savings of a solar array, it may be useful to use kilowatt-hours per day rather than megajoules per day.
        1 kW h = 3.6 MJ.