solar irradiance unit defintion

[mah]

what does the solar irradiance unit W/m2 mean? it is written in datsheet of the panel that it gives maximum power at 1000 W/m2 . what does 1000 W/m2 mean?

[pleppik]

what does the solar irradiance unit W/m2 mean? it is written in datsheet of the panel that it gives maximum power at 1000 W/m2 . what does 1000 W/m2 mean?

It means that the power in the sunlight hitting the panels is 1,000 watts for every square meter of panel area. This is approximately the average power from the sun at sea level. The solar panel can only convert a fraction of this power to electricity (usually 15% to 25%; this is the efficiency of the solar panel).

So for example, if you had ten square meters of solar panels and you aimed them directly at the sun at noon, the sunlight hitting the panels would have about 10 KW of power. 15% efficient panels would convert this to 1,500 watts (1.5 KW) of useable electricity.

For the typical homeowner most of this is not especially relevant. Solar panels are usually sold "by the watt" (where the output is measured under standard conditions of temperature and light intensity). A more efficient solar panel might let you cram more generation capacity onto your roof, but it probably won't be any cheaper per watt.

[carultch]

what does the solar irradiance unit W/m2 mean? it is written in datsheet of the panel that it gives maximum power at 1000 W/m2 . what does 1000 W/m2 mean?

1000 Watts/ meter^2 is nothing more than a nice round number that is used as the industry standard for testing solar modules in the factory. The rating of the module in Watts, that you see on its nameplate, is based on how it performs when tested under a lamp that produces light at an intensity of this nice round number (as well as controlled 25C cell temperature and a few other conditions). These conditions are known as standard test conditions (STC).

It does relate to something realistic, because this is approximately the amount of sunlight intensity that hits a surface at a perpendicular incidence, when there is a clear sky and no clouds, and no background reflected light beyond the sun's direct rays.

It is possible to get more than 1000W/m^2, and this is why we have to design around the first 1.25 safety factor on current. Preparing for an event such as glare from a nearby glass building, or possibly white sand, or snow, or something similar. Most of the time you get much less than 1000W/m^2.

The way to interpret this is the flow rate of energy (i.e. power) per unit area. Watts is a unit of power, meters^2 is a unit of area. Power means the flow rate of energy, i.e. energy per unit time.

Consider a panel of dimensions that are 1 meter by 1 meter. When the sunlight strikes it directly at a perpendicular angle on a day with clear sky, the amount of power striking its surface could be 1000 Watts. Does it produce 1000W? No, not really, because there is a power conversion inefficiency. Probably closer to 150W, per today's technology. If it does produce 150 Watts while being tested under the factory lamp, it will be sold as a 150 Watt module.

[russ]

The 1000 watts/m2 normally only comes at solar noon for a few minutes on a clear bright day - the rest of the time it is less and even far less. The exception being a cloud event or reflected light.

[inetdog]

1000 is a nice round number, but it is also very close to the actual incoming power for the average location on the earth's surface at sea level with the sun directly overhead.
Not sure how that coincidence was arranged.

[J.P.M.]

1000 is a nice round number, but it is also very close to the actual incoming power for the average location on the earth's surface at sea level with the sun directly overhead.
Not sure how that coincidence was arranged.

Start with a solar constant of ~ 1367W/m^2, +/- about 3% for orbital eccentricity and an average clearness index of something like .70 to .73. or so under cloudless skies with a relatively clean atmosphere. Point the device at the sun and voila - irradiance of approx. 1,000 W/m^2. It's mostly a coincidence and a serendipitous, convenient approximation.

[carultch]

1000 is a nice round number, but it is also very close to the actual incoming power for the average location on the earth's surface at sea level with the sun directly overhead.
Not sure how that coincidence was arranged.

I think it is ultimately just our luck of how we defined the kilogram, meter and second. And our distance from the sun / the sun's luminous power.

That's all that unit of Watts/meter^2 contains. It really reduces kilograms/second^3, even though you don't think about kilograms and seconds when you use it. Oh, it looks like the meters don't really even matter.