wattage versus time

[Bill g phillips]

on solar panels how is the watt output based on time
hours of sunlight, days of sun

for estimating purposes only

[Rob Beckers]

Hi Bill,

The "rated output" that's on the nameplate on the back of every solar panel is the output when the panel is 25 degrees Centigrade and the sunlight is 1000 Watt per square meter. Those conditions rarely happen in real life, so most of the time panels don't reach rated output.

How much energy a panel produces depends on the number of hours and intensity of the sunlight, and those numbers are available for an "average day" for just about any location on earth. To make this easier to figure out there's a solar calculator, named PV-Watts:

http://pvwatts.nrel.gov/

Put in the location and other info, and you get a pretty good estimate for the energy production for grid-tie solar for that location (per average year).

-RoB-

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[Chuck Roberts]

Take your solar panel rating, say 1000watts. To get the real life rating, multiply by 0.8. This is the peak output on the clearest sunniest day of the year with the most optimal angle of the sun hitting the PV panels. So, on a perfect day, your panels will output only 800 watts.

Throughout the summer the angle of the sun changes, reducing the power generated on the panels, rain comes and goes, clouds come and go, all reducing the power generated. So to get an average output over time you might multiple 800w by 0.7 or 0.6 depending on your region.

In my area of west Michigan we have very overcast winters so for 3 months straight a panel rated 1000w might only generate 80w for 3 months of winter due to the constant overcast conditions.

[Alton Root]

wow..it's interesting to know..
is it the same watt(1000 watts per square meter) out-put from solar-panel in low-light Environment?

[Dave Schwartz]

1000 watts per square meter is the theoretical maximum solar insolation with the sun directly overhead (mimimum atmospheric absoption/dispersion between you and the sun) and orthogonal (directly above the panel, i.e. suns rays perpendicular to panel in all directions). Panel wattage is rated under these ideal conditions. A tracker is a device intended to adjust for the best orthogonality but nothing can compensate for the atmospheric factor as the weather changes and the sun shines through more or less atmosphere in its daily east-west and annual north-south trek.

I have seen my theoretical 4140 watt roof-mounted array producing 4100+ watts (indicated on the inverter display) under ideal conditions. I've also seen it producing only a few watts even at solar noon under severe cloud (so thick my satellite TV loses signal).

[Mabel Smith]

The rated output that is placed on the back of every solar panel is the output when the panel is 25 degrees centigrade and the sunlight is 1000 watt per square meter. These optimum conditions are very rarely met in real life .The energy produced by a panel depends on the intensity of sunlight .On finding out the numbers on an average day, you calculate an estimate of the output using a solar calculator http://pvwatts.nrel.gov/