I have a 3.68 kW array with a samil power 3300 inverter but i've never seen more than 3.1 kW since it was installed in December last year. Today we have full sun and a clear blue sky and its noon - the array faces almost due south east, with a roof pitch of about 30 deg, so the sun is full on the array. The power reading is 2.7kW Now the output of the cells is guaranteed within 90% for the first ten years. How can I tell if the array is out of spec?
Ecko2012-05-28T06:23:30Z
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The figure for 3.1kW/3.68kW = 0.842. This is 15.8% low. I don't know exactly where your measurement is, but here are some points:
The rating is for a standard sun of 1000W/m^2 of sunlight. This is very close to what occurs in practice on a clear, cloudless day with exact pointing and the sun high in the sky (more than 50 or 60° above the horizon.
The power rating applies at a standard temperature of 25°C. The operating temperature is more like 60°C, so the power output is de-rated at the rate of -0.485% / °C = 17% typical. Make sure the panels are well ventilated. This is probably most of your losses.
There are losses in the wiring (not more than 2% according to some standards). The voltage drop is probably calculated with wire temperature of 75°C, so it will probably be slightly better than the standards imply.
The panels can be less than rated output by up to 5% in some sort of sample distribution.
Being off point by 30° in one axis will make this difference too. Check the shadow from a set square on the panel in both axes to see the point accuracy at the zenith (sun at its highest point). As the panel is south east, not south, this means the peak won't occur at the zenith, but some other time, which may or may not have an effect on the daily production (I am not sure your location, it seems not too far North of the tropic in the Northern hemisphere). Noon may or may not coincide with the sun's highest point, where you are in the time zone. You could look into this, but probably it is just something to accept.
The sun moves one degree in four minutes, so will be 1% down from a true point in 30 minutes (8 degrees off point).
With 3.1kW you are only 16% down when the expected temperature alone implies 17% low. This implies your panels were well pointed, wiring losses were low, and the temperature was less than expected, on the day that you saw the 3.1kW figure. I don't know whether the figure reported is; before or after the inverter, but the inverter can add losses of up to 10% maybe, depending on power, voltage and so on. You could see the 3.1kW figure after the inverter if the panels were cool enough. Inverters are often specified for efficiency at their rated power, which is probably also the best efficiency operation.
Your current reading of 2.7kW is 73.4%, or 26.6% low. This is an additional 10% loss from that expected due to temperature alone. It sounds reasonable if after the inverter, but if before, I would suspect: shading (even of 1 cell in a series string) dust, a bad day (not full sun clear sky, clean air) off point by 25 degrees in one axis, or less in two axes.. It is likely a combination of several of these. Be very careful if you decide to clean panels, as high voltage is involved. Check how to do it safely. I wouldn't bother, just wait for rain.
The link below shows sample specifications. The output power temperature co-efficient is basically the same for all silicon panels as far as I know.
Facing southeast, you're missing the opportunity to point straight at the sun at noon, when the light is most intense. The sun may be coming straight at the panels in the morning, but will be less intense at that time. 20% below peak is not that uncommon at noon.
If you are in a location that gets hot, you can try the following. Wait for noon on a bright summer day, then spray the array with the nozzle of a hose. This is not to clean the panels, but to chill them. You should see the power output jump.
The installer should have given you an estimate of how much energy you would get per year. It would be worth seeing if you're on-track to meet that.
In simple terms, you have been had.The rating system is such that it states an output which the system will never be able to meet. My 6 kW system never puts out more than 4.
Proof of my statement lies in the rebate which the State of California gave me. It was based not on the rating of the system, but a figure quite a bit lower. It has been explained to me that this is due to line losses and inefficiencies in the inverter.
You could measure the DC voltage and current from the panels, to see if their output is close to their rating.
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