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Discussion in 'Photovoltaics' started by AK, Mar 22, 2005.

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  1. AK

    AK Guest

    Hi all;

    I have a 5.9 KW solar system installed. I use 2 Sunny boy 2500
    inverters and my panels are BP.

    According to my contractor, I should be generating at least 7000 KWH
    for the year. I'm standing at apx 5000 now, 10 months after the

    Any ideas what could be the problem? My house has a very good south
    facing angled roof with no trees and obstructions. I thought I would
    be generating more than 7000kwh...

    Also, is there a difference between differrent panels, say between BP
    and sharp? Is one more efficient than the other?

  2. Ecnerwal

    Ecnerwal Guest

    Have you had rain recently? If not, you might have a severe case of dust
    (so wash your panels and see if your daily production improves), or you
    might have a problem your installer should correct (such as some panels
    that are not properly connected).
  3. Well, figuring losses for temperature (20%) and inverter (10%) you're
    looking at something like (5.9 kW * .80 * .90 = ) 4.25 kW actually
    reaching the grid. I don't know your location but let's say you get
    around 4.5 'sun hours' a day, on average. This works out to some
    (4.5 * 365 * 4.25 = ) 6980 kWh/year.

    How much sunlight you get any particular year will vary, of course,
    and you can expect more during the summer than during the winter.
    You might not have a problem. Your system installer may have simply
    used a generic estimate rather than actual weather data from your
    location. This year may have weather slightly different from the last
    couple of decades or you may have an accumulation of dust and dirt
    on your panels.

    You might try the PVwatts program and see if it's estimate is close
    to your contractor.
    No. They all use the same technology (crystalline PV cells) so they
    all perform almost identically. They may fudge their test standards
    though. It's always better to go with the PTC rating for a panel than
    the STC rating.

  4. Some questions:

    1. Where are you located?
    2. Tilt of the array?

    With this I may be able to estimate average annual output
    (if I have sunshine and temperature data for your location).

    A comparison of the last years rain and/or snow to historic
    averages may indicate a better/worse than average year.

    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
  5. Judging from the headers he is on the east coast and near NY.
    So 4.25 sun hours a day may be a bit much. 3.5 would be much more realistic.

    4.25 * 365 * 3.5 = 5429.375

    I'd say he is operating on par if thats the case.

    Also these are AVERAGE insolation numbers, this may be a below average year for his area.

  6. I'm told it can be as much as 30% to 40% depending on several
    factors. Works best in the summer, less well in the Winter.

  7. MrE

    MrE Guest

    Do you use an MPPT charge controller? Otherwise you are throwing away the
    evening and morning power. Voltage drops and charge current doesn't reach
    the batteries. Also to get most power from panels, especially a huge array
    like that, you'd use a mechanical tracker.
  8. Who has the space to have these platforms moving around all over the yard or

    Use the space for stationary panels and get more.

    30% - 40% doesn't stand up in most practical/actual testing.

    It can spread your peak charge out over a longer period though if you can't
    take the shorter "surge"
  9. Would you sacrifice 90% of your house space to get 30-40% more heating

    You didn't read very closely. The 30-40% doesn't happen in real life and is
    easily acheiveable with other methods with less maintenance.

    Will the 30-40% pay for the fence you have to put around your yard to keep
    the kids from getting wacked in the head with moving panels? Will it pay for
    all the extra gas to mow the lawn around these idiotic panels mounts and the
    concrete cow patties everywhere for the mounts?

    How much will that so-called 30-40% cost you?
  10. Not everyone lives cheek to cheek with their neighbors in high density
    housing. Even so, pole mounts are common and the panels can be mounted
    above people's heads. True, they don't integrate in with a building
    structure as aesthetically as, say, PV roofing tiles but you can't have
    If you don't have the space for moving panels then where do you find
    the space for stationary ones? There is one advantage to fixed arrays.
    They don't have moving parts that might breakdown in the 25 to 100 years
    the panels are expected to work and fixed mounts are slightly cheaper.
    I wonder where you got that idea. It all depends on a lot of different
    factors, not the least where you are located and your local weather.
    Generally speaking, for most of the United States, the 30%-40% figure
    seems to be in the ballpark.

  11. First off PV panels are cheap when you compare them to a tracker for a
    couple of $K
    To track the sun here you need to "wobble" these panels over 230 degrees for
    sun up to sun down.

    I got these figures from the Univ of Oregon computer website programme fo
    Michigan state.
    I kind of figured they had some information and research that gives them a
    little more accuracy than a snake oil saleman selling monster mechanisms
    that would get blown off my roof everytime I have a windstorm

    Heyyyyyyyy !!!! Anybody seen a wind turbine with PV blades?.

  12. OK to reciprocate, where do people get this 30-40% figure from?

    Anybody say this, based on testing, and not have a hidden (or not so hidden)

  13. I would also be rather nervous about the higher risk of
    weather damage to panels on trackers. A fixed, flush-
    mounted panel on the roof will have a low profile
    during a storm. But a tracker-mounted panel seems much
    more vulnerable in high winds.

    Although, of course, some fixed position panels are
    also vulnerable. Like those mounted on poles, or on a
    roof at a very different angle than the roof-slope
    itself. But being fixed would give the possibility of
    extra reinforcement, compared to something that has to

    I'm not taking sides either way, but I would consider
    the local weather pattern risks in choosing between
    fixed or tracking.
  14. daestrom

    daestrom Guest

    This is an interesting point. I think it depends on what type of hazard one
    is thinking of. Certainly a fixed collector can have a stronger mounting to
    withstand high winds. But a tracker could position the panels to minimize
    damage from hail storms.

    I guess it would depend on the type of weather in the area as to what kind
    of 'protection' an installation would need.

  15. I would too, if I could get it.

    Cannot get it with a tracker.

    First, you have to pay more to get a tracker. Even IF your tracker got
    40% more power from the PV, since the tracker is NOT free, it is NOT a
    40% return on the investment.

    Every time I've checked, a tracker cost more than the PV it would
    replace -- better investment just to buy more PV. In other words,
    instead of tracking 1000W of PV, buy 1400W instead.

    Finally, by definition, a tracker has moving parts, so is less reliable.

    Personal example: I considerd about 800W on a tracker. The 8 panels
    was all the tracker would hold. Tracker cost so much that instead, I
    purchased 1200W of PV and built a shed (put the panels on the roof) for
    less than buying 800W plus the tracker.

  16. Do you know what my roof would look like with a dual axis tracker that
    actually fully tracks? The sun travel a full 230 degrees some days here. I
    would have to raise the panels up another 6 feet to clear the roof peaks and
    the neighbours would be calling Muldar and Scuzzy for UFOs.

    I would also be out another $20k for mechanical nightmares that take
    maintenance and would be damaged by wind gusts. It was bad enough repairing
    the panel in the last wind storm. Luckily it only came out of it's frame and
    was OK but the banging on the roof lost me sleep for 48 hours until the snow
    melted off the slopes. Luckily it only lasted about 36 hours. Tough panel.

  17. daestrom

    daestrom Guest

    You both seem to be using RMS values for the area under the sine wave of
    insolation in the fixed collector case. Why?? Root-Mean-Square values
    would be appropriate *if* the power of the panel was related to insolation
    squared, but it is not. (of course, RMS is also valid when the power of a
    resistive load is related to current squared, its much more common use)

    The area under a sine wave is not 0.707 * peak value, it is more on the
    order of 0.637 * peak value. This would dictate a much higher improvement
    (Duane's calculation for summer might yield 18hr/(12hr*.637) = 2.35 or a 35%

    And as Duane points out, this is *before* reflection loss adjustments.

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