Interpretation of PVWatts' Numbers

a0128958
a0128958 Solar Expert Posts: 316 ✭✭✭
I have a question with respect to what exactly is 'Nameplate' performance, and how to compute it using numbers supplied by PVWatts (ver. 1).

When someone refers to 'Nameplate' performance of his or her solar PV system, for a given month, would this be computed by:

Solar Radiation (actually Insolation, from PVWatts) * # Days in Month * Panel Nameplate Wattage / 1000 * Number of panels?

I.e., for my PVWatts location (Ft. Worth) and system (225 W / panel, 36 panels), for this past month Mar., is my 'Nameplate' performance:

5.50 * 31* 225 * / 1000 * 36 = 1381 kWh

Many thanks!

Best regards,

Bill

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Interpretation of PVWatts' Numbers

    I've never heard of "nameplate performance" - at least not in that context.
    But then I don't get out much. :p

    Nameplate ratings of PV's are put into the PV Watts program along with other data from which you get a reasonable prediction of real-world performance. The panel's 'nameplate' rating is set under fixed, laboratory conditions and tends to be a high peak of output. In the real world we derate this to a more reasonable, expected average output. Usually about 80% of the rating over 'good sun hours'.

    Probably someone else has a better response. :blush:
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Interpretation of PVWatts' Numbers

    Nameplate is the panels sticker on the back values, also what your paying for in watts. So for your system, its 8100 watts. ( 36 * 225 )

    Panels are verified in factory for 1000wmsq ( watt meter squared ) @ 25C cell temperatures irradiance, its called a flash test, as its only last for a second or two to limit the panel temperature rising.

    When one posts they are getting name-plate ratings, could be its on a tracker or the time of year the angle is favorable and cold temperatures.

    PVwatts is just a calculator that uses historical data to provide a realistic estimate of performance, which can, at the users option be further lowered due to balance of system efficiency and environmental factors. My systems regularly do about 90% of PVwatts without any maintenance, the 77% factor is silly low.
  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Interpretation of PVWatts' Numbers
    Nameplate is the panels sticker on the back values, also what your paying for in watts. So for your system, its 8100 watts. ( 36 * 225 ) ...

    My systems regularly do about 90% of PVwatts without any maintenance, the 77% factor is silly low.

    SG, much appreciate the note.

    I think you meant to say your systems do 90% of Nameplate (without any maintenance).

    Noting this, a utility company's expectation of 80% of a PVWatts' 0.77 Default (to issue a rebate) means the utility is expecting less than two-thirds of Nameplate (62%).

    It's getting easier to see that the utility company's expectation for rebate is not particularly strong (and can be met with considerably compromised design).

    Many thanks!

    Best regards,

    Bill
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Interpretation of PVWatts' Numbers

    More than likely the utility company doesn't understand what is behind the PVwatts calculations, specifically the default derate to 77%. To further reduce the expected performance proves this as all the utility would do is provide what values it expects for the PVwatts result ( its all user adjustable ) which it doesn't.

    If bet if this was presented to the Utility, one would find they expect 80% of PVwatts with NO deratings, so PVwatts = 77% , utility wants 80% , not 80% of 77%
  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Interpretation of PVWatts' Numbers
    More than likely the utility company doesn't understand what is behind the PVwatts calculations, specifically the default derate to 77%. ...

    If bet if this was presented to the Utility, one would find they expect 80% of PVwatts with NO deratings, so PVwatts = 77% , utility wants 80% , not 80% of 77%

    I went back and checked the (Oncor - TX) utility's documents, to make sure it's 80% of 77% (= 62% of Nameplate, and it's worse (< 60%) when taking into account panel temp).

    "To be eligible to receive an incentive payment, the ... annual ... output ... must be at least ... 80% of the estimated annual ... output for an optimally-sited, unshaded system ... ."

    "The estimated annual ... output of an optimally-sited, unshaded system can be determined by selecting an appropriate location, entering the system capacity in kWdc, and accepting default parameters for tilt (latitude tilt), orientation (due south), and derating factor (0.77) into ... PVWatts ... "

    In fact, using PVWatts, I can specify Shading to be all the way to 80% and obtain an overall 62% DC-to-AC Derating factor. I.e., rebate can still be obtained even if fully one-fifth of the panels are in constant shade.

    So for solar businesses wanting rebate from the (TX) utility ($25K for 10KW systems), the 'hurdle' to obtain the rebate is not substantial, from a PV system performance point of view at least.

    Best regards,

    Bill
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Interpretation of PVWatts' Numbers

    Good to know and luck for your Texan's!
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Interpretation of PVWatts' Numbers

    Anybody who is reporting 90% or more of nameplate rating with any consistency is probably not doing their calcs ver well, IMHO.

    Nice low over night temps (-20c/0f) and perfect sun reflecting off the snow or ice, I every once in a while get above 90% and with edge of cloud I get more than 100%, but only momentarily. As soon as the PV warms up a bit, production drops off to the mid 80's, and after an hour or so, ~80% is a pretty good average for the rest of the day. My Rogue controller has a input watts (as well as amps/volts) so it is pretty easy to follow input production. It also has a peak wattage note in the daily record so you can see what your peak for any given day is. My 400 watts usually runs ~320-330 watts, with early cold morning peaks of ~350. The best I have ever recorded is ~420 watts.

    Tony
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Interpretation of PVWatts' Numbers
    icarus wrote: »
    Anybody who is reporting 90% or more of nameplate rating with any consistency is probably not doing their calcs ver well, IMHO.
    Here's a system in Tuscon, Arizona which is fairly regularly peaking close to 100% nameplate rating right now.

    https://enlighten.enphaseenergy.com/public/systems/tL4K292

    System consists of 21 Schott 170w panels all aimed south tilted at 40*. 15 of the inverters are their last gen inverters, the owner added another 6 panels of the most recent revision. Peak output will be going down until next year, though as the tilt and hot summer temps will keep things from reaching 100%.

    Click on the time-lapse 7-days, pause the playback and scroll over to the two peaks a week ago. You'll all panels well over 160w, most in the mid-upper 160w and a couple over 170w. Ambient temps were around 75*F.

    With edge of cloud events, I've seen panels hitting 180W+! (look at yesterday when the system peaked over 3.8kW - nameplate = 3.57kW)
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Interpretation of PVWatts' Numbers

    Fascinating.
    These are 180 Watt panels putting out 160 Watts? That's about 88%.
    What are the atmospheric conditions? Elevation, temperature, humidity.
    And how accurate is the measuring system? We know that some systems report some quite generous numbers that aren't rooted in reality.

    It's hard for us skeptics to believe that such efficiency is possible when repeated real-world results show 80% is a good number. This appears to be 10% better than that. Panels in general aren't very efficient, and all sorts of things knock their output down. Furthermore, inverters are doing good to break 90% efficiency.

    And they laughed at me when I reported 107.5% http://forum.solar-electric.com/showthread.php?t=5646&highlight=edge+cloud

    We'd all be interested in anything that can get efficiency up.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Interpretation of PVWatts' Numbers

    My panels are all ten years + old, some more. How much they have degraded over time I can't tell, but it seems that they put out about what they should all things considered.

    I will continue for the sake of design calcs to count on 80% as being fairly optimistic.

    Tony
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Interpretation of PVWatts' Numbers
    Fascinating.
    These are 180 Watt panels putting out 160 Watts? That's about 88%.
    What are the atmospheric conditions? Elevation, temperature, humidity.
    Tuscon's elevation is about 2600ft. Temperature around 70*F, humidity around 20-25% when the system I referenced earlier got to 100% nameplate.
    And how accurate is the measuring system?
    Enphase's monitoring system supposedly has been tested to +-2%.

    100% is not "normal" for the system I mentioned, but 90%+ is managed with regularity and getting to 100% is not uncommon.

    Compared to my own system where the panels are mounted flat, normal peaks are in the low-mid 80% nameplate rating (even with a bit thin, high clouds). High peak I've seen so far was around 93% nameplate thanks to edge-of-cloud events and a cool day (60*F). But then again, I've only got about 5 days of data so far, and with the flat mounting I should see significantly higher numbers between May-July.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Interpretation of PVWatts' Numbers
    drees wrote: »
    Tuscon's elevation is about 2600ft. Temperature around 70*F, humidity around 20-25% when the system I referenced earlier got to 100% nameplate.

    That's ideal atmospheric conditions. I've noticed that being at 3,000 feet helps, as does not being near a city full of polluted air.
    Enphase's monitoring system supposedly has been tested to +-2%.
    100% is not "normal" for the system I mentioned, but 90%+ is managed with regularity and getting to 100% is not uncommon.

    Compared to my own system where the panels are mounted flat, normal peaks are in the low-mid 80% nameplate rating (even with a bit thin, high clouds). High peak I've seen so far was around 93% nameplate thanks to edge-of-cloud events and a cool day (60*F). But then again, I've only got about 5 days of data so far, and with the flat mounting I should see significantly higher numbers between May-July.

    Well it certainly isn't any less reliable than any other. We're really talking about a 10% improvement over typical, which may not sound like much but we off-gridders try to squeeze out every Watt we can! :p

    As such, we're always interested in the fine details of any system that's performing above expected 'norms'. I think there's good, clean air helping this one along. There is also a benefit from the Enphase set-up; each panel being MPPT to the output. Not sure it's worth the extra cost, though. That's up to the individual, IMHO.