Net metering

gino45gino45 Registered Users Posts: 12
I have sunnyboy 3000 with 14 220 panels. At peak production the sunnyboy shows I'm making 3060 momentarily; however, my net always seems much lower than what the inverter shows.

Show, my questions are: what percentage of dc should I show as net ac production from my system. And, is it possible that the inverter is adjusted so that I'm not getting my full production.

Thanks!

Comments

  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    Re: Net metering

    So 14 X 220 = 3080, that really can be expected in perfect lab conditions, generally AC output is about 0.77 the name plate panel ratings.
    Have a look at PVwatts http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/ to get a pretty good estimate of what you should be seeing on the AC side.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Net metering
    gino45 wrote: »
    I have sunnyboy 3000 with 14 220 panels. At peak production the sunnyboy shows I'm making 3060 momentarily; however, my net always seems much lower than what the inverter shows.

    Show, my questions are: what percentage of dc should I show as net ac production from my system. And, is it possible that the inverter is adjusted so that I'm not getting my full production.

    Thanks!

    Your panels have a total nameplate rating of 3080 watts. It is possible that you are actually getting that much or more from your panels on very cold clear days, because the panel voltage increases with lower temperature.

    What numbers are you trying to compare here? Your inverter production should be equal to the difference between your loads and the amount you buy from POCO. Usually you have only two meters, so you only know two of the numbers. If you do not know your loads, you cannot compare what you see on the two meters.

    If you mean that the total production as shown on the inverter does not match the total generation as shown on a utility grade meter connected directly to the inverter output, then there could be simple accuracy issues.

    Example: If, on one day, your inverter produces 10Kwh, and you use 15Kwh, the utility net meter will show 5Kwh purchased by you.
    If the next day your inverter again produces 10Kwh, but you only use 8Kwh (not laundry day :-)) the the utility meter will show you sold 2Kwh.
    If the utility meter displays separately the amount purchased and the amount sold back, that is simply reflecting that at some times during the day your loads are greater than your production and at other times during the day your loads are less than your production. You should not expect the amount sold to match your production.
    SMA SB 3000, old BP panels.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Net metering

    You have a 3080 Watt array. The Sunny Boy is around 95% efficient, however the panels are not. Typically they will average about 80% of their nameplate rating, or 2464 Watts. Factor in the conversion and you get 2340 Watts average. That would be average over the day's "equivalent good sun" not peak. If you are seeing peaks of 3060 Watts there are several factors that can cause this, not the least of which is the accuracy of any meter taking the readings. Edge-of-cloud events can focus light and push panel production over the nameplate rating. Cold temps can increase panel Voltage which the inverter can turn into more current output.

    So, normal expected average: 2464 Watts. The inverter itself will limit output to 3kW +/- a bit, which is what you are seeing some times. If you want the inverter's output to average its maximum Wattage through the good sun hours of the day you'd need more panel. about 600 Watts more.

    What really matters with a GT system is the kW hours it is producing. You should be getting at least 9 kW hours per day from that on a sunny (but short) day.
  • gino45gino45 Registered Users Posts: 12
    Re: Net metering

    Well, my inverter won't handle more panels. That I know for a fact as we tried it with 15 and had to remove one. This was because of an 'occasional' overload--it did it twice before I was convinced.
    This system has produced as much as 17.5 K in a day, which according to the .77 ratio would be over 13. That would be great, but now, in winter it's doing 11.5 which nets more like 9, fwiw.
    Which is why I was wondering about another row of panels going around the inverter.
    Thanks for your informational posts!
  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    Re: Net metering

    I am confused about what you mean by "going around the inverter".

    Do you mean add another inverter? That is possible. I run 2 5200 watt PVPowered inverters into a AC combiner before the final line goes to the main breaker panel.

    What is your location BTW?
  • gino45gino45 Registered Users Posts: 12
    Re: Net metering

    To answer your question, I had previously started this thread:

    http://forum.solar-electric.com/showthread.php?18046-combine-dc-amp-ac

    which was about adding some more panels with ac inverters to the maxed out dc inverter system.
  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    Re: Net metering

    So using some micro inverters you would like to add some capacity. It is very doable, depending on your AHJ and permitting requirements incremental adds might be expensive. I know in my jurisdiction only one generation connection is allowed in the main breaker panel, it must meet the NEC 120% rule for the buss bar. The incremental adds are then made with a AC combiner panel between the inverters and the main panel.

    You can kind of get an idea of how it is done from the pictures attached.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Net metering
    solar_dave wrote: »
    So using some micro inverters you would like to add some capacity. It is very doable, depending on your AHJ and permitting requirements incremental adds might be expensive. I know in my jurisdiction only one generation connection is allowed in the main breaker panel, it must meet the NEC 120% rule for the buss bar. The incremental adds are then made with a AC combiner panel between the inverters and the main panel.

    You still must conform to the 120% rule in the main panel. The total of the ratings of the backfed breakers in the subpanel plus the rating of the main breaker in the main panel must not exceed 120% of the main panel busbar rating.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Net metering
    ggunn wrote: »
    You still must conform to the 120% rule in the main panel. The total of the ratings of the backfed breakers in the subpanel plus the rating of the main breaker in the main panel must not exceed 120% of the main panel busbar rating.

    That is perfectly logical, and therefore suspect. A literal reading of the code section would make the critical number for the main panel the sum of the main breaker amps and the amps on the breaker feeding the subpanel, even if was either larger or smaller than the total of the individual backfed breakers in the subpanel. And in the subpanel, the 120% rule would apply to the sum of the individual circuit breakers and either the main breaker in that panel or the breaker in the main panel which is feeding it if there is no main in the sub.
    SMA SB 3000, old BP panels.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Net metering
    inetdog wrote: »
    That is perfectly logical, and therefore suspect. A literal reading of the code section would make the critical number for the main panel the sum of the main breaker amps and the amps on the breaker feeding the subpanel, even if was either larger or smaller than the total of the individual backfed breakers in the subpanel. And in the subpanel, the 120% rule would apply to the sum of the individual circuit breakers and either the main breaker in that panel or the breaker in the main panel which is feeding it if there is no main in the sub.
    You are correct with respect to the 120% rule calculation in the subpanel, but I'm pretty sure there is an explanatory note that says that says that the rating of the first breaker encountered by the reverse feed is the value to be used in the calculation of the 120% rule for the main panel.

    If I am remembering and interpreting correctly, if you had two inverters with max current X1.25 = 16A, they would each take 20A breakers in the subpanel, and you could (not that you would, necessarily) connect the sub to a 35A breaker in the main, but you would need to use the sum of the two backfed breakers (40A) for the 120% rule calculation in the main. If there are loads in the subpanel, the breaker feeding it could be larger, even much larger than 35A, but you would still use 40A for the 120% calculation in the main panel.

    It makes sense; if you had a 200A main panel and a 100A subpanel fed from a 100A breaker in the main, and you fed the subpanel with a 15A PV backfed breaker, it would make no sense to have to upsize the main to 500A+ to accommodate the 15A from the PV coming in through the 100A breaker feeding the sub.

    My brain hurts.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Net metering
    ggunn wrote: »
    You are correct with respect to the 120% rule calculation in the subpanel, but I'm pretty sure there is an explanatory note that says that says that the rating of the first breaker encountered by the reverse feed is the value to be used in the calculation of the 120% rule for the main panel.

    If I am remembering and interpreting correctly, if you had two inverters with max current X1.25 = 16A, they would each take 20A breakers in the subpanel, and you could (not that you would, necessarily) connect the sub to a 35A breaker in the main, but you would need to use the sum of the two backfed breakers (40A) for the 120% rule calculation in the main. If there are loads in the subpanel, the breaker feeding it could be larger, even much larger than 35A, but you would still use 40A for the 120% calculation in the main panel.

    It makes sense; if you had a 200A main panel and a 100A subpanel fed from a 100A breaker in the main, and you fed the subpanel with a 15A PV backfed breaker, it would make no sense to have to upsize the main to 500A+ to accommodate the 15A from the PV coming in through the 100A breaker feeding the sub.

    My brain hurts.

    Mine too. I will have to look for that explanatory note.
    But although using the first breaker makes sense in allowing you to use the smaller number at the main, I just cannot see that if you use a smaller breaker (35 amp) in the main you still have to assume that 40 amps could be coming back through it. (And don't confuse the issue with the argument that the 35 amp breaker could hold 40 amps for awhile. If you add that into the equation you would end up having to do assume that you could get 50 amps from the two 20 amp breakers.) :-)

    What I would like to see is an acknowledgement that if the GTIs are only capable of producing 16 amps each you could never have more than 32 amps of backfeed.
    For some RE sources, it might make sense to go with whatever the breaker size is, but a GTI is just not going to be capable of an output overload!
    SMA SB 3000, old BP panels.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Net metering
    inetdog wrote: »
    Mine too. I will have to look for that explanatory note.
    But although using the first breaker makes sense in allowing you to use the smaller number at the main, I just cannot see that if you use a smaller breaker (35 amp) in the main you still have to assume that 40 amps could be coming back through it. (And don't confuse the issue with the argument that the 35 amp breaker could hold 40 amps for awhile. If you add that into the equation you would end up having to do assume that you could get 50 amps from the two 20 amp breakers.) :-)

    What I would like to see is an acknowledgement that if the GTIs are only capable of producing 16 amps each you could never have more than 32 amps of backfeed.
    For some RE sources, it might make sense to go with whatever the breaker size is, but a GTI is just not going to be capable of an output overload!
    I'll look for that language in the Code when I get a chance. I'm pretty sure it says that the rating of the first breaker that the inverter sees must be used all the way up the line.
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: Net metering
    ggunn wrote: »
    I'll look for that language in the Code when I get a chance. I'm pretty sure it says that the rating of the first breaker that the inverter sees must be used all the way up the line.

    It's 705.12(D)(7) in the 2011 code. I believe it was moved around from the 2008 code, but was still in 705.12. May also appear in 690.64(B) in previous codes, don't specifically recall.

    Incidentally the draft of the 2014 code has this changing from the OCPD rating to 125% percent of the inverter output rating. But it's only the draft.
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