grid tied inverter size.

peteoldman
peteoldman Registered Users Posts: 5 ✭✭
in Solar Beginners Corner #1
I have 32- 255 watt solarworld panels and 2 -3500 pv power inverters.It seems to me that I have8.16 kw of panels with only 7 kw of inverters .the installer said that some times it works out that way. The inverters seem to peak at 3514 watts . Is this a good combination or should it be more inverters?
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #2
    Re: grid tied inverter size.

    I have 24 of the same panels, and my peak production was in excess of 7kW, so yours may go over 9kW. This rarely happens though.

    If you do not have enough inverter capacity, you simply lose all the energy that is produced above that.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: grid tied inverter size.

    Panels do not produce their rated Wattage. It is normal to "over-panel" a GTI so that its output remains close to peak for longer during production time. This results in a better value in terms of inverter output for the money.

    Your 8160 Watts of array will likely produce an average of 6500 Watts during good sun hours. Yes, it may peak at 8kW + and the inverters will 'clip' power during that time. But for much of the day the output will be lower. Adding additional inverter capacity to grab that 1-1.5 kW peak performance that is missing for however long (one hour? two?) is probably not worth the money.

    And on less-than-sunny days it would gain you nothing at all.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #4
    Re: grid tied inverter size.

    My grid tie system has 12.5 kw of panels and only 10.4 kw of inverters. I saw mine clip only 2 or 3 times on a cool day in April. The rest of the time is falls short because panel temps tend to derate the panels.
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  • jaggedben
    jaggedben Solar Expert Posts: 230 ✭✭
    #5
    Re: grid tied inverter size.

    It comes down to this... Why pay for more inverter capacity when it is only going to be used a small fraction of the time, when conditions are really good? With panel prices so cheap these days, it is usually considered economical to add more panel capacity, which will always produce more power.

    Your power ratio (inverter/panels) is about 86%, and that is a little on the low side for residential; I wouldn't normally go lower than that when designing residential systems. But commercial systems are often around 80% and utility scale systems can be as low as 60%.
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #6
    Re: grid tied inverter size.

    Even with two 3500w inverters, you've maxed out the backfeed capability of a 200Amp service. With the 20% backfeed rule, the largest your backfeed breakers can be is 40A or 7,680watts of continuous power. (80% derate). Going to a larger inverter would have meant either upgrading your 200A service, or derating its main breaker size. Your array will rarely generate the full capacity of the inverters - so why go beyond this backfeed threshold? Using an inverter that is 90% of the array wattage is pretty standard anyway.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #7
    Re: grid tied inverter size.
    solarix wrote: »
    Even with two 3500w inverters, you've maxed out the backfeed capability of a 200Amp service. With the 20% backfeed rule, the largest your backfeed breakers can be is 40A or 7,680watts of continuous power. (80% derate). Going to a larger inverter would have meant either upgrading your 200A service, or derating its main breaker size.
    derating the main breaker is not necessarily such a bad thing, although you would need an electrician and having the meter pulled to do it safely.
    But you could also go to a line-side tap and not be limited by the panel size at all. Just could not go above the 200A (48kW) x .8 = 38.4kW of the service itself.
    SMA SB 3000, old BP panels.
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