single vs microinverter

stevehull
stevehull Registered Users Posts: 20 ✭
in Solar Beginners Corner #1
Am making the plunge soon to install a 10kW GT array on large barn roof here in central Oklahoma. All electrical infrastructure comes into livestock barn (240 main from pole transformer, cutoffs to houses and to barn as well as transfer switch all in panels). Wiring under roof would be easily accessible. No shading on roof and it is within ~15 degrees of due south at 3/12 pitch (yes, a bit low for winter). Roof is not high at eve (10 feet) with solid wooden purlins/rafters, plywood, foam insulation then ag metal roofing panels.

Everything is on one meter.

Question is to plan for single big inverter or go with multiple microinverters? Looks like cost is higher with micro, but then redundancy if one inverter goes out (or if you believe Enphase, they last forever . . . ).

Looked into wind, but expensive, long payback, output that doesn't match my kWhr monthly useage, need pros to put up big tower, etc. Can do the PV panel installation labor myself.

I use a tractor PTO (10 kW) for occasional outage back-up and have done all reasonable conservation issues (insulated houses, geothermal heat pumps, CFL/LED lighting, air to air heat exchangers, good windows, etc).

The 10 kW (likely 7-8 with net losses) GT was determined to do about 75% of my yearly kW consumption (used PVwatts), but not throwing away too much in months with high PV kW production, but low kW useage (March-May and October-November). Net metering here with coop is on a 30 day cycle.

Hard to believe but no permitting is needed (rural county) or inspections needed except for coop to check it out before throwing the switch.

Thanks in advance
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #2
    Re: single vs microinverter

    Enphase, Fiddle Factor = 0. All you get is KWH output = Priceless. If your looking for a system without any aggravation it can be a little boring to just look at it's output on Enlighten Interface. I don't know how hard it would be to change a inverter on your roof if one ever did go out, then again if one goes out of 40 or so, you wouldn't miss it, till you could change it on your time. One of my systems is Enphase ( 8.2 KW ) and I couldn't be happier with it after 2 years, never a glitch.
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #3
    Re: single vs microinverter

    On a larger system like this, I would go single inverter unless you have shading issues that need module level optimization. Electronics that will last forever on a roof? I don't think so.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,590 admin
    #4
    Re: single vs microinverter

    One other issue--Enphase, you should have live Internet connection to monitor your system output. In the old day, Enphase charged for Internet monitoring (after the first few years for free)--But today, there is no charge (that I am aware of).

    Generally, central inverters have some sort of LCD you can read and/or connection you can make with a local computer. No Internet required for day to day monitoring.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #5
    Re: single vs microinverter
    stevehull wrote: »
    Wiring under roof would be easily accessible.
    You probably don't want to run wiring under the roof.
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  • stevehull
    stevehull Registered Users Posts: 20 ✭
    #6
    Re: single vs microinverter

    OK, then . . . Let's talk costs and specifics. About 40 microinvertrers (@$150 each) is $6000. Would one central inverter be cheaper (and more reliable?). Also, don't I have to run thicker G DC wire from panels to the central inverter? Or do I gang together panels in series to get up to 240 V DC or so. 240 DC has the same resistance drop as 240 AC . . . .

    Can someone provide me with a set of central inverters that will do the trick? This can be mounted inside barn protected from rain/snow etc.

    Thanks again in advance.

    Steve
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: single vs microinverter

    A central GT inverter usually has an input Voltage between 280 and 480 Volts, so you could actually have less V-drop for the wire run.

    40 200 Watt micro-inverters would be 8kW for $6,000.
    SMA Sunny Boy 8kW inverter <$4,000 http://www.solar-electric.com/smasuboy8080.html
    Fronius 10kW inverter <$5,000 http://www.solar-electric.com/frigpl10uni1.html

    You need to do up total system costs for each design, including installation, in order to determine which is the cheapest.

    NAWS has some GT system examples (with costs) here: http://www.solar-electric.com/grtiesyexqu.html
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,590 admin
    #8
    Re: single vs microinverter

    I have given up the central vs micro inverter fight (my generic opinion is to prefer a central inverter)... The central inverters fail (I had one fail after less than a year)--and one entire PV array replaced due to multiple panel failures). And, a few so far, micro inverters have failed.

    The central inverters should be installed in a shaded location (mine is in a pretty cool garage). Micro inverters are on a hot roof--But at this point, I am not sure that a hot roof but under shade of the panel/array is the worst thing in the world (animals find it pretty friendly place to live).

    If you have problems with little chewy creatures--Mounting wiring in metal conduit and keeping the area below the panels screened (and/or easy to clean/clear of nesting materials) can be important too.

    At this point, the whole industry is so fluid (changing panel vendors and specifications, changing inverter manufacturers, tax credits possibly going away, off shoring, business climate, etc.), I would just get the best price/service/support you can and an installer with a good local reputation--And cross your fingers.

    Make sure the economics make sense and you have done a good effort at conservation in your home. 1 month net metering is not the best plan (from a customer point of view). In California, we have highly tiered rates (flat and time of use--From $0.09 to $0.50 or so per kWH)--So, for us, reducing usage to the ~300-1,000 kWH per month range (from higher usage--Such as summer A/C and water pumping) can be a huge cost savings... If you have relatively flat power rates--The economics may not be as good.

    Also--Check out with your roofer on the recommendation for sealing against leaks and flashing (mine was a simple sealant on asphalt tile roof--no leaks after almost 10 years). My roofer spends a fair amount of time fixing/remounting solar arrays because of leaks. And (at least at one time) was even thinking of getting into the solar power installation themselves--From their point of view, the electrical part is the easy portion of the job.

    And--The whole leasing/buying set of options... I don't like leasing--But it is so cost effective (and if it includes maintenance), that it is difficult to ignore (there are so many goverment credits and tax breaks/tax tricks for leasing companies--They can give you ridiculously cheap deals over all). Read the contracts closely--The whole guarantee and buyout sections can be complex.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: single vs microinverter

    Speaking as an off-grid guy I have to point out that having a grid-tie inverter fail is not a catastrophe; you do not lose power, you just have to pay for it until the inverter is fixed/replaced.

    This is why off-grid guys have spares; no inverter = no power. :cry:
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  • stevehull
    stevehull Registered Users Posts: 20 ✭
    #10
    Re: single vs microinverter

    What is the problem with running wires under the roof?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: single vs microinverter
    stevehull wrote: »
    What is the problem with running wires under the roof?

    How do you get them there? Punch a hole through for each and every panel? Not a good idea.
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  • stevehull
    stevehull Registered Users Posts: 20 ✭
    #12
    Re: single vs microinverter

    I assume that you gang together PV panels along the roof top mounting rails and have as few penetrations through metal roof as possible going to central inverter. Is there something I am missing about not running wires under the roof?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: single vs microinverter
    stevehull wrote: »
    I assume that you gang together PV panels along the roof top mounting rails and have as few penetrations through metal roof as possible going to central inverter. Is there something I am missing about not running wires under the roof?

    Maybe we need to define what you mean by "under the roof".
    Normally a central inverter has a high Voltage array created by connecting the panels in series. If there is more than one of these strings they are usually combined in a weather-tight box. This is often mounted on the panel mounts or some other convenient location very near the array. At that point you have one set of wires coming from the combiner box and going to the inverter. It doesn't matter if this is run down the side of the building in conduit or threads its way through the attic and inside walls so long as the route meets regulations regarding safety and identification.

    What you can not do is run wire in the raised channels of metal roofing or directly below any type of roofing (sandwiched between it and sheathing or furring strips) for example. In short, any method where there is a likelihood of penetrating the wire insulation by fasteners or eventual abrasion.
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  • stevehull
    stevehull Registered Users Posts: 20 ✭
    #14
    Re: single vs microinverter

    Yes I certainly DO understand that you cannot run wires directly under the metal sheathing roofing material (like you say under the raised channel ridges). I was going to use a metal roof conduit to bring the wires to central inverter via one through roof point and then have those wires run "underneath the roof".

    Now I understand the confusion!

    Thanks!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #15
    Re: single vs microinverter

    Sorry about that. Sometimes it seems we do have to state the bleeding obvious, but we only do it because sometimes things get expressed in ways not everyone grasps and sometimes it's necessary to make sure anyone else reading the thread isn't mislead or confused.

    At least that's what we try for. Maybe I didn't explain that well. :p
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