Micro inverters

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  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Micro inverters
    Every part of that statement is debatable. I just wish people would stop declaring any one particular inverter (or any other part of a system for that matter) as being the vastly superior, one-and-only, solution to all situations. 'Tain't so.
    Not going to get any argument from me on that!
  • Hoonose
    Hoonose Registered Users Posts: 16
    Re: Micro inverters

    I am looking to expand my home PV system where i have 2 central SunnyBoy inverters, and will get bids from an installer that uses the enphase panels.

    My previous installer, who is also a personal friend, tells me has has heard (or seen) some of the enphase inverters failing early. A few out of a few dozen panels, that he attributes to our harsh heat here in Lake Havasu.

    Another friend of mine is having the enphase panels installed as his home system, so we will see this next year.

    It's all somewhat of a 'crap shoot' with early adoption, and I like to keep it simpler, if given a choice. And if the total costs end up being similar, I will probably go with a 3rd SunnyBoy.
  • System2
    System2 Posts: 6,290 admin
    Re: Micro inverters

    If you use a 190W micro inverter with a 230 W panel, aren't you wasting 40 watts of power? I mean shouldn't you use a panel with a power rating that is lower or equal to the inverter?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Micro inverters
    MrFye wrote: »
    If you use a 190W micro inverter with a 230 W panel, aren't you wasting 40 watts of power? I mean shouldn't you use a panel with a power rating that is lower or equal to the inverter?

    Nope. Panels rarely actually put out their rated power, whereas the inverter is quite capable of doing so - if you feed enough into it. Something like this:

    230 Watt panel operating @ 80% efficiency (typical) = 184 Watts.

    The more you can keep the panel's true output up at levels which will allow the inverter to produce its maximum power the better off you are. Those occasions when the panel really does put out its rated Watts will be few and short in duration.
  • System2
    System2 Posts: 6,290 admin
    Re: Micro inverters

    Wouldn't the 230 watt panel ever produce 230 Watts? If it was hooked up to a 190Watt microinverter at that moment wouldn't you lose out on 40 watts of power?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Micro inverters
    MrFye wrote: »
    Wouldn't the 230 watt panel ever produce 230 Watts? If it was hooked up to a 190Watt microinverter at that moment wouldn't you lose out on 40 watts of power?

    Yes it would - occasionally. Sometimes it might even produce more than 230 Watts. If the sky is right and the temperature is cool and the clouds re-focus the sun's rays ....

    But you can't plan on that. Most of the time it is going to run well under rating, as I said. So if you went down to a 190 Watt panel your inverter would put out approximately 150 Watts, and then you'd really be losing 40 Watts!
  • System2
    System2 Posts: 6,290 admin
    Re: Micro inverters

    Thanks for the info Cariboocoot. I know that the panels are priced based on wattage so a loss of 40 watts at 2.50/watt would be $100 wasted. Should I shop for an inverter that is above the rating of the panel? In that way I would capture all the power generated by the panel. I am wondering if a larger micro inverter would be more cost effective in the long run. If it cost $50 more, but captured 100% of the panels output, I think it would be worth it.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Micro inverters

    Solar panel output varies with temperature. At very cold temperatures, the panels can generate near rated power or even more that rated power in some conditions (subfreezing weather, snow on ground/frozen lake reflecting light onto array, etc.).

    You are near Boston? So, you certainly have some cold weather...

    I live in a very temperate climate (near San Francisco, Ca--Virtually no freezing weather) and find that my 3,500 watt array will produce around:
    • 2,500 watts / 3,500 Watt Array = 71% typical max
    • 2,700 / 3,500 = 77% good day max (happens sometimes)
    • 3,000 / 3,500 = 86% a few times a year
    My system produces around 4,800 kWH per year. Lets say that I design my system with solar panels of a difference between 71% rated power and 77% rated power... And that, on average, my system will produce 77% rated power 1 hour per day, 100 days per year:
    • 6% * 3,500 watts * 1 hour * 100 days = 21,000 WH = 21 kWH per year extra
    I have time of use metering, and the utility credits me with ~$0.18 per kWH (winter/summer/peak/off peak rough average):
    • 21kW * $0.18 per kWH = $3.78 per year extra power
    Lets say the micro inverter costs around $1.00 per rated watt (power output):
    • $1 per watt * 6% extra size * 3,500 watt base array = $210 extra costs
    • $210 extra costs / $3.78 per year extra power = 55 year payback
    So--just on a pure $/Watt payback--It does not look like a great deal to by extra large/oversized inverters to pickup that last little bit of lost power for those rare times that the panels exceed the inverter's output capabilities.

    Even if my estimates are wrong and I would obtain 3x greater power output than my estimate/guesstimate above, it still would take almost 20 years to pay off the extra costs. Around the time you would be replacing the inverters anyway as they are nearing end of life (guessing).

    For me, I have time of use metering and seasonal power pricing... So, even if I could collect a bunch more power in the winter--I would only get $0.09 to $0.12 per kWH... In the summer (when my peak power is lower and I have more sun), I will get credited around $0.09 to $0.27 per kWH.

    Of course, inverters come only in certain sizes--so you don't have the ability to get exactly the size you want--Run your cost differences and how much more power you will obtain in a year and see what your rate of return would be.

    Personally, I would not feel bad if my inverter was ~77% to 86% of array capacity... But I would not go out of my way to rate my inverter >86% of array capacity...

    Roughly, inverters are $1 per watt, and solar panels are ~$2-$3 per watt... I would probably collect more power over an average year if I spent the extra money for a larger solar panel than a larger inverter... The larger panel will collect XX% more power during the whole sunny day, vs the XX% larger inverter which will only collect the extra power for limited time during the day with very cold/clear days.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Micro inverters
    MrFye wrote: »
    Thanks for the info Cariboocoot. I know that the panels are priced based on wattage so a loss of 40 watts at 2.50/watt would be $100 wasted. Should I shop for an inverter that is above the rating of the panel? In that way I would capture all the power generated by the panel. I am wondering if a larger micro inverter would be more cost effective in the long run. If it cost $50 more, but captured 100% of the panels output, I think it would be worth it.

    No. Most of the time the panel will be running under its "rated" capacity. This applies to all panels, that's why they are usually sized larger than the inverter. You can never get that perfect balance of panel/inverter because of this.

    Take a look at the data Bill has provided. You have to go for averages, because you can't expect full (or better) output all the time nor low output all the time. If I were to go for what the panels would put out up here in the Great White North (-40 Winters) I'd have such a severe output drop come Summer that it would never work. Instead I have to fret over not being able to "harvest" the "extra" capacity when available.

    You have to pick a panel/inverter combination that will work best with the expected efficiency rating in your are. It does get cold in Boston, but only in Winter! And compared to this place it's still tropical! :p
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Micro inverters

    Bill summed it up well. Also keep in mind that at least the Enphase M190 inverters will push out up to 199W (according to the built-in monitoring) so are slightly underrated there.

    Installing up to 230W panels on the M190 inverters is very common and even recommended by Enphase. Depending on your situation you can even go up to 240-250W without losing any significant amount of harvest.
  • System2
    System2 Posts: 6,290 admin
    Re: Micro inverters

    Very helpful Gentlemen and much appreciated.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Micro inverters

    You are very welcome.

    I try to "show my work" so you can see what assumptions I have made when doing "back of the envelope" estimates. And you can adjust them to meet your particular needs/application.

    Try for "close enough" to get a rough answer--and we can workout a more exact answer if needed (for example, PV Watts website includes "average" hourly data--so you could load a years worth of data and do more accurate calculations in a spread sheet).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Micro inverters

    I have an Enphase M190 micro inverter setup. I LOVE IT. I have been able to slowly expand our system on the cheap by taking advantage of deals on the panels and inverters as those deals happen. I got a few more M190 inverters on Ebay for just $90 each....and I picked up a panel here and there in ranges of 205 to 230 watt. Just last night I picked up a great SunPower E18 230 watt panel for just a few hundred off a Craig's list ad.

    It is rather fun to see how the different panels perform. The 205 size panels rarely max the M190 production....the 230 watt panels get there regularly. I would like to try some 250 watt or larger panels so as to max the inverter production for more of the day....particularly in the less productive seasons.
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Micro inverters
    Brian wrote: »
    I have an Enphase M190 micro inverter setup. I LOVE IT. I have been able to slowly expand our system on the cheap by taking advantage of deals on the panels and inverters as those deals happen. I got a few more M190 inverters on Ebay for just $90 each....and I picked up a panel here and there in ranges of 205 to 230 watt. Just last night I picked up a great SunPower E18 230 watt panel for just a few hundred off a Craig's list ad.

    It is rather fun to see how the different panels perform. The 205 size panels rarely max the M190 production....the 230 watt panels get there regularly. I would like to try some 250 watt or larger panels so as to max the inverter production for more of the day....particularly in the less productive seasons.
    Got a link to your Enlighten monitoring site? I think most others would also find it interesting to see how all your different panels perform, too.
  • spurlocktools
    spurlocktools Registered Users Posts: 11 ✭✭
    Re: Micro inverters

    This is the first I have heard of these, and am wondering if they have an automatic shot off in case the grid is down. Say if I was to hook up a micro inverter to a spare PV panel I happen to have and, hypothetically speaking of course, hook it into my house wiring to grab a few extra KWH of power, if the grid went down would the inverter output shut down and thereby not endanger utility repairmen?
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Micro inverters

    Yes. I don't know of any UL listed grid-tied inverters that don't automatically shut down when the grid is offline.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Micro inverters

    The "Micro" Enphase inverters are pretty much exactly like the standard 2-10kW GT inverters. Just ~200 watts of output instead--One "micro" GT inverter per panel instead of 10-20+ panels per "full sized" GT inverter.

    To do Off-Grid or Emergency power--You need a battery bank and a Off-Grid inverter (or a Hybrid GT+OG inverter with battery bank).

    There is a fancy way of connecting a GT inverter to an OG inverter and sharing loads (plus charging a battery bank) behind a transfer switch--But it is not for the faint of heart to attempt for their first solar install.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • spurlocktools
    spurlocktools Registered Users Posts: 11 ✭✭
    Re: Micro inverters

    Thanks for the info. I'm thinking I could just plug this single panel & inverter combo into one of my shop receptacles for a bit of guerilla - sans permit - power. Aside from being illegal, any reason not to do this?
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Micro inverters

    Just make sure your shop receptacle is a 2-pole 240V with separate neutral and ground.

    You might want to get something that will let you measure current/power as well.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Micro inverters

    Technically illegal and unsafe... The unsafe part is, you have, for example a 15 amp breaker on the branch circuit--With the addition of the GT Inverter, you are now placing more current into the circuit and have >15 amps available for a short circuit/overload... That is why GT inverters are supposed to be connected to their own branch circuits instead of "plugged" into a wall outlet.

    Obviously, one GT inverter is not a huge issue--Adding 10 would be.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Micro inverters
    BB. wrote: »
    Technically illegal and unsafe... The unsafe part is, you have, for example a 15 amp breaker on the branch circuit--With the addition of the GT Inverter, you are now placing more current into the circuit and have >15 amps available for a short circuit/overload... That is why GT inverters are supposed to be connected to their own branch circuits instead of "plugged" into a wall outlet.

    Obviously, one GT inverter is not a huge issue--Adding 10 would be.
    Does anyone have 15A 240V circuits in their shop? I'd actually think it'd be 20A-30A+ for welding equipment and stuff.

    I actually highly doubt he's got the appropriate socket sitting unused in his shop so will likely need to run another circuit, anyway.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Micro inverters

    It was just an example (SWEA GT inverters are 120 VAC).

    I believe the vanilla Enphase inverters are defined for use on a 20 amp circuit (12 AWG wiring for Enphase install?)--so a 30/50/etc. branch circuit would be too much for the Enphase (without an additional 20 amp breaker/fuse).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    Re: Micro inverters
    BB. wrote: »
    I believe the vanilla Enphase inverters are defined for use on a 20 amp circuit (12 AWG wiring for Enphase install?)--so a 30/50/etc. branch circuit would be too much for the Enphase (without an additional 20 amp breaker/fuse).
    The M190 (most common) specs a dedicated 15A circuit with up to 15 inverters.

    You are definitely right about needing a separate breaker/fuse if plugged into a larger circuit.