Enphase module sizing

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heynow999
heynow999 Solar Expert Posts: 80 ✭✭✭✭
I am going to be putting some more panels on my house. I am going to add about 3 kw to the 2.5 I already have. I sell to the grid in Ontario with the Feed-in-tariff program we have where they buy power at 80.2 cents/kw. Because I already have a central inverter that is maxed out, and the area I will be using has some shading issues it seems that Enphase in the right choice.

I see that Enphase will allow up to a 235 watt module to feed a 190 watt inverter. I always questioned this as it seems to limit the output of the panel. They have a paper on module sizing here.

http://www.enphaseenergy.com/downloads/Enphase_White_Paper_Module_Rightsizing.pdf

Essentially, it says that a module will likely only put out its PTC rating, and the module will quickly degrade to a point where it is not an issue. The calculation they used is %1 per year for degradation. They did simulations for Palm Springs, Phoenix and Denver. The worst loss was in Denver where the simulation says they will only lose %.61 so they are ok with that.

I am in Ontario, which I guess would be as cold as Denver or colder. I think I would lose much more than %.61 because we get many cold bright days in the spring. Our best month for solar is usually April.

I know I am answering my own question, but my plan is to use a 185/watt mono module into a 190 watt Enphase inverter to get every last possible watt.

Any thoughts?

Peter

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Enphase module sizing

    Look at the $$$/Watt for system installed cost (including the raw panel costs)... Sometimes the larger panel is actually a bit less expensive.

    Also, if you want to see, roughly, how many hours a year/kWH (on average) you lose... Use PV Watts and output the hourly summary. You can load that in a spread sheet and add up the "missing" power (where available output is > inverter limit) and see what the loss kWH may come too.

    Even if you get 4 hours a day and 20 "extra watts" for 100 days a year:
    • 4 hours * 20 watts * 100 days / 1,000 w per kW = 8kWH per year "Lost"
    Or about $0.80-$1.60 worth... (made up numbers for example).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
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    Re: Enphase module sizing

    There seems to be a lot of discussion implying putting too high a wattage panel on an Enphase may damage it.

    I also don't believe there can be a UL regulation that links the Enphase unit to a particular set of UL approved PV panels. The panels should be within the operating range specifications of Enphase unit. Enphase could specify particular panels, if they desire to for some reason. A UL approved system must have UL approved PV panels.

    From the mentioned Enphase document:

    "The two schools of thought can be summarized as follows:
    1) The PV module(s) should be sized so that the inverter never limits its
    output power.
    2) The PV module should be sized so that the inverter limits its output power
    frequently, possibly every clear-sky day."

    #1 prefers not to waste panel power capability.
    #2 prefers not to waste the power capability of Enphase inverter.

    Neither statement implies a PV panel of too much wattage capability should not be connected to an Enphase unit.

    The only real limit is the cold temp Voc of the panel not exceed the max input DC voltage rating of the Enphase unit.

    The higher the wattage of the panel the more time the Enphase will be operating at its maximum power rating. This will increase time of greater heat generated within unit and therefore theoretically accelerate its MTBF.

    Panels of lower voltage, at rated panel power, will likely result in lower conversion efficiency within Enphase unit and therefore higher heating within unit.

    I don't see where putting a 230 watt rated PV panel on a 190 watt Enphase unit will do anything other then waste some of panel power capability at high noon. It will likely still end up giving you more overall generated kWH during the day then a lower wattage panel.

    A 210watt Enphase unit would do better for overall kWH generated then a 190 watt unit fed from a 230 watt PV panel.

    The argument is how to yield the most kWh's from the lowest $ spent for panel + inverter.
  • mr.radon
    mr.radon Solar Expert Posts: 158 ✭✭✭✭✭
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    Re: Enphase module sizing

    The Enphase will not be damaged by hooking up a panel that is over 230W. The device limits itself according to an Enphase engineer I talked with.

    Their web site has a list of compatible panels.

    Enphase has a 0.77 AC - DC derate factor for PV Watts. Also, 190/230 = 82.6%
    A 230 panel would rarely cause the Enphase inverter to derate itself.

    I would not hook the Enphase up to a non-certified panel unless I called them up and asked, you don't want any hidden surprises.
  • heynow999
    heynow999 Solar Expert Posts: 80 ✭✭✭✭
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    Re: Enphase module sizing
    BB. wrote: »
    Also, if you want to see, roughly, how many hours a year/kWH (on average) you lose... Use PV Watts and output the hourly summary. You can load that in a spread sheet and add up the "missing" power (where available output is > inverter limit) and see what the loss kWH may come too.

    I see your point

    I looked through several years of hourly irradiance and I could only see a few instances where there was more irradiance than the 1000/w/sq/m, at least on an hourly basis.

    Still, my point is with all the Enphase 'hullabaloo' about efficiency it seems like a poor system design to not be able to capture all available irradiance.

    I will still do a 185 watt panel into an M190 Enpase inverter though. I have some other factors that come into play. The M190 is made here in Ontario and is the only model available to me. I can get the 185w module at about the same price as a larger module and the 185w module fits better into the available space I have on my roof, plus they look great, all black with black frames.

    In a separate project, we are installing a one axis tracker with 225w modules and M190 inverters. It is a done deal, sold and paid for and it meets the Enphase requirements for module compatibility. We do have a record low of -35C which is outside the temperature range of the compatibility list, but that will not produce panel voltages above the 56v limit. I should double check that. I think what is going to happen is on the first cold clear day you are going to see the output jump straight up to the max and then go straight across the chart as the inverters limit the output of the panels. It will be interesting to see.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Enphase module sizing

    Regarding what size should an inverter be--It is the old engineering/marketing problem of designing for 90% vs 99% vs 99.9/99.99/etc. of anything. At some point, those extra .9999's are going to be so expensive it is simply not economically worth paying for them (solar generation, phone / power network up time, etc.).

    However, I would suggest one avenue to research--I do not believe that PV Watts takes solar panel/ambient temperature into account--Plugging in the correction factor for power increase at low temperatures may actually bump the winter numbers up significantly for your system...

    However, there is always the downside that way up north (of course, how far north is the question), reduced hours of winter sun is just does not generate that much power, let alone a few percent increase of a low number.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset