Can you use solar panels larger than recommended?

ratherbeflyen
ratherbeflyen Registered Users Posts: 2 ✭
in Grid Tie and Grid Interactive Systems #1
Long story made short, I have ~10 year old Enphase M210 inverters. Enphase recommends an input of 31-50 volt and 240 watt solar panel. However, I cannot find any new panels that are ~240 watts in anything but 12 volts. I did find a good deal locally on some 305 watt panels. I know that it's not optimal efficiency, but would the 305 watt panel damage the inverter short or long term?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #2
    You should contact Enphase support (or your original installer). If your panels match the requirements in the manual of 10 Amps maximum and are either 72 cell or 84 cell (Vmp~36 volts or 42 volts nominal)--I guess it would be "legal".

    https://enphase.com/en-us/support/m190-m210-installation-and-operation-manual
    https://enphase.com/sites/default/files/Enphase_M190_M210_Installation_Manual.pdf

    A quick read of the manual does not list a maximum input wattage. And MPPT controllers/inverters should safely limit output current/power to the device's capability.

    Using nominal math:
    • 36 volts * 10 amps Imp * 0.8 Imp to Isc conversion (estimate) = 288 Watt maximum 72 cell (Vmp~36 volts)
    • 42 volts * 10 amps Imp * 0.8 Imp to Isc conversion (estimate) = 336 Watt maximum 84 cell (Vmp~42 volts)
    The above are just guesses... Usually "damage" occurs when maximum input voltage is exceeded (hard limit). Exceeding input current should be less of an issue (micro inverter will simply draw less current than panel could supply).

    Depending on Enphase specifications... They could be Isc=10 amps as their limit, or Imp=10 amps as the input limit (or they could be squishy and not tell you which limit is the 10 Amps).

    Again, I do not work for Enphase or UL/etc... I am just reading the specifications and trying to determine what would work for you.

    This is your decision (hopefully with input as needed from Enphase).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ratherbeflyen
    ratherbeflyen Registered Users Posts: 2 ✭
    #3
    Thanks for the reply. I talked to Enphase and they said anything over 240 watt would damage both the inverter and the panel. I'm sure the legal department has some influence in that answer, especially when panels degrade over time. I'm thinking the easiest thing to do would be to install a current limiting device or maybe a resistor between the panel and the inverter. A 72 cell 12 or 15 amp panel limited to 10 amps would be the same as a 10 amp panel, or am I missing something?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #4
    It would be a mess to install a resistor/diode. Both just to install something cleanly and waterproof, and the device would need to dissipate heat.

    I would not do it... Instead, just look for a 240 Watt panel--used or new (or, for example, 2x ~120 Watt 36 cell panels in series).

    Not sure what else you can do other than buy a different/newer model of micro-inverter (and the issue of how to remote monitor--compatible with what you have or not)..

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #5
    The Enphase data sheet is clear:

    Recommended input power (STC) 240W
    Max. DC short circuit current 12A
    Max. input current 10A

    Of course it's safest to use the lower of this and their verbal statements.  




    I am available for custom hardware/firmware development

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  • Ampster
    Ampster Registered Users Posts: 174 ✭✭✭
    #6
    ratherbeflyen said:
    Long story made short, I have ~10 year old Enphase M210 inverters. Enphase recommends an input of 31-50 volt and 240 watt solar panel. However, I cannot find any new panels that are ~240 watts in anything but 12 volts. I did find a good deal locally on some 305 watt panels. I know that it's not optimal efficiency, but would the 305 watt panel damage the inverter short or long term?

    I found a great deal on some Sunpower panels that were 305W. It turns out they were 96 cell panels and I had to buy new IQ7+96 for those panels, It is important to look at the voltage spec also. Fortunately I only needed to put 4 on micros and the rest did well on my string inverter. 
    8 kW Enphase micros AC coupled to a SolArk 12K
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