Offgrid module V Grid tie?

solarabcdsolarabcd Solar Expert Posts: 38
Using an MPPT Charge controller you can obviously use the larger grid tie panel voltages which in turn has the potential to reduce wiring costs.

What about things like; junction boxes on off grid modules instead of the MC connector/cables. Do they out weigh the other advantages?

Lets say the charge controller and NEC rules will allow you to use 3 series 215w panels or 4 or 5 x 135w off grid (non MC cable) panels (for a 48V battery set up).

The 3x 215w panels Vmp = 79.8V, 4 x 135w = 70.8V, 5 x 135w = 88.5V.

So in actual fact I can get more voltage with the 5 x 135w option.

You would obviously need more of the 135w panels and hence more racking cost. They save a little on not needing the MC cabling.

In California is it a requirement to have the MC cables in conduit? If so then does the off grid junction box allowing standard wiring make more sense?

The 135w Modules will work out more expensive but maybe it is worth it for the differences written above?


Edit to add: Also is it more efficient to choose the 4 x 135w to keep voltage closer to nominal battery voltage, saving the controller less conversion?

Comments

  • solarabcdsolarabcd Solar Expert Posts: 38
    Re: Offgrid module V Grid tie?

    Anyone installers have an opinion on this? Any help is appreciated.
  • BB.BB. admin Super Moderators, Administrators Posts: 29,634 admin
    Re: Offgrid module V Grid tie?

    The answer is always--it depends.

    MC4 are locking connectors (require tools) so that they do not have to be "locked up" in a box for "safety" requirements. Since, for the most part, >100 watt solar panels are designed for use with Grid Tied systems--you are probably going to find most of them are MC4 connectors and very few "box" type solar panels (and some of those box type available now seem to be going away from what I have seen here). For long term compatibility, I would probably stick with MC4 for expansion/replacement as needed.

    It also depends on the Brand/Model of MPPT charge controller as to "optimum Vmp array voltage". The Xantrex XW should be just above the maximum battery voltage for more efficient operation.

    The new Morning Star MPPT 45/60 amp charge controllers, have a more efficient step-down architecture and therefore can run Vmp Array >> than battery voltage and still be very efficient (per Solar Guppy's posts).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarabcdsolarabcd Solar Expert Posts: 38
    Re: Offgrid module V Grid tie?

    Thanks Bill.
    Kind of confirmed what I was thinking. I think even though stringing 5 x 135w is allowed it makes the controller have to convert a little more. Keeping somewhere near the battery voltage sounds best. In this scenario the cable runs are all pretty short and not running long distance to a pole mount away from the loads for example.
    So can you think of any benefits of sticking with the off grid modules if you use a MPPT controller? They seem mainly applicable now to a basic setup with the old school controllers which still have their advantages with less EMI issues I guess.
  • BB.BB. admin Super Moderators, Administrators Posts: 29,634 admin
    Re: Offgrid module V Grid tie?

    135 watt panels are about as big as you would want to handle for setup around a camp. 175 watt panels are about the limit for a 1 man install on a roof.

    Big panels means fewer connections and probably a bit more reliable in that regard vs a bunch of 60 watt panels.

    In the end, larger panels tend to be cheaper per watt.

    Not that many controllers have FCC class A or B approvals (Xantrex and some MorningStar MPPT controllers have Class A/B). Even the PWM controllers can still make for RF noise... I think Morning Star has some PWM controllers that have a a "low noise" mode where the PWM is cycle is much longer (seconds?) vs 1,000-10,000+ Hz).

    If you are less than 400 watts of solar panels and have short wiring runs--a good quality PWM controller is probably hard to beat for $$$/performance.

    If you are going with >400 watts--a good MPPT controller is probably the better choice. Or, if your install will be in weather extremes (Hot or Cold).

    Another advantage with High Power Grid Tied intended modules--they generally are rated for 600 VAC minimum operational voltage and have UL/CSA/NRTL approvals. The Off-Grid modules (typically under 100 watt) may not have NRTL listings and may only be rated to 70 VDC (if rated at all). With many MPPT type controllers, they are rated to 150 VDC maximum and can exceed the 70 VDC limit of the panels.

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