Upgrade my current system

I currently have 4 Shell SQ75 (12V 75W parallel) panels on my boat. I'd like to up my array by adding a couple more panels with today's technology and power. Any restriction on what type of panel I can add?  I plan to have an MPPT controller.


  • PPLepewPPLepew Posts: 5Registered Users ✭✭
    Here are my current panels' specs
    STC: irradiance level 1000W/m2, spectrum AM 1.5 and cell temperature 25°C
    Rated power Pr 75W
    Peak power Pmpp 75W
    Peak power voltage Vmpp 8.5*/17V
    Peak power current Impp 8.8*/4.4A
    Open circuit voltage Voc 10.85*/21.7V
    Short circuit current Isc 9.6*/4.8A
    Series fuse rating 15A
    Minimum peak power Pmpp min 70W

  • BB.BB. Posts: 27,914Super Moderators, Administrators admin
    In general, you would replace the 4x small panels with one (or two) larger format panels + whatever extra panels you want to add... However, you can keep the existing panels+charge controller (assuming all is working well) and add a new array+controller (in parallel to the existing solar system).

    So--to basics. How much energy do you use in a day (Amp*Hours @ 12 volts or Watt*Hours per day). Do you use 12 VDC / 120 VAC / both?

    How big of battery bank do you have/want to install next (Amp*Hour, flooded cell vs AGM/Sealed, etc.)?

    Where do you use the RV (figuring hours of sun per day)? Seasonal (weekend, winter vs summer, full time off grid)? Backup genset?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PPLepewPPLepew Posts: 5Registered Users ✭✭
    My panels are working fine, but I need to replace my defunct controller. 
    The 4 panels are in a custom frame that would not fit any modern panel. Siemens makes a 80W panel that a replacement drop in for the Shell SQ75. But that's a lot of money for practically no gain. Hens the add panels upgrade thoughts.
    If I add two panels and create 2 parallel arrays of 3 panels in series, given the higher voltage, I'd be able to use my current wires.
    I have an Epever MPPT Tracer 4210A on order that should be able to manage them.

  • PPLepewPPLepew Posts: 5Registered Users ✭✭
    Didn't answer your questions:
    I live on a boat full time, US North East (Chesapeake bay) in summer, down to Florida Keys & Bahamas for winter, so sun availability is not a problem.
    6 x 100Ah AGM batts
  • BB.BB. Posts: 27,914Super Moderators, Administrators admin
    What is the voltage of your batteries and battery bank (i.e., 12 volt batteries and 12 volt battery bank)?

    Assuming 12 volt batteries and 12 volt battery bank--You have a 12 volt @ 600 AH battery bank. If you are planning on living 100% from solar power with genset for just bad weather backup/high power needs for tool use, our off grid rule of thumb is ~5% to 13% charging for your battery bank. Lead Acid batteries have a minimum charging current for proper chemical reactions during charging and to keep up with self discharge. AGM batteries are better suited for 5% rate of charge (more charge efficient, lower self discharge)--10% is the typical recommendation for off grid living in a cabin/home--But for RV+Boats, this may be a larger array than you can fit on your place (not a lot off roof/deck space). So, the math would look like this.
    • 14.4 volts charging * 600 AH * 1/0.77 panel+controller drating * 0.05 rate of charge = 561 Watt array minimum
    • 14.4 volts charging * 600 AH * 1/0.77 panel+controller drating * 0.10 rate of charge = 1,122 Watt array nominal
    • 14.4 volts charging * 600 AH * 1/0.77 panel+controller drating * 0.13 rate of charge = 1,459 Watt array "cost effective" maximum
    Depending on your boat--You probably would have a hard time to fit more than ~561 Watt of solar panels.... Let alone trying to get >1,000 Watts.

    In the above, you see that we have a recommended solar array based solely on the size (AH*Voltage) of your battery bank. Which now begs the question is your battery bank "appropriately" designed for your needs. For Lead Acid batteries, we typically size the battery bank for 1-3x your daily load with ~2x daily load as an "optimum" battery bank (cost/functionally a good match to loads/needs). And since we suggest you avoid going below 50% state of charge very often, that ends up with 2x daily load 2x for 50% maximum discharge or ~4x daily load as recommended battery bank size for an off grid cabin.

    But for boats, many folks seem to put a relatively large battery bank in to support many days without genset power (quiet time at anchor, bad weather during sailing cruse). But, as you can see, this "forces" a larger solar array--But you have limited space for solar--Which may tend to force you to use the genset/fuel more than you want.

    Anyway--My first guesses as to what your needs may be.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PPLepewPPLepew Posts: 5Registered Users ✭✭
    I have 12V batts by 12V bank.
    I have a 4kW generator on board, but only have used 2-3 times in the 4 years living on board (too noisy, too much fuel)
    Also have a wind turbine giving me alternate power when sailing and at anchor.
    The two boat engines load up another 90Ah (combined) whenever I fire them up.
    Just need an bit of a solar boost in the morning after windless night and wont be moving for the day.

  • BB.BB. Posts: 27,914Super Moderators, Administrators admin
    OK--So the bank estimate I made seems to be accurate (600 AH @ 12 volt battery bank).

    A "suit case" type genset (Honda eu2000i or eu1000i) can be pretty nice (small, quiet, efficient, reliable)--However, a small solar array (or even a small genset) is not going to make much of a dent on the battery bank state of charge...

    Do you have an estimate of how much Amp*Hours you want to replace in the battery bank with solar and/or a "smaller" genset?

    For example, say you use 1/4 of the 600 AH per day (or night)--~125 AH per day. If you have a 45 Amp battery charger on a Honda eu2000i:
    • 125 AH / 45 Amps = 2.8 hours of run time minimum (realistically, if going for >90% state of charge, + few hours of taper/Absorb charging).
    • and use ~1/2=3/4 a gallon of fuel per day (really rough guess)
    And if you want to replace that full amount with a solar array with 5 hours of day of sun--that is a good amount on a boat?
    • 125 AH / 5 hours = 25 Amp charge current
    • 25 amps * 14.4 volts charging * 1/0.77 panel+controller derating = 468 Watt array to "break even" with 5 hours of sun per day
    Of course, I have no idea of how much energy (or Amp*Hours) you actually use in a day--And "any solar" power can help--But you need substantial amount of solar to really help extend your "quiet time" on the boat.

    "Deficit" charging on a lead acid battery bank is really tough on the batteries themselves (not replacing the AH used every few days--But charging to 90% one day then 80% the next day, 70% the third day, etc.). Cost of a "early death" battery bank vs more money for solar/fuel to keep the batteries "happy" is part of the ownership cost equation.

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