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.


  • PPLepew
    PPLepew Registered Users Posts: 5 ✭✭
    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. Super Moderators, Administrators Posts: 33,412 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
  • PPLepew
    PPLepew Registered Users Posts: 5 ✭✭
    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.

  • PPLepew
    PPLepew Registered Users Posts: 5 ✭✭
    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. Super Moderators, Administrators Posts: 33,412 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
  • PPLepew
    PPLepew Registered Users Posts: 5 ✭✭
    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. Super Moderators, Administrators Posts: 33,412 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