Panel Calculations

CATravelerCATraveler Solar Expert Posts: 98 ✭✭✭✭
The following formulas are often posted on these boards. What voltage and losses would you recommend for my RV?
4 Lifeline GPL-4CT for 12V with bulk voltage 14.2-14.4V at 440AH. 460W panels flat mounted (5 degrees). The panels are 30V Vmp and I'm certainly considering a serial connection. A Morningstar MPPT controller would be about 95% efficient from their graphs.

•14.5 volts charging * 225 AH * 1/0.77 panel+controller losses * 0.05 rate of charge = 212 Watt minimum
•14.5 volts charging * 225 AH * 1/0.77 panel+controller losses * 0.10 rate of charge = 424 Watt nominal
•14.5 volts charging * 225 AH * 1/0.77 panel+controller losses * 0.13 rate of charge = 551 Watt "cost effective maximum"


  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    Re: Panel Calculations

    I am the guy that keeps posting those. :roll:

    RV's are certainly limited in roof space and battery capacity--so there are trade offs. Running the same math on your system:
    • 440AH * 14.5 volt charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 414 Watt array minimum
    • 440AH * 14.5 volt charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 829 Watt nominal
    • 440AH * 14.5 volt charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,077 Watt "max cost effective"

    With AGM batteries, they can operate closer to the 5% rate of charge because they are more efficient and have fewer losses... So your 460 watt array is a good start.

    We always like to look at loads--How many AH per day are you planning on using? Too large of battery bank is a cost too (need more solar panels/charging, more batteries to replace X years down the road, more weight to haul in the vehicle, more fuel to haul those chunks of lead around, etc.).

    A 460 Watt array running on a MPPT controller could use a minimum sized controller of:
    • 460 Watt * 0.77 derating * 1/14.5 volts = 24.4 amps

    So, a minimum of 25 amp MPPT controller. Some of your options would include (NAWS's charge controller page for MorningStar/Midnite/Outback):
    • Rogue 12/24 volt 30 amp MPPT controller
    • MorningStar 45 amp TS MPPT controller
    • MorningStar 60 amp TS MPPT controller
    • Midnite Classic 80+ amp MPPT controller
    • Outback FM/FX 60 to 80 amp MPPT controller

    The Rogue will only take the Vmp~35 vdc maximum (no placing panels in series). The others will take around Vmp~100 VDC (Midnite even has some higher voltage options).

    For an RV, usually you don't need to go with high Vmp-array options (need a bit heavier copper wire).

    If you were, for example, going to stake some panels beside the RV (park RV under trees, stake panels in sun), then using higher Vinput solar charge controllers will allow to use reasonable size/cost "extension cords" from the array to the RV.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CATravelerCATraveler Solar Expert Posts: 98 ✭✭✭✭
    Re: Panel Calculations

    I was wondering if a lower efficiency number should be used for flat mounted panels.

    A battery bank size change isn't likely due to space considerations. I've given this some thought and while more batteries are possible they would not be adjacent to the current set.

    I want to allow for adding a third 230W panel so that narrows down the controllers. The Rogue with remote is $430 and won't support 3 panels. The MS MPPT 60 with remote is $568. The Classic with remote is $800 (plus temp probe). For my application I just don't see the justification for the Classic.

    Over a 6 day period we discharged to 198AH (55% SOC) and gen recharged to 66Ah (85% SOC) or 132AH daily use. My objective with solar would be a much higher finish SOC as the sun goes down. When necessary we can run the gen early AM and get a 80A bulk charge and then let solar finish.

    I have no doubt that adding solar will be a learning experience but hopefully my initial install will be in the ballpark.
  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    Re: Panel Calculations

    Flat mounted panels will not affect peak power that much--at least during summer. You are welcome to use a lower efficiency factor--for your calculations, it won't hurt anything. Just make your suggested array sizing numbers larger--but you are limited by roof space anyway.

    To see how flat vs tilted to latitude (from horizontal) would affect your power--Using PV Watts for Yakima Washington (getting away from the marine layer). Both tilted for latitude and flat:

    First tilted to 47 degrees:
    Month    Solar Radiation (kWh/m 2/day)
    1      2.35     
    2      3.73     
    3      4.98     
    4      5.27     
    5      6.09     
    6      6.39     
    7      6.83     
    8      6.59     
    9      6.21     
    10      4.84     
    11      2.51     
    12      2.24     
    Year      4.84

    Second, 0 degree tilt:
    Month    Solar Radiation (kWh/m 2/day)
    1      1.33     
    2      2.27     
    3      3.68     
    4      4.81     
    5      6.30     
    6      7.06     
    7      7.24     
    8      6.13     
    9      4.74     
    10      3.00     
    11      1.50     
    12      1.15     
    Year      4.11

    During winter, it can make a big difference (2x amount of sun collected)--But 2x a small number is still a small number.

    If you snow camp in the far north, bringing panels to vertical can be a big help with refections from snow. Adding improved cold weather solar panel performance with MPPT charge controllers, I believe a few "far north" posters have reported higher peak power in dead of winter vs summer.


    PS: If I had the choice between a remote charge controller panel and a battery monitor (not that much difference in price), I would go with the battery monitor. It is much more useful. Of course, if you need to program the charge controller, you need the display (and/or computer interface--which is not cheap either).

    However, it sounds like you already may have a battery monitor--so that will be covered anyway.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CATravelerCATraveler Solar Expert Posts: 98 ✭✭✭✭
    Re: Panel Calculations

    BTW The reason I favor the larger higher voltage panels is $/W. The savings is significant. I can easily install 2 of these panels but they will not be adjacent - about 48' 10ga wire to the controller. Using 8 ga wire for a second parallel and 4 ga from the combiner box the loss is about the same.

    So serial or parallel??? I have yet to find any information on a direct comparison of partial shading on serial vs parallel and partial shading can be signifcant for a RV. I'm actually planning to do some ground level testing before installation. By partial shading I mean covering up 1 or more cells with a understanding of just what cells are covered by the bypass diodes.
  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    Re: Panel Calculations

    Serial vs Parallel... With MPPT controllers, it usually does not make a big difference until you go above ~2x Vbatt charging for Vmp-array--Then the charge controllers do become a bit less efficient (ignoring wire lengths and gauge requirements).

    For shading--It is a tough one. Some folks would like to use everything in parallel (Vmp-array=~17.5 volts) and others suggest placing everything in series (for grid tied inverters, that is Vmp-array~400 VDC).

    It is a complex issue in the sense that shading moves and is otherwise variable (winter/summer, RV parked in different locations/angles to sun, etc.).

    One person here mounted his array in "landscape" because he would get snow on the lower frame of the panel. His cells were two groups in parallel, so the upper group was in full sun generating 1/2 panel power while the lower group was waiting for melt or a snow broom to clear.

    More or less, even "minor shading" on a solar panel will take out "that string" of cells. If two strings in parallel, that will be 1/2 the current of the panel (a 12 volt and 6 volt battery in parallel with blocking diodes--The 12 volt will supply the power).

    When you have panels in series... then you can lose the "whole panel" worth of power, but the bypass diodes allow the rest of the string to function (say two 30 Vmp panels in series with a 12 volt battery bank. Basically, Vmp-array drops from 60 volts to 30 volts when one panel partially shaded).

    These are probably "worse case" examples, but, to me, indicate why a person should try to limit shading as much as possible (relocate panels/vents, cut vents short, raise panels, etc.).

    If you end up with three panels (or other prime numbers that don't split well)--Just placing them all in series for Vmp-array~90 volts and be done with it. Keeps wire diameter down.

    Another option of two in series and one in parallel--that takes two charge controllers to manage--Not cost effective.

    Otherwise, three panels in parallel (assuming Vmp>Vbatt charging) will be OK--Just uses larger diameter wire and a combiner box with fuses/breakers for safety (3 or mare parallel panel strings need series protection fuses). If one panel is a longer wire run, use heavier cable--But current sharing is less of an issue with solar arrays. They are, more or less, current sources and will share current even if the wiring is not "identical" between parallel solar panels, assuming your voltage drop is not excessive (unlike batteries or voltage sources which do not share well with even slight differences in wiring resistance).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CATravelerCATraveler Solar Expert Posts: 98 ✭✭✭✭
    Re: Panel Calculations

    Thanks for the information on tilting. Yes I have a Trimetric 2025RV battery monitor installed earlier this year as a precursor to understanding my solar desires/requirements.

    My understanding is that the MS MPPT 60 MSView software allows the controller to be programmed with a PC. But I'll recheck. I have a PC and LAN cable so no new costs.

    BTW I don't plan to tilt the panels or intentionally camp in the snow. :D
  • CATravelerCATraveler Solar Expert Posts: 98 ✭✭✭✭
    Re: Panel Calculations

    Bill - Thanks again for the information.

    There are a number of posts on boards concering serial/parallel panels that favor parallel. I've come to believe that a number of these posts are based on older (or current) panels that do not have bypass diodes. I'm favoring serial at this point.
  • BB.BB. Super Moderators, Administrators Posts: 30,037 admin
    Re: Panel Calculations

    I believe that all panels these days have internal bypass diodes. The cells would otherwise be ruined by shading (any "forward voltage" over ~12 VDC breaks down the diode barrier and will damage the junction--either from voltage generated "on panel" or from other panels in a series string).

    What most panels don't have these days are "blocking" diodes. These were originally used on solar panels that were directly attached to batteries (without charge controllers). This blocked the battery from feeding leakage current back through the array at night (I don't think it caused damage--just a bit of charge loss from the battery bank).

    Diodes and how they are implemented in solar panels--We have few posts here about bypass diodes that have failed and are replaced. I am not sure that diodes are properly installed in the average solar panels for a long and reliable life. Diodes generate heat when passing current--and they are never mounted to a metal box/heat sink that I have seen. So--I would worry that shading a panel "a lot" in full sun/current, would stress the bypass diodes (diodes fail both shorted and open--not good in either case for our needs). I would just avoid shading as much as I could--And, otherwise, let the warranty happen...

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