RV Solar Sizing

I am new to solar and am trying to determine what size solar panel(s) and related equipment to order for a small class B van.

I have estimated that I will use no more than 65 amps per day at 12 VDC. 66% of the usage will be at night. I have a bank of two 100 amp 12VDC flooded batteries. My van has a 130 amp alternator and I drive a minimun of 1 hour per day while camping. I will not exceed five days of camping, typically 3 days. I would prefer to discharge the batteries about 25% most of the time and never more than 50%. I have enough flexibility in my 65 amp/day budget that I could reduce usage by 20 amps/day in an unusual situation (ie rain etc).

In my modelling I came up with a 205W Kyocera panel and Morningsrat Mppt controller, which would be expensive and perhaps overkill. On the lower side I came up with a 135W Kyocera panel with Morningstar Sunsaver which is significantly less expensive, but might require some power management in some cases.

Can someone validate that I am on the right track and help with the judgement of which option is the best, or a better option?

Comments

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: RV Solar Sizing

    You are indeed on the right track,,, and I congratulate you for doing your homework and doing your math,,, thanks

    First, your batteries , (using rough numbers throughout) with a 65 ah load would use ~32% of capacity each day, assuming no charge. That sounds about right for the battery sizing.

    Second,, it is recommended that Solar charging amperage should be between 5-13% of capacity. That would mean with your 200 ah of battery you would be looking for between 10-26amps of charging capacity, so your 205 panel would be something like 20 amps, the 135 perhaps 11, both with in the range.

    I am always a fan of mppt controllers,, although the advantage gets less as the panel temperature increases and/or the batteries are closer to fully charged,, so if you are in a hot climate, and you only draw your batteries 20% it may not make sense.

    Let's assume that when you drive, you are able to get 100 amps out of the alternator. That alone would keep the batteries pretty well fully charged,,, although I don't know how a car alternator will deal with deep cycle batteries with regard to bulk, absorb, float voltage and amperages.

    If you assume that the 40% of your usage that comes during the day is a wash (solar in/load out) then you are really only needing to come up with~ 40 ah/day. Without driving, your 135 panel at 11 amps would do that in a bit under 4 hours, The 205 in something like 2 hours.

    As I said before,,, these are all rough numbers, and I'm sure bill will chime with more real numbers. (The general rule of thumb is,,, that you can guestimate panel output by taking the name plate rating, cut it in half, multiply that by the numbers of sun you expect, and that gives you about the number of watt/hours you can expect. That takes into account, panel eff, charge controller ef, battery ef, inverter ef, and wiring loses).

    Shout if you have other questions,

    T
  • kevinhenrycalgary
    kevinhenrycalgary Solar Expert Posts: 33
    Re: RV Solar Sizing

    Here is a link to my Youtube channel.

    http://www.youtube.com/watch?v=QrjW8gnKSkY

    Also see my post, in this forum "Best Vmp for use with the SolarBoost 2000E"

    Have a look at all of my solar videos as this setup will be similar to yours and may answer some of your questions.

    The guys on this forum helped by guiding me in the right direction.
  • System2
    System2 Posts: 6,290 admin
    Re: RV Solar Sizing

    Icarus,
    Thanks for the reply.

    I do have a question. How did you arrive at the numbers of 11 amps for the 135 watt panel and 20 amps for the 205?

    Also, I do know that the circuit breaker from the alternator to the battery bank is rated at 80 amps. So the charge from the alternator must be less than that.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: RV Solar Sizing

    Don't know how he did it, but I go by the charging voltage of about 14V, and assuming a MPPT controller:
    135/14= 9.5A
    205/14= 14.6A
    and in real life, plan on 20% less, as few panels produce nameplate output, except in the lab they were tested in.

    Mounted flat on a RV roof, they will never be properly aligned, and so you will see even LESS output. Go for the 205W panel and a MPPT controller. You will still have marginal PV capacity, which you could supplement with either running the RV engine (sucks fuel), or a little inverter style, variable throttle genset, run to make your morning coffee, and toast, and bulk charge your battery. Let solar top it off in the day.
    Or you could install wires to allow paralleling a 2nd panel that you can hang out and orient when parked.

    To top off your house batteries, will need 6 hours of engine run time (automotive alternators are not set to the proper full charge voltage) but that's what the solar is for.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • System2
    System2 Posts: 6,290 admin
    Re: RV Solar Sizing

    Mike,

    Thanks

    I had been using 7 amps for the 135 watt panel and 11 amps for the 205 watt panel. If I take your numbers

    135/14= 9.5A
    205/14= 14.6A

    and derate by 20% then

    135/14= 9.5A x .8 = 7.6
    205/14= 14.6A x .8 = 11.6

    we get to the same place.

    Also, I had been using 10 amps per hour as a charge rate from the alternator. This assumption is based on just my experience and not a measurement.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: RV Solar Sizing

    As Mike points out,, guestimating has it errors,,, but the principal is right just the same.

    205/12=17.83
    135/12=11.25

    Who knows where I was yesterday!

    I realize that using 12 volts is not an accurate way of measuring out put,, but the number is blazed into my head! Batteries are 12 volts right? Not 14.6! lol!

    Even with MPPT controller,, very low battery voltage, and very cold weather I confess you wouldn't get the numbers I suggest,, even if you derate them 20%....My bad!

    As for how much the alternator will put put out,, your real life experience will tell.

    So,, while your numbers may vary,,, if you plug your real life numbers into the equation you can begin to answer your question(s) As they say,, you mileage may vary.

    Tony

    PS Sorry for any confusion. In any situation that you are looking for real numbers,,, trust Bill,, or Mike or Neil or someone. I have some theoretical idea of what I speak,, but math was never my long suit,,,, and neither was attention to detail. I guess that is why I will always be a tinkerer rather than an engineer.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: RV Solar Sizing

    Generally, solar panel Watts are based on Vmp*Imp=Pmp... Vmp is typically around ~15-18 volts for "12 volt" panels... And the current does not increase at 12 volts... You will be much more accurate to use Vmp=~17 volts to find Imp (or look at the data sheets).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: RV Solar Sizing

    BB.

    Thanks
    Here are the specs

    135 Watt Panel
    vmp = 17.7 x imp = 7.63 = 135 Watts

    205 Watt Panel
    vmp= 26.6 x imp 7.71 = 205 Watts

    How is this better than just dividing the panel wattage by 14V and derating by 20% to esitmate how much amperage goes to the bank?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: RV Solar Sizing

    The ~20% derating is just another rule of thumb that says the "205 watt STC" rating from the manufacturer is only true on a cool, bright, windy day, with the solar panel stuck up in the wind.

    When mounted in 80 degree weather, 5" inches off the roof, full sun, no wind, the silicon cell temperature soars... When the temperature rises, the Vmp value goes down (Imp stays almost the same)... So Pmp=Vmp*Imp will drop.

    There is another rating called PTC -- which takes into account the "normal" operating temperature of a solar panel--and it is ~88% of the STC Power Rating...

    Add dust/dirt, a "bad" mount (low air circulation) and a a bit of ageing--then you will see closer to ~80% of Pmax for an ongoing period.

    In general, we try to be conservative here... With Solar, you have so many variables between the hardware, batteries, loads, and users--that it is just easier to use the rules of thumb to first size the systems... Then later figureout how to improve performance or make exceptions for an installation (difference between a PWM charge controller in a hot climate during a dust storm vs an MPPT charge controller mounted in the Canadian Tundra with a layer of reflecting snow).

    At times, with so many derating factors--the system can be a bit over priced for the application--but it works. Vs designing for the exactly "right" design, failing miserably at its desired job with a cloudy/stormy winter that the other system might have rode through better (or the expectations where just too high).

    For me, I am pleasantly surprised every evening when I check my daily production on my Grid Tied system--it is magic! :D

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: RV Solar Sizing

    BB.

    At this point I am trying to estimate the size of the solar panel I will need to replace about 65 amps/day in southern california with an RV setup. Is there a better method than

    Amperage = Panel Wattage / 14 * .8

    assuming a mttp controller?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: RV Solar Sizing
    DLS wrote: »
    135 Watt Panel
    vmp = 17.7 x imp = 7.63 = 135 Watts

    205 Watt Panel
    vmp= 26.6 x imp 7.71 = 205 Watts

    How is this better than just dividing the panel wattage by 14V and derating by 20% to esitmate how much amperage goes to the bank?

    Just to make a point here... How different designs have radically different results.

    Take perfectly good PWM controller (Pulse Width Modulation)--more or less the equivalent of turning on and off a light switch to regulate power to the battery. Assume the controller has a 2 volt drop. And you want to charge/equalize the battery to 15 volts (normal spec.).

    Vpanel>Vbatt+Vcont=15 volts + 2 volts = 17 volts minimum panel voltage...

    Both your examples meet that requirement.

    How much power will get into the battery from the panel:

    135 Watt
    vmp = 17.7 x imp = 7.63 = 135 Watts ... So we know 7.63 amps Imp

    But the battery is charging at ~14.2 volts (long night of reading and a fan running):
    Pbat=Ibat*Vbat=7.63 amps * 14.2 volts = 108 watts into the battery
    How about that big old 205 watt solar panel with a PWM controller:

    vmp= 26.6 x imp 7.71 = 205 Watts
    Pbat=Ibat*Vbat=7.71 amps * 14.2 volts = 109 watts into the battery
    Remember, a PWM controller just passes the current through from the panel to the battery...

    Now, lets look at a MPPT charge controller... Basically, it does Imp*Vmp=Ibat*Vbat with a little taken off for internal losses. Assuming 92% efficiency:

    135 watt panel: 17.7V; 7.63amp
    Imp*Vmp*0.92= 7.63*17.7*0.92=Pbat=124 watts to battery

    Ibat=Pbat/Vbat=124 watts / 14.2 amps = 8.73 amps to battery
    205 watt panel: 26.6v; 7.71amp
    Imp*Vmp*0.92= 7.71*26.6*0.92=Pbat=188.7 watts to battery

    Ibat=Pbat/Vbat=188.7 watts / 14.2 amps = 13.3 amps to battery
    So--Depending on how we setup the two example panels, we can either waste power (using PWM with miss-matched panel Vmp), or using an MPPT controller--recover about 92% of the energy available in the system...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: RV Solar Sizing

    BB
    Thanks

    Now I am comfortable with the panel sizing for 1 hour of sunshine and have decided to go with an mppt controller. Next I would like to do this calculation

    Amp/Day = Panel Wattage/14 * .8 * Sunlight Hour per day

    Can you give me some guidance on the number of Sunshine hour /day in Southern Ca?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: RV Solar Sizing

    DLS,

    You have a couple of ways... Some really nice PDF documents that list, by month and by year, the amount of sun for various reporting stations in the US... I like to look at this--because they have a small graph that also shows how variable the solar power vs 20 year average due to weather/smog/fires/etc. may be.

    And there is a website that takes the above data and lets you predict how a solar PV system will perform in your area. PV Watts. I have found the results predicted my GT system (near San Francisco) very well. Remember solar power is very climate specific... 8 miles to the west of me, on the coast--4 days of fog, 2 days of sun in the summer. On my side of the coastal range--no fog in summer.

    Most of the fields are pretty self explanatory... Note that the minimum panel size is 1kW... So, if you have a 200 watt system, just multiply the results by 0.2 x (I will sometimes paste into a spread sheet).

    The Derating Factor... Use 0.77 for GT system. For a full off-grid system, with flooded cell battery, and an AC inverter... You can use 0.52 as the derating factor (may be a bit conservative--but that's what I do... ;) ).

    Assuming you are near the coast around Long Beach, a 1kW array, 0.52 derating, fixed mount, will produce:
    Month
    Solar Radiation (kWh/m2/day)
    AC Energy (kWh per month)
    Energy Value at $0.125 per kWhr ($)
    
    1      4.43          99         12.38   
    2      5.00          102        12.75   
    3      5.52          123        15.38   
    4      6.21          132        16.50   
    5      6.12          133        16.62   
    6      6.09          128        16.00   
    7      6.63          141        17.62   
    8      6.79          144        18.00   
    9      6.18          129        16.12   
    10     5.46          119        14.88   
    11     4.81          103        12.88   
    12     4.34          96         12.00 
    =========================================  
    Year 5.63 sun hours     1449kWhpyr $181.12
    

    You can take the Monthly kWhr column and divide by 30 days per month for kWhrs per day.

    Note that the "Hours of Sun" per day--is if you add up all ~12 hours of sun in a day, and made it the equivalent of "high noon" sun (4-6 hours of noon-time sun).

    You would take this "noon-time" sun hours and multiply by the handy-dandy derating rules of thumb (0.77 for typical Grid Tied system, or by 0.52 for an off-grid flooded tired cell lead acid battery averaging 80% efficiency, with an AC inverter averaging 85% efficiency).

    That will bracket your system pretty well... If you have local issues (farther from coast, more sun; in a valley on the coast that captures summer fog; etc.)--you will have to take a guess there.

    Also, if you have other issues (tree/building shade, no south facing location, blocked by hills morning/evening)--all of this really kills solar output. The only time the panels produce any meaningful energy is when they are in full sun. Period.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: RV Solar Sizing

    BB.,

    Thanks Again

    OK So far here are my assumptions

    RV application fixed top mount

    Need 65 amp per day 12vDC, 66% used at night.
    Can get by on 45 amps/day in unusual situations.

    Battery bank 2 100ah flooded cell batteries.
    Drain batteries to 25% mostly; never more than 50% occasionally

    Amps/day = Panel watts/14*.8 * Noon Time Sun Hours Eqv + 20 amps from driving

    Mppt controller

    Based on the latest information you sent me, again thanks. Here is the formula I am thinking of to project average derated noon time daily sun hours in the southwest on an RV

    Noon time sun hours = 6 * .52 = 3.12
    You had mentioned this was with an inverter. I have no inverter. All my power is DC unless I am grid tied. Will this change the .52 derating factor? Also, can I add alternator amps to solar amps? Am I double counting the derating factor by using the .8 factor in the amps/day along with the .52?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: RV Solar Sizing

    You can see how they did the Derating Factor here.

    Basically, I just took their derating factor and multiplied it by 80% for for battery and 85% for the inverter:

    0.77*0.80*0.85=0.52

    If we take out the inverter:

    0.77*0.80=0.62

    If we use AGM batteries which are better than 90% efficient (new ones are probably 98% efficient:

    0.77*0.90=0.69

    In the end, if we get within 5% of predicted performance--we are doing really well... Even the weather is more changeable year over year than that...

    Don't bother with the 0.80 derating--the 0.77 (roughly) take into account the solar panel derating and MPPT controller losses...

    Yes, your alternator would simply add to the charging... Does not matter where the 20 Amp*Hours (1 hour * 20 amps, or 2 hours * 10 amps, etc.) comes from--solar or alternator.

    If you drive that much--you probably should look into attaching an inverter to your vehicle's battery and run a good quality AC charger on the house bank. You will get a much higher potential charging current (and higher voltage at the house batteries) than if you "run a 12 vdc wire" from the vehicle battery to the house batteries.

    Also, just from a fuel use point of view--if you need to top off the batteries while camping--get a small inverter generator (like the Honda eu1000i or eu2000i) to charge your bank... Don't run the van--usually very wasteful of fuel. Size the generator to the load--most "cheap" generators below ~50% rated load (or ~25% in the case of the Honda eu family) don't consume any less fuel.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: RV Solar Sizing

    BB

    Thanks.

    OK. I am beginning to get a handle on the solar sizing issue, the inverter is a new wrinkle.

    I already have a converter, which is an Intellipower 30 amp with a charge Wizzard, which is used when grid tied. Here is a description

    The Charge Wizard is a microprocessor-controlled device designed to operate exclusively with Progressive Dynamics 9100 Series Inteli-Power Converters. It constantly monitors the RV battery, then automatically adjusts the converter output voltage based on its charge status. The Charge Wizard has four (4) operating Modes (BOOST, NORMAL, STORAGE AND EQUALIZE). Each Mode is automatically selected by the microprocessor and ensures a fast yet safe recharge for your RV battery.

    If I attached an inverter to the van battery could I used the Intellipower/Wizzard converter to charge the banks while driving? What happens if I am plugged into shore power? Shore power plus starting the engine? Would the bank ever draw more than 30 amps?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: RV Solar Sizing

    If I have the correct device:
    Input: 105-130 VAC, 500 Watts
    Output: 13.6 VDC, 30 Amps

    Then the inverter you use should be around 750 to 1,000 watts maximum...

    Perhaps a Samlex 600 Watt 12 Volt Sine Wave Inverter would work (around $250)... But I do not know.

    I cannot tell from the PD1xxx specifications if it is power factor corrected or not-- Non-PFC devices usually require a larger inverter to power them.

    The Xantrex TC xx series should have Power Factor Correction -- but they are not out yet... So can't tell you much about them. And you already have the PD1xxx--so not much interest in spending money you don't have to.

    If you have a Kill-A-Watt meter, you can see how your PD1xxx works--how much power, PF, current, etc. and decide which inverter would work for you...

    You would want to make sure you only run the inverter on your van when the motor is running. Drawing a few hundred watts will quickly kill your battery (real problems after 15-30 minutes?).

    I am sorry I cannot be more helpful--there is so much differing hardware out there--and sometimes the only way you will find out if a combination will work is to try it... Not easy if you don't have it in hand or can borrow from a friend.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: RV Solar Sizing

    BB,

    Thanks Again.

    I think I am going to put the invert/charger while driving on the back burner and concentrate on the solar. The addition of an inverter an perhaps a converter upgrade is a complexity that I would rather address later. I plan to stick to adding solar first.

    Given what I have learned I plan to go back and put a fine pencil to my amperage requirements. Thanks Again for giving me the education to solve this problem effectively.
  • System2
    System2 Posts: 6,290 admin
    Re: RV Solar Sizing

    I have polished up my modeling and would appreciate a quick review see below

    Solar Worksheet for Sizing Solar on a Small Class B

    Average Amps/day used
    Appliance Amps Hours Amp/Hours
    Fridge 3 12 36
    Furnace or Fan 2.8 2 5.6
    TV/DVD Laptop 4 2 8
    Lights 4 2 8
    Misc 2 1 2
    Total 59.6

    Assumptions/Requirements
    Battery Bank 2 100Amp Hour Batteries
    Batteries only drawn down 25% (50 amp hours), except in unusual situations and then only to 50% (100 amp hours)
    Driving charges batteries at 15 amps/hour
    Average driving time per day 2 hours
    66% of electrical usage at night
    Each trip started with full battery bank
    Need to be able to leave Van unattended for several days with Fridge running to support Side trips (channel islands for example)

    Modeling
    Normal Day - down 59.6 amps (usage) up 30 amps (driving), net 29.6 amps down
    Side trip – down 36 amps up 0, net down 36 amps
    Conclusion – panels must generate 36 amps per day

    Average full power sun hour equivalents in southern ca = 6 hours
    Average loss factor (panels, wiring, controllers etc) 33%, assumes no inverter

    Calculation
    Panel size (Amps/Day) = Daily Usage Amps/Sunlight Hours* Loss Factor
    Panel size (Amps/hour) = 36/6*1.33 = 7.98
    Panel Size (Watts) = Panel size (Amp/Hours) * Average Panel Charging Voltage
    assumes Mttp controller
    Panel size Watts = 7.98* 14 = 111

    Result
    Kyocera 135 Watt Panel with Morning Star Mttp Controller
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: RV Solar Sizing

    DLS,

    You are getting there... A question, to you plan on having your solar panel flat or tilted up? A flat panel (vs tilted to ~34 degrees) will cost you power in the summer -- basically another derating factor of ~67%...

    You can run the numbers through the PV Watts program and see how it affects your output.

    Measure how much area you have available for a solar panel, and see what is the maximum size you can put in there (or multiple panels) and if you will be tilting them or mounting them flat. If you go up "North", the derating with panels laid flat will even be more--especially in "non-summer" months.

    You may want to look at some sort of battery monitor (Xantrex can set signal contact on XX % battery capacity) or power cut-off to prevent the fridge draining your batteries flat (really a trade off between saving your food and rotten food in fridge vs saving your battery bank). A propane fridge may be a better choice (propane, with all of its drawbacks...)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: RV Solar Sizing

    BB
    Thanks

    I had assumed a flat mount, but could easliy mount with a tilt capabilty.

    I have measured the area available and can accomodate a 205 W panel. Two 135W Panels or three 85's. Since this is a pop top I wanted to keep the weight at a minimum so would prefer to stick with one panel.

    The battery monitor sounds like a good idea I will investigate.

    I had already checked on a propane fridge and it has been ruled out due to the additional venting with regard to the Van layout.

    The previous modeling assumed a full recharge every day. If this does not happen, ie flat panel mount, rain etc there are areas of extra margin. The first is battery draw down. I could go two days with no solar and be down only 25%. The second is that the sizing resulted in an 111W panel and the 135 watt panel is 22% over that. I was hoping that this would be sufficient.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: RV Solar Sizing

    DLS,

    You can always add a panel later (and depending on panel size, another charge controller)--if the area/mounting allows it.

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