I'm soooooo confused

System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I am installing a solar system for a camper trailer. I have a 135 watt panel and an 85 AH battery (AGM). This will be for running lights, a refrigerator, and small appliances such as computers, cell phone charges, etc. What size controller do I need?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #2
    Re: I'm soooooo confused

    The minimum sized solar charge controller... Assuming 135 watt panel with Vmp=17 volts:
    • 135 watts * 1/17 volts = 7.94 amps Imp
    • 7.94 amps * 1.25 * 1.25 NEC safety factors = 12.4 amps
    So, a 12 volt PWM charge controller with a minimum current rating of 10-12.5 amps would work.

    Also, you would need a fuse/breaker between the battery and the other loads (and generating sources)... For example, the fuse/breaker for the above would be around 15 amps on a 14 AWG copper wire (as a minimum). And remember fuses/breakers protect the wiring from short circuits. And fuses/breakers/circuit capacity should be at least 1.25 your maximum continuous load(s) you are planning for.

    The other important point is to measure your loads (Amp*Hours or Watt*Hours) to figure out how much available power per day from the panel and you can draw from your battery bank (and if you need to bring along a small genset for loads/bad weather).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: I'm soooooo confused

    get one much larger in capacity because you are going to find that you will need much more battery and pv capacity to run all of the items you are listing.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #4
    Re: I'm soooooo confused

    In an attempt to "do the math"

    A 135 watt panel, under "normal" circumstances might put out the following: As a quick, easy calc. you can take the name plate rating of the panel, divide it in half to account for cumulative total system loses, then multiply that number by 4 to represent the average number of hours of good sun one can expect. On an RV application, this total is more likely than not to be even less as the panel orientation is likely to be less than perfect.

    135 watts /2 *4= 270 wh/day

    So, under the best of conditions you could expect to get ~270 watt/hours out of your system per day, or stated another way, enough power to run a 135 watt TV for 2 hours.

    Assuming you have other charge sources (vehicle/shore power etc) your 85 ah battery, at a 50% DoD will give you ~500 wh of power. Personally I wouldn't routinely draw the battery that low if I expected it to last very long, but that is a trade off one can make.

    You might find the following links useful

    http://www.batteryfaq.org/
    http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

    There are a series of mistakes that folks tend to make. They tend to over estimate the amount of real power they can harvest(amount of sun/orientation etc), they couple that with under estimating the total loads they draw (including system loses!) and they don't realize that loads will almost always grow with time,

    Good luck and welcome to the forum,

    Tony
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  • Crotalus
    Crotalus Solar Expert Posts: 26 ✭
    #5
    Re: I'm soooooo confused

    Here is something that may help you out. If you have not changed out your incandescent bulbs to LEDs, do so. They are not cheap. I believe that the 1141 bulbs use about 1.5 amps to light. There are direct replacements for them from places like superbrightleds.com or other places. Depending on the color temperature of the lamps they draw 100ma or less. I am assuming that the refrigerator is running on propane and not DC. If you have one of the 3-way Dometic refrigerators the DC current draw is 75 watts. You don't want to run it on DC. If you have one with the electronic control panel then the current draw is very little. It takes about an amp to energize the gas solenoid valve and use the electronic ignition to ignite the gas. It will then draw very little current to keep the propane lit, about 800 ma or less. Your water heater will require more current than the refrigerator. The biggest current draw will be your furnace when it is running, about 2 amps and the water pump, about 8 amps. However those two items don't run very often. I have 70 watts of PV with a 20 amp 3 stage MPPT controller that has a battery temperature probe and have never had any problems keeping the battery charged even with a few hours of sunshine. My battery is a 105AH rating. The controller is a little overkill but I have room for growth. Enjoy your solar you will like it.
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