What do I need for my off grid solar system?

SystemSystem Posts: 2,511 admin
Hey

I have been doing some researching into solar power energy and such.

I am working towards building an off-grid system for my shed. I looked on google for information about what i need for an off-grid solar system, but the only results were those about the science and benefits of solar energy.

I know that I need to have solar panels, an inverter, battery, and a charge controller. Is there anything missing from that?

I want to start off with lower priced solar panels until I see the benefits and understand what I am doing.

I was just going to install the solar panels onto the roof of the shed and keep all the other stuff inside the shed on a table, then just let it work its magic. Is there any specific method I should do for putting together my solar power system?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    Re: What do I need for my off grid solar system?

    Your solar power options really depend on the size of the off-grid system you want to build... A "good size" for "cabin/small home" is around 3.3 kWH per day type system for ~9 months of the year... But that is a fairly big system (maybe around 1,500 watts of solar panels + a ~1,000 AH 12 volt battery bank (should probably be a 24 or 48 volt system).

    If you are OK with a smaller system (a few hundred watts of panels and a couple hundred AH battery bank)--take a look at this two threads:

    Emergency Power

    Basically a very long thread that starts from the beginning with a few vague requirements through design and assembly for a "portable" solar RE off-grid power box.

    And here is another example by Mike90045 called the Solar Monolith:

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    Update pictures/information here.

    Here is a nice thread with video from Kevin in Calgary Canada that shows designing and installing solar PV in a small RV trailer.

    In the end--we usually recommend:
    1. Conservation--Minimum sized loads you need to support
    2. Measure your loads
    3. Define backup power requirements--if any (genset, AC battery charger, etc.)
    4. Plan out system against load, location, seasonal use (summer, winter, etc.)
    5. Select Hardware (ask more questions)
    6. Open wallet
    You can approach it "side ways"--How much money do you want to spend and what system could you build with it.

    Note that solar PV systems do not tend to "scale" well... If you go >2x up in size--usually it is better to almost start from scratch. Very little of the hardware can be reused (new/larger batteries, higher voltage battery bank means new inverter, new panels may not match existing Vmp/Imp panels, may want to upgrade solar charger, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What do I need for my off grid solar system?

    To expand on what Bill said, it's a good idea to have a target for the amount of power you want to supply - even for a small "trial" system. The whole key to a good off-grid design is to know how much power you need/want and then design the system to provide it. For instance the Watt hours per day you hope to get will dictate how big the battery bank needs to be which in turn will determine how much panel is necessary to recharge it. Inverter choice is largely based on the maximum amount of power to be supplied at any given time. The worst thing you can do, even for a set-up you're just experimenting with, is to buy a bunch of stuff willy-nilly and then agonize over trying to get it to work together.

    As far as what is missing off your list of components, don't forget the fuses. Both the inverter and the charge controller need to be connected to the battery bank with proper circuit protection. Right size wires, right size fuses = safe system.

    You can kill quite a few hours by reading through the FAQ section of the forum. :D
  • System2System2 Posts: 6,290 admin
    Re: What do I need for my off grid solar system?

    I'm planning to use the solar power in the shed to power

    3x60w lightbulbs

    Maybe a computer other electronics later on

    These are the stuff that I am planning to buy to start out with.

    Instapark® 20W Mono-crystalline Solar Panel

    B004FOIDU6.01-AU5MW0P13QZ5V._SX420_SCLZZZZZZZ_V176982349_.jpg

    Cobra CPI 480 400-Watt 12-Volt DC to 120-Volt AC Power Inverter with 5-Volt USB Output

    31meleqoZQL.jpg

    12 Volt, 7 Ah Sealed Lead Acid Battery with F2 Terminals

    41Yl2Gj32PL.jpg

    HQRP 20A Solar Panel Battery Charge Controller / Regulator 12V / 24V 20 Amp 300W with PWM Type of Charging plus HQRP Coaster

    51W54GfzuZL._SS400_.jpg
  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    Re: What do I need for my off grid solar system?

    Most people tend to underestimate their loads and overestimate the power output of an off-grid solar power system...

    In your case--Assuming a minimum of 4 hours of sun per day, an AC inverter, and related hardware with a ~52% efficiency:
    • 20 watt panel * 0.52 derating * 4 hours of sun average = 42 WH typical per day
    Your 12 volt 7 AH battery:
    • 12 volts * 7 AH = 84 WH of storage (from 100% to dead)
    3x60 watt light bulbs for 3 hour per day:
    • 3x60 watt * 3 hours = 540 WH per day
    400 watt AC inverter running at 12 volts, 85% efficiency, at 10.5 volt cutoff:
    • 400 watts * 1/10.5 volts * 1/0.85 eff = 45 amps on a 12 volt battery
    A 7 AH batter would last:
    • 7AH / 45 amps = 0.16 hours = 9.3 minutes
    Realistically, it would probably kill the battery the first time you tried to run the inverter at full 400 watts for over 5 minutes.

    Going back to your 20 watt panel--It would take roughly:
    • 20 watts / 17.5 volt Vmp = 1.1 amps Imp current rating
    • 7 AH battery / (1.1 amps * 4 hours of sun per day) = 1.6 days to recharge
    Any way you cut it--Way too much load and/or way too little panel/battery bank.

    Some good ways to measure your loads:
    For occasional power usage--a Honda eu2000i (1,600 watt) or eu1000i (900 watt) inverter-generator set is hard to beat... Quite and does not use too much fuel (1,600 watts uses 1.1 gallons for ~4 hours of runtime; 400 watts 1.1 gallons for ~10 hours of runtime).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What do I need for my off grid solar system?

    Well three 60 Watt bulbs is 180 Watts. Certainly a 400 Watt inverter will handle that.
    But a 7 Amp hour battery will only provide about 42 Watt hours at best, not including conversion losses. That means your three lights won't be on for more than about 13 minutes before the battery is completely drained. The inverter's stand-by power consumption will probably knock that figure down to less than 10 minutes.
    You certainly won't need that 20 Amp charge controller for one 20 Watt panel; it will probably put out slightly over 1 Amp under ideal conditions.

    How to improve things:
    1). switch to CFL lights: three 13 Watt CFL's is only 39 Watts. (LED bulbs use less power but cost a lot more and may not be readily available).
    2). increase the battery capacity so there's enough power to run those lights for a while. This will necessitate increasing the amount of panel to recharge the bigger battery.

    You see it's all connected together; the choice of one item influences the choice of others. I don't know what you're budget is like, but you may want to shop around a bit for some better value in things. For instance small panels tend to be pretty pricey on a per-Watt basis, and that sealed battery may be not such a good value in terms of Amp hours per dollar either.
  • System2System2 Posts: 6,290 admin
    Re: What do I need for my off grid solar system?

    So I assume that I would need a better battery than the one I choose. Its only around $20.

    What would be a good battery to get to make my lightbulbs last longer, say around 2-6 hours and less than $100? And have a longer lifespan of maybe 3-6 months.

    My budget for this is around $300. As mentioned earlier, I just want to experiment with this for the time being, until I learn more about it and how to fully utilize everything.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What do I need for my off grid solar system?

    Basic mathematical relationship: Watts * hours = Watt hours.
    If you want to run 180 Watts of lights for 2 hours that's 360 Watt hours. For 6 hours it's 1080 Watt hours. Without getting in to the efficiency losses inherent in any system, the Watt hours divided by the system Voltage equals the minimum number of Amp hours needed to provide that power. Since you don't want to take a battery below 50% capacity (or else it may not recharge) you have to multiply that by at least 2. So:

    360 Watt hours / 12 Volts = 30 Amp hours, * 2 = 60 Amp hour battery
    1080 Watt hours / 12 = 90 Amp hours, * 2 = 180 Amp hour battery

    Again, that does not include power losses that will happen in the system (like the amount the inverter uses).

    The next problem is that to recharge a battery you want at least 5% of the capacity as a peak charge current. 10% is better. So 5% of 60 Amp hours is 3 Amps. To determine how much panel you need for that you have to multiply by the charging Voltage (usually 14.4 on a "12 Volt" system): 43.2 Watts. But panels don't put out their "nameplate" rating, rather an average that is somewhat less. Typically you can expect 77% efficiency, so that 43.2 Watt need becomes a 56 Watt panel, minimum.

    These aren't precise calculations: I'm just trying to demonstrate the relation between one thing and another in the system.
  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    Re: What do I need for my off grid solar system?

    MR16 LED lights may be a better option for power (4-7 watts at 12 volts)... Or other LED lamps.

    You do have to be careful with 12 VDC lights on a 12 volt battery system... The range of a solar PV system is around 10.5 to 15+ volts (dead battery to equalization charging)--That is a pretty wide voltage range and not all "12 volt appliances" will run nicely on them.

    A MR16 runs on a 12 VAC transformer--so how any particular LED bulb will work on "12 VDC" is a roll of the dice as they do not seem to spec. them out...

    If you need to send the power some distance, a small 120 VAC inverter with LED or CFL lighting may be a good alternative too. The inverter takes care of the 10.5-15 volt DC battery power and will send the 120 VAC power much larger distances with much smaller gauge wiring.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What do I need for my off grid solar system?

    You could consider an all DC system: no inverter = no inverter expense or power consumption. Lots of things available for 12 VDC from automotive/RV supply.

    Another option: look for used equipment. If you read through our host's web site and familiarize yourself with what's available and what it's worth you can then peruse the local ads like Craigslist and see if anyone is looking to sell some panels and other equipment for a reasonable price. Too often what I see there is people asking "more than new" prices for quite old stuff. But every now and then you could get lucky. Just be sure you can check it out to see if it works. I'd avoid anything that isn't "famous maker" (Xantrex, Outback, Morninstar, et cetera) and try to see how old it is too; elderly electronic equipment has a tendency to quit shortly after you but it. Don't bother with used batteries unless they are going for scrap metal price; biggest pig-in-a-poke there is.
  • System2System2 Posts: 6,290 admin
    Re: What do I need for my off grid solar system?

    From the information you guys gave me, I'm going to do the math and see what I really need to make it work.

    I didnt expect to see many expensive items needed for better power, so i'll keep looking for the best quality and priced battery and panels needed for my project.
  • mike95490mike95490 Solar Expert Posts: 8,466 ✭✭✭✭✭
    Re: What do I need for my off grid solar system?

    a 20W panel, is really not much power. There are 12V CFL lights too, a little less pricey than LED lights.
    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 ,

  • bsolarbsolar Solar Expert Posts: 103 ✭✭✭
    Re: What do I need for my off grid solar system?
    Mihawk wrote: »
    What do I need for my off grid solar system?


    .. a less technical and precise answer is to achieve something alittle more useable you'll have to spend more money. A couple of ~150w panels, a big deep cycle walmart special battery or two, and a 400w or so pure sine inverter if you want to be putting pc's, laptops or anything like that on it + cabling ect, your charger would work.

    so the batts and inverter put it around $250 and charger and cabling if your thrifty maybe your at 300 and then you hit the brick wall of panels. Im sure you know the pricing of factory panels .. panels in the 150w range can be made at home if theyre done right for alittle less than 200 a panel but its alot of work and there arnt too many ways to make them right where they will perform and last but it can be done i did it ..

    .. and the way i see it having some experience but sucking at precise figures like these other guys is it would take ~$600 bucks minimum to put something together useable to run a couple of lights and a small pc. What you are looking at with the little system you are envisioning, just to be blunt, is charging a cell phone and running a couple of small on-the-dim-side LED's.
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