Solar NEWBIE :)

joeplumber
joeplumber Registered Users Posts: 6 ✭✭

Hi all,

I'm just getting interested in solar power and have MANY questions. First is what size solar setup do I need? I purchased an old road maintenance message sign that has six 70watt solar panels and 8 12 volt batteries. Would this be enough system to run a small 256 sq ft cabin for a weekend? Just basics like lights, laptop, etc


Thanks All :)

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin

    Energy usage is a highly personal set of choices. I will give you some idea of what a small cabin (no refrigerator) might look like--But you need to decide if it, smaller/larger system is closer to your energy needs (and wallet--Always a concern that lots of expensive stuff in a remote cabin can grow legs).

    LED lighting... There are some good 12 and 24 VDC lamp options, or go with an AC inverter and 120 VAC. And use the 120 VAC for powering the laptop, LEDs, cell phone chargers. And use 12 vdc to run an RV type water pump (if needed).

    Get a Kill-a-Watt type energy meter and measure the 24 hour per day usage of each of your major loads. You will probably find a laptop is your "largest" load (12-24 hour per day loads typically use the most energy):

    https://www.solar-electric.com/kiacpomome.html

    Call it 500 to 1,000 WH per day (at 12 volts, that would be 42 to 83 AH per day at 12 volts--Using Power=Voltage*Current, or I=P/V).

    As always, using the smallest/most efficient devices you can... LED lighting, using a tablet/chromebook/small laptop at 12 watts or less, vs a full size laptop at 30-60 Watts, or a desktop computer at 300 Watts.

    Really quick 1,000 WH per day system using our rules of thumbs for a conservative system design:

    • 1,000 WH per day * 1/0.85 AC inverter eff (If used) * 2 days storage * 1/0.50 max discharge * 1/12 volt battery bus = 392 AH @ 12 volt battery bank

    Note, used 2 days of storage... However, for a weekend cabin, 1 days of storage (and 1/2 the battery bank AH rating) can work OK--Especially during non-winter/sunny conditions. 2 days of storage for "full time" usage suggested.

    For solar array, two calculations, one based on battery bank size (more batteries, more solar panels). And the second based on hours of sun at your location.

    For charging 5% to 13% rate of charge recommended. 5% for weekend cabin, 10%+ for full time off grid (and really what lead acid battery Mfg's request). Say 400 AH @ 12 volt battery bank (2x 6 volt @ 200 AH "golf cart" batteries in series for 12 volts, and 2x parallel for 400 AH):

    • 400 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 377 Watt array minimum
    • 400 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 753 Watt array nominal
    • 400 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 979 Watt array "cost effective" maximum

    And then based on hours of sun for your location. Say fixed array, tilted to best year round harvest (53 degrees from vertical), facing south for Virginia Beach Virginia:

    http://www.solarelectricityhandbook.com/solar-irradiance.html

    We get a range of 3.53 hours of sun (December) and a minimum of 4.10 hours of sun (ignoring December and January--Long term average):

    • 1,000 Watt*Hours per day * 1/0.52 off grid system eff using inverter * 1/4.10 hours (Feb "break even") = 469 Watt array minimum suggested

    So, somewhere in the 469 to 752 Watt array would be "nice" for a 1,000 WH per day system. 753 Watt to 979 Watt would be nice if you spend a few weeks or more at the cabin.

    469 Watt array (based on hours of sun) and 200 AH @ 12 volt (2x golf cart batteries) and a 300 Watt (or smaller) AC inverter would work and be the "minimum" system (weekend/sunny weather visits). Cut power usage during bad weather, and/or (more) use of a genset when needed.

    Questions/corrections to my guesstimates?

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • joeplumber
    joeplumber Registered Users Posts: 6 ✭✭

    Great info Bill, THANK YOU!

    If I understand this, it looks like my 420 watt array will be too small for my needs. I think if I add another 300 watts I should be good as this is just going to be a weekend getaway setup. I currently have 96 volt battery setup, so I should be good there also and with a 1000 watt inverter to run the lights and laptop (or led TV) and a 2000 watt inverter generator for backup use I think I'll be good for a weekend.

    Finding this forum is going to provide me with LOTS of good info.


    THANKS! :)

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭

    The 96V battery is not a common nominal voltage, rearranging to 48V would open doors to finding an inverter suitable for the loads, don't get ahead of yourself, figure out the loads first, then work on the array needed to support them and recharge the battery. Bill has furnished a great amount of information, ponder this, ask questions and remember haste makes waste.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • joeplumber
    joeplumber Registered Users Posts: 6 ✭✭

    Yes, sorry. I was just saying I have 96 volts worth of batteries. (i have 8 12 volt batteries available for use). As a newbie to solar power, and really knowing nothing about it, I'm just at the tinkering stage right now.


    I have LOTS of reading to do on this forum! :)

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited March 2019 #6

    Regarding the batteries, make sure that they are kept charged. For flooded cell lead acid, they need to be charged at least once per month (or on a good float charger). AGM batteries need to be recharged every 3-6 months (or good float charger).

    Need to know battery type (FLA, AGM, etc.) and their type (deep cycle, automotive, etc.), and their Amp*Hour capacity too.

    Unless you really need a large inverter (running a microwave oven, electric tea kettle, etc.), I would suggest you keep down to a 300 Watt 12 volt AC inverter. Larger inverters have higher "tare" current (amount of power they take just being "on") and for bigger inverters, you need a bigger battery bank (roughly for every 100 AH @ 12 volt battery bank, the maximum reliably supported inverter is ~250 Watts).

    For larger loads that are not used often (power tools, etc.), just fire up the genset when you need to power the larger genset.

    The size of your array (and battery bank inverter, etc.) is really based on your needs. I am giving you a target--But "knowing your loads" and your needs--You have to make that decision (i.e., 500 or 1,000 WH per day, or some other different value).

    Once you decide on the size/capacity of your system (on paper), then research equipment. In general, 70 Watt panels are pretty small these days. Your system is large enough where you can use 140 to 200+ Watt panels. Larger panels (200+ Watt) are typically "different" voltage (Vmp~30-36 volt typically) than the smaller panels (Vmp~18 volts). You have to match your panels to your battery bank voltage (12/24/48 volt) and charge controller type (less expensive PWM vs MPPT more expensive).

    Very roughly, PWM are less expensive on 400 Watt and smaller system, and MPPT are better for >800 Watt arrays. Sort of "more expensive 140 Watt/Vmp 18 volt panels with cheaper PWM... Vs much cheaper 200+ Watt/Vmp 30-36 volt panels and more expensive MPPT controllers.

    MPPT controllers (can take a higher voltage array and lower current, and efficiently "down convert" to lower voltage/higher current for battery bank charging) are good when the array is mounted some distance away from the battery bank/charge controller (smaller AWG cables from "higher voltage" array to the charge controller).

    Also, your 70 Watt panels (hopefully) are Vmp~18 volt panels, and if you add (for example) >200 Watt panels, they do not usually mix/match well on a single charge controller--It may make more sense to simply give/sell the 70 Watt panels to somebody that is building a smaller solar system (or if you want a smaller system somewhere else). Similar with your existing batteries--They may or may not be a good match for your needs.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • joeplumber
    joeplumber Registered Users Posts: 6 ✭✭

    The solar panels are - Siemens SP70

    I don't have a lot of info on the batteries but will check them out this evening. They are currently charged directly from the solar panels as they are still being used on the road sign (portable message board).

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin

    Here is a specification listing for those panels:

    http://www.a1solar.com/sp75_sp70.html

    At Vmp~16.5 volts, that is kind of low for charging a 12 volt battery bank through a PWM charge controller.

    In hot weather, the array voltage may not optimally charge your battery bank (lower voltage when panels get hot, lower charging current through the controller+wiring resistance/voltage drops).

    Having lots of "smaller" solar panels and lots of smallish (AH) batteries means lots of connections. More expenses for connections/fuses/breaker and more things to go wrong.

    We try for a simple/reliable off grid power system. When you try to mix in old+new, you can end up chasing your tail over time and one after another "little thing" goes wonky. Electronics last ~10+ years, batteries may last 3-5 years in these types of applications (wide temperature ranges, iffy charging, poor maintenance (flooded cell batteries need watering, terminals need cleaning, etc.).

    You can certainly do it.. But I suggest that your do a couple different pencil and paper designs (before you buy more equipment) and see what ends up working best for you (and your wallet).

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • joeplumber
    joeplumber Registered Users Posts: 6 ✭✭

    OK, so the batteries are

    Prevailier pv-4d

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin

    They appear to be this

    http://www.batteryplex.com/prevailer.cfm/m/PV4D

    Or 12 volt @ 165 AH batteries... Probably GEL. No further specifications listed.

    Typically, GEL batteries require around 14.2 volts to charge (vs 1.48 to 15.0+ volts for Flooded Cell Lead Acid Deep Cycle Storage batteries).

    I would not be suggesting them for use with solar power systems (GEL are typically not great for solar charging--GEL require many hours of lower voltage charging to fully charge). The are like AGM in that this store well (3-6 months between charges) and can supply very high surge current.

    But, GEL (and AGM) batteries are pretty expensive compared with FLA batteries.

    You an certainly setup a system to use them. Just need a charge controller with a GEL (or sealed) setting, and plan on replacing them sometime (in the near future?).

    I would suggest you avoid 12 volt batteries in a 12 volt system. Is a bit more difficult to monitor your battery bank. I prefer 6 volt batteries because you can take a quick voltage check on each battery to see if everything is balanced (all batteries should have nearly the same voltage, if one or more battery is "different" then you need to figure out why). with 4-6 batteries in parallel, all 12 volt batteries on a 12 volt bus read the same voltage--Hard to find a "problem battery or connection).

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • joeplumber
    joeplumber Registered Users Posts: 6 ✭✭

    Thanks again Bill for all this info :)


    To recap, I got a road sign that has the 6 solar panels and the 8 gel batteries for $100 bucks. I understand the panels are old and the gel batteries aren't the best for solar, however, as I'm taking the road sign apart to use the trailer, I figured I could get my girlfriends 14 year old son interested in learning how to live without some things and still have some comforts. Hence, I thought about using the panels and batteries to produce power in a cabin I have that doesn't have any electric currently. As I won't be putting a bunch of money into this, I thought it would be a good way to get him thinking about solar power.

    Yes, these batteries appear to be expensive, but since I really have no money invested in them and no other plans for them, why not use them :)


    If he seems to get interested, we can start looking at a real solar setup. :)