Help With off the grid Solar Project on my university campus!!

gdandrews54gdandrews54 Registered Users, Users Awaiting Email Confirmation Posts: 1
I am currently working on a lighting solution for an outdoor classroom I and other architecture students are currently working on for our university. The current plan for our lighting is to have an LED strip as well as a couple pathway lights around the site. In total the system required 27 watts (I calculated 2.25 watts at 12V) of energy and this will run over night every night. I calculated that this will draw around 25Ah during the winter months and around 12Ah during the summer(based on the length of the nights). I was thinking about getting a 35Ah battery and connecting it to a solar panel. The university has stated that they will not be providing much maintenance to the site so i'm expecting months to go by without anyone working at the site. I have a few questions though as I have never worked with these systems before.
1.) what type of battery would you recommend? I was looking at an AGM battery. I need something that will require no maintenance and can withstand daily cycles where it will have 30 percent discharge. I understand the life span will be somewhere around 2 or 3 years. Also would you recommend a higher Ah batter? Lastly, the system will be kept inside a box we will construct. During the summer and Winter months should I be concerned the battery will be ruined from these temperatures (up to 90 in the summer and as low as 10 occasionally in the winter).
2.) What wattage solar panel would you recommend that will fully charge the battery every day?
3.) What type of controller would you recommend that could sense the temperature?

Anything else you think I should take into consideration I would be very grateful to know

Thank you,



  • mike95490mike95490 Solar Expert Posts: 7,492 ✭✭✭✭
    I assume the lights would run from 12V and not household power ?
      Will the lights and control withstand 16V if the battery charges at that voltage ?
    I would use a much larger battery, something that is readily available in your are, like a deep cycle marine battery 80ah at least
    AGM would cost more that flooded, but does reduce maintenance costs

    I would suggest the Morningstar Sunsaver MPPT with updated firmware (DIY) for Lighting control on the Load terminals
    firmware update

    You can use somewhere around a 200w panel to charge the battery,  If the panel you select has a 21v output, you could use the PWM Sunlight controller  Chose the model you need.

    Connect the LEDs to LOAD terminal, set the controller for the battery type and Dusk to Dawn operation and lock it all up so nobody steals it.

    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

    gen: ,

  • SurfpathSurfpath Solar Expert Posts: 350 ✭✭
    The system run time should vary, as you say, depending on the time of year - so I suggest you base your consumption figures and choice of battery size based on the longest anticipated run.

    You say that "the system required 27 watts (I calculated 2.25 watts at 12V)." I believe you mean 2.25 Amps at 12V.

    This off-grid calculator might be of help in calculating your panel/battery needs:

    Outback Flexpower 1 (FM80, VFX3048E-230v, Mate, FlexNetDC) 2,730watts of "Grid-type" PV, 370 AmpHrs Trojan RE-B's, no genny, 100% off grid.
  • BB.BB. Super Moderators Posts: 27,129 admin
    Where will the system be installed (nearest major city)... Up north (Chicago and above), don't usually have great sun in the winter:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 48° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 33° angle from vertical:
    (Optimal winter settings)
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec

    For example, if you are running 25 AH * 12 volts per day (winter):
    • 25 AH * 12 volts = 300 WH per day
    • 300 WH per day * 1/0.61 typical off grid system eff * 1/2.77 hours of sun December = 178 Watt panel minimum for typical December "break even"
    Generally, you should derate output by 65-75% for full time loads (vs optional loads you can cut during bad weather):
    • 178 Watt panel / 0.65 derating = 274 Watt nominal worst case panel
    Of course, if you have more than 2 days of bad weather--A backup genset may be good to have.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 1,910 ✭✭✭✭
    If I was doing this (assuming Chicago), I would size an AGM battery for 5-7 days autonomy to 50% SOC, so 25ah x7days x2=350ah. At 10°f the apparent capacity will be reduced by ~1/3. PV panel mounted vertical to shed snow.

    With a wide temp range, you will definitely want a controller with a remote battery temperature sensor so charging voltage is appropriate for battery temp, which could be quite different than internal controller temp.

    For unattended/low maintenance, I'd consider some sort of IoT type of "phone home" system, maybe a nano-computer that can access campus wi-fi to say "help me"?
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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