Very small solar set up - diagram

beachmanbeachman Registered Users Posts: 5
I am setting up a very small solar power installation and I have included a diagram of the set up herein. I am new to this forum and was recommended by someone in another forum. This looks like a great spot to gather information and get ideas. I must admit that I know very little about electricity so I post this diagram with a warning that it may be incomplete or misleading. I would like feedback from any body as to their thoughts on this system. I realize that I will need a larger fuse on the plus wire to the 12v system - say 8 AMP, and will need a large breaker or fuse on the plus cable to the inverter - say 63 AMP.


  • solar_davesolar_dave Solar Expert Posts: 2,362 ✭✭✭✭
    Re: Very small solar set up - diagram

    You have a pair of L16 batteries @ 12V/370 ah you are going to need lots more than one 235 panel to achieve a 10% rate of charge. Those tall batteries need it to keep them stirred as well.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Very small solar set up - diagram

    Welcome to the forum.

    In addition to what solar_dave said, skip the Blue Sky controller. They aren't very good. The 230 Watt panel is likely to be Vmp around 30 so an MPPT type controller would be needed to use it on a 12 Volt system without wasting lots of power.

    You may want to down-size the batteries as well. The L16's are not cheap, and if you want a small system to experiment with they are not as practical as GC2's - standard golf cart batteries.

    A common first system set up would be two GC2's giving 220 Amp hours @ 12 Volts and around 400 Watts of PV to charge it with. You could use standard '12 Volt' panels (Vmp 17-18 range) and a PWM controller or a couple of 'GT style' panels (Vmp around 30) and an MPPT controller (MidNite's Kid 30 Amp is a good value there).

    Some more detail as to your intentions for use will help guide the advice.

    Wire sizing and appropriate over-current protection would be determined last, after equipment selection and configuration is finalized.
  • beachmanbeachman Registered Users Posts: 5
    Re: Very small solar set up - diagram

    OK. I hear you about the panels and the CC. I actually have some of the equipment already and I will explain where I am at and what my plans are. The Blue Sky is MPPT but cannot do the equalizing which I would have to do with my generator and a battery charger from time to time. I have the solar panel, the grounding wire and plate installed, and I have the batteries which I keep at home during the winter with periodic charging to keep them fresh. Right now, the Blue Sky seems to keep the batteries topped up but I want to wire the cabin and install a battery monitor -hence the complication with the shunt and I will need another SPD and a controller box. I may need to get a different CC or add a solar panel or both. Will this system work? I have calculated a daily need of about 600 to 700 watts.
  • AuricTechAuricTech Solar Expert Posts: 140 ✭✭
    Re: Very small solar set up - diagram
    I have calculated a daily need of about 600 to 700 watts.

    "Watts" or "Watt-hours"? The difference is huge.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Very small solar set up - diagram

    The amount of power needed to keep batteries charged and the amount needed to recharge them is quite different.

    Let's assume you mean 600-700 Watt hours per day. Divide by a single 230 Watt panel and you get a need for that panel to produce 100% power rating for three hours straight just to produce the amount of electric consume. This does not happen. What does happen is an average output of about 77% of the rating over possible four to five hours, and then the inevitable losses as you try to store the power in the batteries and take it from them. The result is an end-to-end efficiency around 52%, or: 230 Watt PV * 5 hours per day * 0.52 = 598 Watt hours AC at best.

    The more you can use 'directly' the better, but deep cycle batteries do not like to be shallow cycled or floated. So you are better off using around 25% of capacity daily, and being able to recharge at a good enough rate to complete the charging on a good day.

    Let's look at it forwards from the load demand of 700 Watt hours AC:
    700 / 0.85 inverter efficiency & tare = 824 Watt hours DC.
    824 / 12 Volts nominal = 69 Amp hours used.
    50% maximum DOD makes that 138 Amp hour battery bank, 25% DOD makes that 276 Amp hour battery bank.

    On 370 Amp hours the 69 used is 18% DOD and might work.

    But then we have the problem that the current peak from the panel will only be about 15 Amps, or a peak charge rate of only 4% which is less than half the minimum recommended for those L16's. Enough to keep it charged, but will not really exercise the batteries and keep the electrolyte mixed.

    The shunt & battery monitor aren't much of a complication, but beware it is easy to screw up a battery monitor especially on an existing system. if it is not programmed properly to begin with (batteries need to be at 100% SOC when the monitor is told they are at 100% SOC for one thing) then it is wrong forever. Frankly a hydrometer is far more accurate.
  • beachmanbeachman Registered Users Posts: 5
    Re: Very small solar set up - diagram

    Thanks so much. I do mean watt hours btw. This gives me more to go on and I do not think I will be using that much power but I want to watch the monitor to see what is happening before going any further. Good info on having the batteries at 100% when setting up the monitor. I never would have thought about this unless it was spelled out in the directions. I have gone from zero power to having a battery for a satellite radio, to a cell phone booster to now wanting a few led lights and to have power to run the sparks on our gas stove (no glow bar) and we have a propane fridge. Also want 12v to run a water pump for a shower. Unless I hear much to the contrary, I will probably forge ahead with my plans - but not until May when the weather warms up. Thanks again! (Wished I had run wires in the cottage while I was building a couple of years ago - so much easier!)
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