your opinions

I am in the process of building a solar system that wil eventually be completely off grid.

My current opinion is to have the following:

16+ solar panels ( will start with 2 panels and build up)

combiners (1 for every 4 panels with 2 combiners feeding into a second combiner so that I have a total of 8 panels per charge controller)

Blue sky Solar Boost 6024HL charge controller (1 fore every 8 panels)

Inverter (grid tie)

Battery bank (Start small and work up to 4000+ amphours)

I would appreciate everyones opinions and knowledge here.

Should I go 12,24, or 48 volt?
If other than 12 volt, do i need to set my panels in series or can they stilll come in as 12 volt?

Any other information you can give is greatly appreciated. If you see something you do not like about this or ways for me to improve, please tell me all you can.


  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    Re: your opinions

    Woo-boy... What you are asking for is actually pretty difficult to do with Solar PV systems. They really do not "grow" well over time. You can probably design a cost effective system that can double in size without major component change outs--but 8x or larger scaling is quite difficult to pull off...

    Small systems, you can get away with 12 volt components (around 1,200 to 2,000 watt peak power maximum). If you go above the 2,400 to 4,000 watt range--you should probably be looking at a 48 volt system.

    Also, you asked about "Inverter (grid tie)" -- Not quite sure what you are asking/telling me.
    • There are Off-Grid Inverters (take battery power and give you 120/240 VAC output).
    • Grid Tied Inverters (connect directly to solar array and output 120/240 VAC slaved to the utility power--no emergency backup).
    • Hybrid Inverters (connect to a battery bank and can operate in GT mode when the utility power is present and switch over to Off Grid mode when the utility power fails).
    And there are variations in the above (Central GT Tied Inverters in the 3-10+ kW range; as well as "micro-GT inverters" in the 200 watt per GT brick range).

    Rather than working from the hardware out to your system design--Lets look at your loads and requirements.

    Are you looking for an off-grid system, an emergency backup system, or the most cost effective / environmentally friendly system with no backup capabilities (use a standard genset for emergency power if needed).

    Also, need to know how much power you plan on using... There is "peak power" (1,500 watts to start a refrigerator) vs 6 kW inverter to operate an Air Conditioner.

    And there is how many Watt*Hours or kWH per day are you using (i.e., 150 watt load * 8 hours per day = 1,200 Watt*Hours = 1.2 kWH per day).

    Solar power is not cheap... If you want a pure grid tied system (solar panels to GT inverter to AC mains) your electric cost may be in the $0.10 to $0.20 per kWH--very competitive with utility power costs (especially in California).

    If you want off grid power, your power costs may be closer to $1 to $2+ per kWH (all the hardware, conversion losses, battery losses, replacement batteries, etc.)... Or ~10x utility power costs.

    In any case, generally the first step is conservation--Look at reducing your power usage (turn stuff off--computers, entertainment systems, unused lighting), conservation (insulation, double pane windows, energy star appliances).

    Then we look at your power needs (peak, daily kWH use, where you will install the system, backup power requirements, seasonal power requirements, etc.).

    Power usage is a very personal choice. There are folks here that live in the middle of now where on 1kWH per day (computer, satellite modem, few lights, fan, and gasoline powered clothes washer) or less (in the great white north). And there are others who live in the south that use 50 kWH per day for A/C, running a business, etc...

    If you want to learn first before spending the big bucks (not a bad idea)--You can start with a small 2-4 panel off-grid system with a couple hundred Amp*Hour 12 volt battery bank and an 300 watt to 1,500 watt AC inverter.

    One issue with Grid Tied and in-town solar systems is that, generally, building permits and utility approval are needed. If you add 2 panels at a time--technically you will need new building permits/utility approval every time--Permit fees and wasted time swamp any cost savings spreading out the install.

    Anyway--Enough from me--What is it that you are looking for? It is usually easier and less confusing to address your system than to talk in generalities.


    PS: The Vmp-array (solar panel maximum power output voltage) must be > than the battery bank charging voltage + voltage drops.

    For example, a 12 volt battery charges around 14.5 to 15.0 volts, and the Vmp of a "12 volt" solar panel will be around 17.5 volts -- And there are other issues, as panel temperature rises, Vmp falls.

    Normally, for larger systems 400-800 watts or larger, a MPPT (Maximum Power Point Tracking) solar charge controller (or GT inverter) is used. They can take Vmp>>Vbatt-charging and efficiently (>95% efficiency) high voltage / low current and down converter it to low voltage / high current needed to charge the battery bank.

    Anyway, enough from me for the moment.;)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,345 ✭✭✭✭
    Re: your opinions

    First question,,Why are you going off grid? Do you realize that grid tie is far cheaper and is inherently more efficient?

    Second, The Bluesky 60 amp controller is no longer being manufactured, so why would one buy hardware that is now out of production? Additionally, I think there is some consensus that the Outback and Midnight large controllers are better hardware.

    Finally, my sense, without being too harsh, is that I think you are ready fire, aiming on this. If I was to make any other suggestions, I would suggest that you start with your motivation for going off grid, then do a serious load calculation(s) and begin to design a system around this. For example, you have basic questions surrounding system voltage that should be answered in question one, since the answer effects all the others. For example, I don't think the controller you mention will run a 48 vdc system.

    Next, you don't have any listing on how big the system (or the panels) are, nor do you suggest what voltage the 4000 ah of batteries are likely to be. What is critically important is to match the PV to the batteries in size and capacity.

    For example, you 4000 AH of batteries in 12vdc configuration is ~48kwh, in 48vdc configuration is ~ 192 kwh, big by any measure. The Pv would need to be ~ 400 amps of of charge current. 12 vdc would be ~ 4800 watts of PV, at 48 vdc that would ~19 kw, also big by any measure.

    So your "16 Pv panels" would have to be ~ 300 watts for 12 vdc, or 1200 watt (each) for 48.

    Bottom line, avoid the biggest single pitfalls of RE/PV, that is Ready, Fire, Aim!

    Once again, welcome to the forum. There are some very sharp folks here who have forgotten more about Pv than most of us will know. Ask all the questions, and folks will give freely of their opinion and open our eyes to things that you hadn't realized.


    PS Bill got there before me so sorry for any duplication. I would add, just to emphasize the point, Battery based PV comes at about twice the cost (per kw) at half the harvest, leading to a kwh cost net/net of about 4 times.

  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    Re: your opinions

    And Tony is who I was referencing about the small off grid system way up there in Canada. :D

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: your opinions

    The reason for going off grid is that I have purchased land that does not have access to power.

    I will be building a house here and am looking at the power needs.

    The house is going to be 2500 sqft.

    Where I live now( south texas) I am running about 40Kwh per day (1200Kwh per month average). I know this can be reduced by quite a bit since I have 3 computers that are on all the time as well as a the kids tv's, ac usage, and lights.

    I am not firing before I aim, because my whole reason for asking my initial question was to try to learn what I need to do from you people who have experience in it.

    I dont want to build a pile of crap and am asking what the system should have and how it should be configured, things to consider, and what you would advise my system to look like.

    On a note, I am also looking at putting some sort of wind generator in the system so that I can be charging a battery bank at night and cloudy days.

    I would also expect to get a generator for keeping the batteries up when the solar and wind just couldnt handle it.

    Please keep the info coming.

    Once again, if I asked my question wrong then I am sorry. What I need to know is how to go about designing and what should be in a system of this nature.
  • icarusicarus Solar Expert Posts: 5,345 ✭✭✭✭
    Re: your opinions

    My best planning suggestion, is to buy a Kill-a-watt meter and start charting your loads. Your biggest is likely to be your A/C. 40 KWH/day is a hefty load on a battery based system.

    Just for the record, we live off grid, with 400 watts of PV. We use ~ 600-800 wh/day. On a perfect day we can generate 1-1.5 kwh/day.

    So if you can see, if you are using ~50 times as much energy, you are going to need about 50 times as much PV. 20kw Pv might cost ~$150-200,000. As you can rapidly see, conservation is going to be your cheapest energy dollar. Reduce your loads by half and you save a ton of money. Reduce it by 80% and it gets reasonable.

  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    Re: your opinions

    As an example of a system design. Using PV Watts to calculate your solar panels for Austin Texas, fixed array, 0.52 system efficiency (flooded batteries, AC inverter, etc.). Pick 1kW of solar panels (nice round number):
    "Station Identification"
    "Lat (deg N):", 30.30
    "Long (deg W):", 97.70
    "Elev (m): ", 189
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.520"
    "AC Rating:"," 0.5 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 30.3"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 9.7 cents/kWh"

    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 4.32, 65, 6.30
    2, 4.96, 67, 6.50
    3, 5.47, 81, 7.86
    4, 5.52, 77, 7.47
    5, 5.54, 78, 7.57
    6, 5.93, 79, 7.66
    7, 6.21, 85, 8.25
    8, 6.22, 85, 8.25
    9, 5.77, 78, 7.57
    10, 5.65, 80, 7.76
    11, 4.60, 65, 6.30
    12, 3.96, 59, 5.72
    "Year", 5.35, 900, 87.30
    Say you use 1,200kW per month year round average, and that you will only use the generator ~3 months of the year--That gives us February with 67 kWH per month per 1kW of solar panels (again, for an off-grid system):
    • 1,200 kWH per month / 67 kWH per month per 1 kW of panels = 17.9 kW of solar panels
    Battery bank sizing... A good sized bank is 3 days of no-sun and 50% maximum discharge. For this large of system, we are going to assume a 48 volt battery bank:
    • 40,000 Watt*Hours per day * 1/48v battery * 1/0.85 inverter eff * 3 days of no sun * 1/50% max battery discharge = 5,882 Amp*Hour @ 48 volts
    Lets say that your 40 kWH per day is used mostly in an 8 hour period (just making back of the envelope calculations). The average power would be:
    • 40kWH per day / 8 hours = 5 kW average load
    The Xantrex XW 6048 volt Hybrid Inverter is rated at 6 kW load and 12 kW maximum surge. I would guess, for this size system, you will need 2x XW 6048 inverters.

    And for charge controllers. The "standard" largish MPPT charge controller is rated at 60 amps for a 12-48 volt battery bank. For 17.9kW of solar panels and 0.77 nominal derating (practical day to day maximum power output from solar array+charge controller) charging ~58 volts into your battery bank:
    • 17,900 Watts * 0.77 derating * 1/58 volts charging = 238 amps
    • 238 amps / 60 amp max rated charge controller = 3.97 = 4+ charge controllers
    So--A very rough system cost (I am not in the solar business, I do not do estimates for a living, I speak for no solar company, including our host NAWS):

    $50,250 = 75 panels * 245 watts of solar panels @ $670 each
    $2,645 = 5 controllers x Morningstar TriStar 60 amp MPPT solar charge controller $529.13
    $62,690 = 2 batteries per string * 5 parallel strings Crown Deep Cycle Battery 12/85/21 $6,169.00 24 volt, 1060 AMPS @ 20hr rate
    $6,800 = 2 x Xantrex XW6048-120/240-60 Hybrid Inverter-Charger $3,400.00
    $2,280 = 2xXantrex XW Power Distribution Panel, Prewired $1,140.00
    $20,000 misc hardware/mounting/wiring/etc...
    $144,665 rough system hardware costs

    Your cost per kWH, assuming 20 year system life (including industrial batteries):
    • $144,665 / (900 kWH per year per 1kW panels * 17.9 kWH * 20 years) = $0.45 per kWH
    Realistically, you will not use every watt the system can produce--but probably closer to 50% of the average power. And major electronic components will probably last 10-15 years and need replacement. Plus, the batteries may last 10-20 years (maintenance, quality of battery, luck, etc.)...

    So averaging 1/2 your generated electrical power doubles the cost to $0.90 per kWHour.

    If you have " bad luck", you could end up replacing almost everything in 10 years--That will almost double the above costs (plus inflation), so now you are in the $1.50 to $2.00 per kWH cost range.

    Anyway, there were lots of assumptions I have made. Also, I used pricing from our host and mixed and matched components.

    This is not my job--so while the pricing is probably roughly accurate (major solar components are becoming commodity items and similar pricing across vendors)--There are still lots of details to work out.

    But it gives you a rough idea of the scope and scale of such a project based on the information you have provided.

    In "real life" you would have a system this large professionally quoted and possibly even installed (add another 20-40% for costs).

    Also, you talked about wind power--That will be a big project in itself and probably way out of my knowledge base other than to say I would start with Solar PV panels + Genset and look at the wind turbine to reduce your fuel use during bad weather (hopefully it is windy during your poor sun conditions).

    Any way you cut it--this is a large system. You are looking at ~$8-$10 per Solar Panel Watt just for components for this off-grid system.

    Much of this will scale in pricing... If you decide on 20 kWH per day--The system would be pretty much 1/2 the capital cost.

    I hope this helps. Please ask questions--The above is just intended as a starting point for discussions.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solar_davesolar_dave Solar Expert Posts: 2,346 ✭✭✭✭
    Re: your opinions

    If your building your house you have the perfect opportunity to maximize your conservation. Round one I would consider reviewing your building design to maximize passive solar techniques, extreme insulation, efficiency in the layout. Use propane or nat gas (if available) for hot water, heating and cooking. Consider a solar thermal system for your hot water needs. Also geothermal or high efficiency heat pumps should work well in Texas but running them is a pretty high load for off grid. Consider your water pumping needs in the design.

    Once that is done, try to get a realistic estimate of your current usage. Look for ways you would reduce your consumption on you current loads.

    If the new location is getting new appliances use the most efficient one you can afford. Use efficient lighting in the new design.

    From all of that you should be able to start with an pretty good estimate of the loads. Now you can work forward with the solar system sizing and components.
  • mike95490mike95490 Solar Expert Posts: 8,466 ✭✭✭✭✭
    Re: your opinions

    re: wind power

    Unless you have trees that grow at an angle, from being pushed over by the wind all the time, I doubt you have enough wind to make a worthwhile investment. Or unless your hat blows off nearly every time. It can "feel windy" but in actuality, a breeze does not contain enough power to extract.

    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: ,

  • icarusicarus Solar Expert Posts: 5,345 ✭✭✭✭
    Re: your opinions

    The irony with small scale wind is that if you have enough wind to be useful, the chances are that you have so much wind that small scale turbines can't handle it. The problem with small scale wind turbines is that you have a large mechanical machine that lives in a very harsh environment so that failure rates are high, especially in genuinely high wind areas. I suggest that you read all you can on the wind section of this forum.

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