newbe question

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
hi all
i am wanting to set up solar power to small barn will need enough power to run a 40 watt light for about 3 hours daily & power an electric fence charger imput 12v 170 ma about 8 hours daily. question is what size panel ,how many batteries & what kind of controller will handle this
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #2
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    Re: newbe question

    Based on what you asked for:
    • 40 watts * 3 hours per day = 120 Watt*Hours per day
    • 12 volts * 0.170 amps * 8 hours per day =16.32 WH per day
    • 120 + 16 = 136 WH per day
    For the battery, we usually recommend 3 day of "no sun" and 50% maximum discharge. This is a small system, so you probably will go with a 12 volt battery bank and you will be using an 85% efficient inverter for running the lights:
    • 136 WH per day * 1/12 volts * 1/0.85 eff inverter * 3 days * 1/0.50 max disch = 80 AH @ 12 volt battery
    For the amount of solar panels, I will guess you are from around Richmond Virginia. I will use PV Watts website for the amount of sun for your area, 1,000 watts (1 kW) of solar panels for starting point, 0.52 for overall efficiency (77% panel+charge controller efficiency + assuming you are running the lights with an 85% efficient inverter), and an 80% efficient flooded cell battery bank):
    "Station Identification"
    "City:","Richmond"
    "State:","Virginia"
    "Lat (deg N):", 37.50
    "Long (deg W):", 77.33
    "Elev (m): ", 50
    "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:"," 37.5"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 8.0 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.99, 64, 5.12
    2, 4.37, 62, 4.96
    3, 4.96, 76, 6.08
    4, 5.32, 75, 6.00
    5, 5.49, 79, 6.32
    6, 5.54, 74, 5.92
    7, 5.55, 76, 6.08
    8, 5.31, 74, 5.92
    9, 5.30, 73, 5.84
    10, 4.65, 68, 5.44
    11, 3.95, 58, 4.64
    12, 3.51, 55, 4.40
    "Year", 4.83, 834, 66.72

    Your worst month of production (this is a 20 year average of solar for your area) is 55 kWH per month for an off-grid system (52% efficient):
    • 55 kWH per month / 30 days per month = 1.833 kWH per day for December per 1 kW of panels
    • 136 WH per day * 1 / (1,833 kWH per day per 1,000 watts of panels) = 74 watts of solar panels
    So, an 80 AH 12 volt deep cycle battery (not a automotive or marine battery) + 74 watt solar panel would give you enough power, on average, to power your needs 12 months of the year.

    As always with solar, you will have a week or stretches at a time where you have bad weather and will either need to cut back on the power usage or use a back up genset (or take the battery back to the house) to recharge.

    The above is just a starting point--you or I can adjust the assumptions, if needed, to give you a more cost effective system for your needs.

    You may also want to look at your loads and decide if you want to add more panel (or possibly battery bank) to support additional loads (a few power tools, radio, etc.). If this were a cabin--many people would double the battery and double or 4x the amount of solar panel to allow for addition loads (which always seem to grow over time)...

    Solar off-grid power is expensive... Looking at your lights, is 40 watts the best you can do, or should you look at CFL's or possibly LED based lighting. With solar, conservation is your friend.

    Also, now that you are looking at somewhere around $1,000 (just for sake of argument) worth of time and labor to put an off-grid system together--Is there power close enough that you can spend a $1,000 trench a cable to the barn and have 120/240 VAC utility power at the barn. Now you have power 24/7 and can power any major tools/lighting/appliances you may need.

    The solar system will only be generating, roughly, $0.30-$0.40 of electricity per month--Solar may be a good solution for you--or not. Utility power (if anywhere close) is usually hard to beat.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #3
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    Re: newbe question

    For a parts list--here are some items to look at:
    I have listed several options for the major components from our host's webstore (note: neither of us moderators, Niel nor I, have any relationship with NAWS other than volunteering here for Spam Control--You are welcome to purchase from NAWS or elsewhere as make sense to you).

    The 300 watt TSW inverter above is a really nice, low power, flexible unit. Your system may not need any inverters (all 12 volt DC) or you may just get a small/cheap MSW inverter to power a few lights/power tools. TSW is recommended for running battery chargers, small wall mount transformers, cell phone chargers, etc... But, otherwise MSW may be fine for "simple loads".

    For the charge controllers, PWM type controllers are less expensive and fine for smaller systems. If you are looking at 200-400 watt + of solar panels, you may wish to revisit MPPT type charge controllers. In any case, there are many options out there (with/without LCD/LED displays, LVD-Low Voltage Cutoff, landscape timers, etc.) for you to consider.

    Battery wise--They are expensive to ship so finding a local battery distributor may be your best bet for purchasing a "True" deep cycle battery (no automotive or marine batteries unless they are "free").

    I listed a couple AGM batteries for price reference. AGM batteries don't require any water, are more efficient, and very clean (no acid fumes to ruin cabling, etc.). However, they are very sensitive to Over Charging--So a good charge controller (with RBTS-Remote Battery Temperature Sensor-option is highly recommended). NAWS does not list any "small" flooded cell batteries at their store.

    I will stop typing here--enough for the first reply... Questions? Suggestions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #4
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    Re: newbe question

    ok great info SO i need at least a 74 watt panel ? i was looking at pricing of panels & the panels between 100 watt & 200 watt are cheaper than the 50 - 100 watt . if i would go with the higher wattage panels would i have to use more expensive components or would the higher wattage panels be overkill

    THANKS
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #5
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    Re: newbe question

    In general, the >100 watt solar panels are less expensive per Watt...

    HOWEVER, most of the > 100 Watt panels do not have Vmp=~17.5 volts or so...

    Many are 24 volts or higher... Which for a 12 volt battery bank sort of rules out the use of an inexpensive PWM type controller and pushes you towards the (very nice) MPPT type controllers.

    Based on your power usage you first posted... You could easily justify going to a ~192 with the suggested 80 AH battery battery.

    Assuming a 75 Watt panel is the minimum needed to run your loads... You can size the array to any size battery bank you want/need. We use a rule of thumb of 5%-13% rate of charge (20 Hour Rate) for the battery bank. Say you want to get two "golf cart" sized batteries. They are "cheap" and will last 3-6 years or so (with proper maintenance). 2x 6 volts 225 Amp*Hr rated batteries give you 12 volts of 225 AH capacity... The "suggested" size for a solar array would be:
    • 225 AH * 12 volts * 1/0.77 sys derating * 0.05 charge rate = 175 Watts of solar panels minimum
    • 225 AH * 12 volts * 1/0.77 sys derating * 0.13 charge rate = 456 Watts of solar panels maximum
    The above are "suggested numbers"--would not be a big deal if you chose 160 watt or 500 watts of panels. And if you make other choices, we could adjust things accordingly.
    Also, as you increase wattage (charging/loads) to above 1,200 watts or so--you should look at a 24 volt or higher battery bank voltage (cheaper wiring, more efficient use of solar charge controllers).

    The Morning Star MPPT charge controllers are very nice. And the Rogue 30 amp MPPT is also at a nice price/performance level... Depending on how many watts and want battery bank voltage you eventually choose.

    In the end, it is your choice how you wish size your system. Remember there are ongoing charges (replacement batteries banks every 3-8 years or so, electronics that last 5-15 years, etc.). Good quality mono/poly crystalline solar panels should last 25-40+ years. The "cheap" panels (typically inexpensive amorphous panels) would perhaps last 10+ years.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    #6
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    Re: newbe question

    Use these 12V DC CFL bulbs instead of buying an inverter. If you decide to buy them, select "Warm White" option, cool white is unntaturally blue-ish. They will run direct from 12V battery and you will not need to buy inverter. That also bypasses inverter loss and lets your light run 20% longer on the energy that inverter would have wasted itself.
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  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    #7
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    Re: newbe question
    tlkellar wrote: »
    ok great info SO i need at least a 74 watt panel ? i was looking at pricing of panels & the panels between 100 watt & 200 watt are cheaper than the 50 - 100 watt . if i would go with the higher wattage panels would i have to use more expensive components or would the higher wattage panels be overkill

    If you can find the panel that has Vmp in 17 - 19V range, is 75W rated of above and is cheapest, then get it. Bigger panel is not overkill, it will make your system more resistant to cloudy weather. Just make sure your charge controller can handle extra current. For example, if you are getting 190W rated 18Vmp panel, maximum current will be 190/18 = 10.6A. For that get 15A rated controller. You should not need to spend more than $250 on solar panel.
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