need help with size panel and battery

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Need help calculating solar panel and battery capacity needed to run communications equipment. the device runs on on 12vdc and draws about 1.4amps and runs 24/7. I have a 21w panel and type 31 deep cycle battery, which does not appear to be anywhere near enough capacity. I can add panels and/or batteries but do not know how to calculate what should work.

'edit'
I am in southeastern Oklahoma. so we will get some cloudy winter days that may hang around for 2-3 days at a time. the equipment is an internet cable modem (12vdc @ 0.8amp) and an ethernet extender (12vdc @ 0.6amp). both are always on. I put these in a small shed which sits out in a pasture, open and easily accessible. solar panel mounted to sloped roof of the shed.

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: need help with size panel and battery

    Roughly, where is the installation and is this 12 months of the year, generator backup, etc.?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: need help with size panel and battery

    Welcome to the forum.

    If you're drawing 1.4 Amps for 24 hours you're using 33.6 Amp hours per day. That means the battery should be at least 67 Amp hours. A Group 31 battery should be at least 95 Amp hours and as such should handle the load.

    You are correct about the panel being too small. There is no way a 21 Watt panel is going to replenish all that power in the 4-5 hours of good sun available. For one thing, it will in reality only produce about 16 Watts. That's roughly 1.3 Amps. Multiply that by 4 hours and you see the problem: 5.2 Amp hours produced by the panel, 33.6 Amp hours consumed by the loads.

    You want to get enough panel to produce 8.5-10 Amps in that short time (8.5 * 4 = 34 Amp hours). That is at least 102 Watts. And since sun doesn't always shine so bright more is better. A Kyocera 135 Watt panel would be minimal. And if your battery's Amp hours are higher than 95 (at the '20 hour' rate) it would be better still to size the panel at around 10% of that capacity.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: need help with size panel and battery

    i'll give some general speak here. this can be tricky as we don't know the nature of the comm equipment being employed. is there a transmitter included in this power draw and is it also going 24/7 and is it remote in that nobody will be there for long time periods to check on it?

    another aspect might be backup capacity for the time periods there isn't any sun. this can go for a week in some places and i've seen that here at my place.:cry: at a minimum you would want the worst case scenario to be the basis for charging and that would be the amount of sunshine roughly available during the winter solstice and this is somewhat location dependent.

    with all of the backup capacity and necessary pv to charge the batteries, you could have a hefty system going just to insure this little comm thing runs.
  • bluewickedburner
    bluewickedburner Solar Expert Posts: 78 ✭✭✭✭
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    Re: need help with size panel and battery
    JSherbe wrote: »
    Need help calculating solar panel and battery capacity needed to run communications equipment. the device runs on on 12vdc and draws about 1.4amps and runs 24/7. I have a 21w panel and type 31 deep cycle battery, which does not appear to be anywhere near enough capacity. I can add panels and/or batteries but do not know how to calculate what should work.

    'edit'
    I am in southeastern Oklahoma. so we will get some cloudy winter days that may hang around for 2-3 days at a time. the equipment is an internet cable modem (12vdc @ 0.8amp) and an ethernet extender (12vdc @ 0.6amp). both are always on. I put these in a small shed which sits out in a pasture, open and easily accessible. solar panel mounted to sloped roof of the shed.

    How far away Is the comm equipment from the other that it needs an Ethernet extender? Is there actual traffic 24x7 or is it on just to avoid the physical on/off process.

    Here is a piece of information you might not know. Cable modems draw almost nothing unless they pass traffic. If you have a computer that uses the cable modem for access but aren't using the computer 24x7, suspending or hibernating the computer will cause the power use of the cable modem to drop considerably yet it will be constantly available.

    If your Ethernet extender polls the cable modem, that guarantees high power draw on the cable modem.

    if you can, look for efficiencies before throwing panels and batteries at something.