calculation of watts of solar panel

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smhn72@gmail.com
smhn72@gmail.com Registered Users Posts: 1
in General Solar Power Topics #1
Salam Every body

I have 12v 2A DC Fan and a battery of 12v 40AH. i need a solar pannel that should charge the battery fully and run the fan mean while

mean If i turn on the fan in morning, it shoud move full day and in the evening the battery should be charged fully

by looking this condition, How many watts of solar pannel is needed?????????

i m form pakistan
here day is hot enough and my spatial position is 74N 31E.

also please guide me a formula that help me to calculated the watts of a solar panel according to my load

thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: calculation of watts of solar pannel

    Welcome to the forum.

    Since you are doing this with a battery there are two parts to the equation. The first is how long the battery will supply the 2 Amp load. Basically since the Voltages are the same that's 40 Amp hours (battery capacity) divided by 2 Amps equals 10 hours to 50% state of charge.

    This will not be exact as the real battery Voltage is not always at precisely 12 Volts, the Peukart effect will vary the battery's capacity as it discharges, there will be power lost in the system, and temperature will affect everything.

    These same factors as well as the charging efficiency and insolation of the panel come into play with the second part of the equation; recharging the battery. It isn't as simple as applying the panel output current for 'so many' hours to 'replace the used Amp hours'.

    Plus if the fan's use is primarily during the day, at least some of the power to run it can come directly from the panel once the battery is charged.

    As a rule-of-thumb you want panel current to be at least 5% of the Amp hour capacity, or 2 Amps in this case. If you move that up to 10% (which is what I normally recommend) it's 4 Amps.

    You should look for the Imp rating of the panel to be close to that, and use a charge controller capable of at least that much current. In approximate Watts you'd be looking for 4 Amps * 17.5 Volts (usual Voltage of '12 Volt' panels) or 70 Watts.

    Did that help?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
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    Re: calculation of watts of solar pannel

    i'd make that a bigger pv to cover losses and to account for some days of less than desirable weather. say 95w stc or more.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #4
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    Re: calculation of watts of solar panel

    I'll try to guess. Assume you have 4 "dark" hours in the morning, then 8 "sunny" hours, then 4 "dark" hours in the evening and then 8 "off" hours in the night.

    If your battery was fully charged by the end of "sunny" period of previous day, it had 4+4=8 hours of discharge by the beginning of this day. This is 8 x 2 = 16AH. You need to replenish these 16AH, but you'll need to put back at least 20% more 16 x 1.2 = 19.2AH. Then your panel would need to support the fan for 8 "sunny" hours 8x2 = 16AH. So, you need to produce 16+19.2 = 35.2AH every day. Since the "sunny" period is 8 hours, it comes to 35.2/8 = 4.4A. It's hot in Pakistan and you're not tracking the Sun, so panel will only produce, say 70% of its rated capacity. So, you need a panel rated to produce 4.4/0.7 = 6.3AH. The charge 12V battery you need a panel with Vmp around 18V. It should have 6.3 x 18 = 113W.

    You probably can get away with 100W panel, but 130W panel will do it without problems.
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  • SolarPowered
    SolarPowered Solar Expert Posts: 626 ✭✭✭
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    Re: calculation of watts of solar panel
    NorthGuy wrote: »
    You probably can get away with 100W panel, but 130W panel will do it without problems.

    Depends, to many factors, if its monocrystaline or poly crystaline, and on top of everything conversion efficiency.

    I'm willing to bet with (2) 60's- 120 watts in panels mono he could get away with alot of the issues that most polys have.

    I'm averaging 95watts through out the course of my PV watt hours and most importantly temperature is the biggest factor, this average is on 85~90*F days.

    The specs on my mono's are pretty durable 17.3 conversion efficient, it handles a good I-R curve fixed south where I live.

    Attachment not found.Attachment not found.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: calculation of watts of solar panel

    Polycrystaline panels do not have any "issues".

    When it comes to power output, a Watt is a Watt an Amp is an Amp and a Volt is a Volt. There is no difference in the power output from the various types of photovoltaic panels; it is all electrically the same.
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  • lc777
    lc777 Registered Users Posts: 1
    #7
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    Re: calculation of watts of solar panel
    SolarPowered wrote: »
    Depends, to many factors, if its monocrystaline or poly crystaline, and on top of everything conversion efficiency.

    I'm willing to bet with (2) 60's- 120 watts in panels mono he could get away with alot of the issues that most polys have.

    I'm averaging 95watts through out the course of my PV watt hours and most importantly temperature is the biggest factor, this average is on 85~90*F days.

    The specs on my mono's are pretty durable 17.3 conversion efficient, it handles a good I-R curve fixed south where I live.

    Attachment not found.Attachment not found.

    Thank you for this. I was trying to get my head around this earlier.
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  • SolarPowered
    SolarPowered Solar Expert Posts: 626 ✭✭✭
    #8
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    Re: calculation of watts of solar panel
    Cariboocoot wrote: »
    Polycrystaline panels do not have any "issues".

    When it comes to power output, a Watt is a Watt an Amp is an Amp and a Volt is a Volt. There is no difference in the power output from the various types of photovoltaic panels; it is all electrically the same.


    You are correct. However according to conversion efficiency mono can be up to 10% reduced in size and weight. A poly crystalline panel tends to weigh more, and size can some times bece an issue of relevance. If the thread starter is only charging and discharging roughly 40 Ah in size, I would tend to sway in my opinion that the more compact the better, and lugging around poly crystalline for transport can be a serious pain to deal with.

    Now according to name plate you are correct a watt is a watt, that doesn't at all mean that during PV watts with a fixed system that the I-Rcurve between poly or mono replicate within the same curves ot operate in the same manner of conversion.For many reasons I've chosen mono over poly.

    poly is older technology, it's actually harder to manufacture than mono, and pricing on mono is inline, and is becoming lower in cost than poly.

    you may say I am biased about polycrystalline, which I would tell you I am since there is over whelming data that mono is an overall better product.
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