24 v panel and 12 v battery wattage calculation help

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mondo
mondo Registered Users Posts: 3
in Solar Beginners Corner #1
Hi, i have a midnight kid mppt controller with 270 watt 24v panel connected to a 12 volt battery. I am wondering how i should calculate the wattage i am producing. In bulk charge lets say the controller is reading 8.7 amps, 13.5 battery voltage, and 27.8 input voltage. Should i use the input voltage x amps or battery voltage x amps to get watts? The controller seems to use the battery voltage because it is displaying 117 watts.

If the controller is correct then in best case situation i should see almost 20 amps? Assuming 270 ÷ 13.5 = 20. 270 being panel watts, 13.5 being battery voltage, and 20 being amps.

How should watts be calculated when using 24v panel and 12 volt battery?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: 24 v panel and 12 v battery wattage calculation help

    Welcome to the forum.

    Watts is Volts times Amps so the Watts coming out of the controller is 13.5 Volts * 8.7 Amps or 117 Watts.

    The current will only be what the load demand is at a given Voltage as the controller moves from Bulk to Absorb and then to Float. You would usually see your maximum power during Bulk when current demand is at its highest.

    From 270 Watts on a 12 Volt system you might expect (270 * 0.77 derating / 12) 17 Amps peak current.

    Watts aren't really important to battery charging. The peak current and correct charging Voltages are.
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  • mondo
    mondo Registered Users Posts: 3
    #3
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    Re: 24 v panel and 12 v battery wattage calculation help

    The most i have seen is 10.5 amps and about 160 watts since october. Can i expect to see a third more during summer? The panel i have is used so i am concerned with its output efficiency.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: 24 v panel and 12 v battery wattage calculation help
    mondo wrote: »
    The most i have seen is 10.5 amps and about 160 watts since october. Can i expect to see a third more during summer? The panel i have is used so i am concerned with its output efficiency.

    The output will depend on both need and availability. So there must be enough light on the panel for it to produce its maximum power and there must be enough load on the controller to demand that power.

    In Summer you are likely to have more sun hours, but not necessarily more sun. The angle of panel to sun is more important in terms of peak current. Also the cold temperatures of Winter will increase panel Voltage which on an MPPT controller like the Kid translates to even more power than we'd normally expect to see.

    But if the demand isn't there (i.e. battery not deeply discharged) it won't happen. You can try running some loads at the same time as the battery charges and see how that effects current output. At 17 Amps you'd be looking at roughly a 204 Watt 12 Volt load, which is quite a bit for that Voltage level.
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  • mondo
    mondo Registered Users Posts: 3
    #5
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    Re: 24 v panel and 12 v battery wattage calculation help

    Thank you for the welcome and replies. Today was a clear day so last night i loaded my battery down to 12.18, rest, so i can test the power output today. What i forgot to do was turn the controller off before noon so it was charging all morning. I did however loaded it up all morning but that only drew the battery down to about 12.45. While loaded, i still did not see more than 140 watts at about 10.5 amps. I assumed the battery was too near full to up the power. As you mentioned above about the panel angle, i hope that is the main limit in my system as my panel is flat mounted. Next clear day, i will unbolt the panel and tilt it towards the sun, along with draining the battery down to 12.2, and leaving the controller off til the sun is highest in the sky. Tomorrow and the next few days will most likely be cloudy so i will have to postpone further testing.

    I am in no way in need of more power but am just very curious concerning this used panel. I picked it due to half the cost compared to my original choice of solar world 270. Am a bit up in the air if i made the right choice. First experience with solar so i have no real feel of what to expect.

    Thanks again for quick responses, was very helpful.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: 24 v panel and 12 v battery wattage calculation help

    If you want to test just the panel you can disconnect it from the controller and measure the short circuit current of it in full sun. For a 270 Watt panel which probably has a Vmp around 30 this would be dangerously close to 10 Amps which most multimeters can't handle so you'd need a clamp-on DC Ammeter to check it.
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