charge controllers

i have a 40W Panel,advice on the size of charge controller i need.
how do i make the system effective?


  • BB.
    BB. Super Moderators, Administrators Posts: 33,501 admin
    Re: charge controllers

    What size battery do you have (12 volt? How many Amp*Hours?).

    And what is the Vmp and Imp of the solar panel (Voltage maximum power and Current maximum power)?

    For a 40 watt solar panel, I would guess its current output is:
    • P=V*I
    • I=P/V= 40 watts / 17 volts = 2.35 amps
    Any solar charge controller rated for 3 to 10 amps or so would be just fine.

    There are very simple solar charge controllers like this one:

    wind-sun_2119_37537748SunGuard 4.5 amp solar charge controller

    There are more expensive charge controllers out there with volt/amp meters built in, or others with LED status lights (is the battery charging or fully charged and such).

    For long battery life, you need to keep the battery from being fully discharged (a battery taken below 20% state of charge may be permanently damaged).

    So, you need to watch your battery with a volt meter and keep it above ~12.0 volts (when measured with a good/accurate meter).

    For a 40 watt panel, the battery should be around 20AH to 50AH capacity (assuming 12 volts). If you have too large of battery your solar panel will not be able to properly charge the battery.

    In the end, you will need to see what brand/model of small solar charge controller you can get locally. As long as you connect it correctly (keep the wires between the charge controller and the battery short and low resistance--And the wires between the solar panel and the charge controller not too long or small diameter wire)--your system should work OK).

    A 40 watt panel + battery bank will be able to supply around (assuming 3 hours of sun in the winter and 5+ hours in the summer--full "noon time average sun" with panel pointed at noon time sun):
    • 2.35 Amps * 3 hours * 0.62 system efficiency = 4.4 Amp*Hours per day winter
    • 2.35 Amps * 5+ hours of sun * 0.62 eff = 7.3 Amp*Hours per day summer

    So, if you have a light that takes around 1 amp of 12 volt current--then it will run around 4.4 to 7.3 hours a day (4.4Amp*Hours/1amp=4.4hours).

    Most people tend to over estimate how much power they can get from a solar system and underestimate how much power (current) they use.

    The battery is the part of the system that you have to watch out for--If you undercharge/take the battery dead--it will not last very long.

    Also, if you overcharge it (without a charge controller) and do not keep the battery properly filled with distilled water / filtered rain water (or use well/river water to fill the battery)--The battery will not last very long either (sealed batteries cannot be refiled with distilled water).

    You can read about batteries here if you want some more information:

    Deep Cycle Battery FAQ

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset