How much power do I need ?

If I have two devices that draw a total 500 ma and want to keep them up all of the time what size battery and PV pannel do I need?



  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Re: How much power do I need ?


    You did not state at what voltage... So, let's assume 12 VDC... The 12 volt solar panel will typically need a Vmax-power rating of ~17.5+ volts (17.5 volts = 15.5 volt max charge voltage for battery + 2 volt charge controller voltage drop) and an open circuit voltage of around 22 vdc.

    Your load:

    0.5 amps * 17.5 volts * 24 hours / day = 210 watt*hours per day of solar power.

    Now, you can look up on either of these two (PDF File and Point and Click) and estimate "how many hours of sun per day"--but for the sake of argument that you want this to be 24 hours per day and you want this to run during the winter when there is less sun--assume that 2 hours per day with 3 days of storage, wet cell lead acid battery (80% efficient), and you discharge the battery to 50% capacity.

    Battery Size:

    210 W*H per day * 3 days * 1/50% * 1/12 volt battery voltage = 105 Amp*Hour 12 vdc storage battery

    Solar Panel Size:

    210 W*H per day * 1/80% batt eff * 1/2 hours per day of minimum sun * 1/75% solar panel derating = 175 watt STC rated "12 volt" solar panel.

    The above is a bit on the over sized side of the design. You could probably cut this by 1/2 and it would work for 9 months out of the year. And even with this size system, you may need an alternate power source during periods of bad weather (if you get lots of clouds/fog during certain seasons).

    Also, if this is a 12 volt application, in needs to operate over a range of voltage of ~10.5 volts to 15.5 VDC (actually, quite a wide range for most 12 VDC devices).

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