Solar Calculation Help needed!!!

Discussion in 'Homework Help' started by Shapath, Aug 2, 2010.

  1. Shapath

    Thread Starter New Member

    Jul 10, 2010
    Hi all,

    I am now trying to calculate the different parameters of a solar system. for eg, for 150W Load (DC):
    1. what will be the size of the battery bank (Ah)? how many batteries will be needed to run this load for 5hrs per day with 3 days backup?
    2. what will be the specification of the charge controller?
    3. what will be the power of the pv module?

    for problem like this what i did is:
    i. divide the load power by battery voltage and me got the current. Now multiply the current by the operating hours to get the Ah. but now I am confused how to calculate the number of batteries needed? should i use the ratings of the standard ones available in the market?
    ii. i use the output current as the charge controller specification. Is it ok?
    iiii. I multiply the load power by 1.25 to get the PV module power. Is it ok?

    Thank youψ
  2. zxsa


    Jun 11, 2010
    I'm not experienced with solar cells, but I can give you a few hints to get started with:

    1. To decide the battery bank capacity (Ah), you can calculate the load Ah as you suggest. Then, make the load Ah to be about 30% of the battery's capacity. If your application is always using 80% or more of the battery capacity, the battery's life span will be reduced.

    2. 5 hours per day. If these 5 hours are during the best sun period of your day, you would want the charge controller to supply enough current to drive the load plus add a little extra for charging the battery.

    3. Let's see what other people say about this. I'm not sure.
  3. Voltaire

    New Member

    Aug 3, 2010
    Let's say the sun shines for 5 hours min and you need 150W at 12V ≈ 15 amps for 5 hours = 75 amp hours for lights at night. The battery depletion is, say, 75% so you need to store 100 Ah. You'd want to add another 100Ah battery for every day of storage required. I think the charge rate is specified by the manufacturer and is usually the total charge/5 so 100Ah/5h = 20A.

    In your example you'd have 4 batteries and hence 80A controller if required.

    The PV module size depends on you. If the data shows that the power outage is once per year for three days then you could gamble and charge the spares over a week = small panel. If the probability of a power outage is large then you'd want to charge those batteries pretty quickly and you'd specify a larger array. This is typically specified by the customer and based on his experience of the area. You may want to explain the cost implications first and then let him/her decide.

    Consider losses and efficiency as well and look for alternatives e.g. LEDs.

    That's the basics of it.