Solar Calculator

Here is a simple example to show you how off-grid system is calculated, it will help you understand solar off-grid system better.

Given that:

Appliance: DC 12V40W.
Working Time: 8hs/day.
Average Peak Sun Hour: 5hs/day
Days of Self-autonomy: 3 days.
Rainy days interval: 7 days.

Find:

  1. Panel power.
  2. Battery capacity.
  3. Controller current and voltage.

So:

  1. Appliance Current=Appliance Power / Appliance Voltage = 40 / 12 ≈3.3A
    So the panel current=Appliance current x Appliance working time / Average Sunshine Duration = (3.3 x 8) / 5 = 5.28A
    So the panel power=Panel current x Panel voltage = 5.28 x 17.2 = 91W
    Normally, the voltage of solar panels for 12V battery is 17.2V.
    As rainy days interval is 7 days, and days of autonomy are 3 days, we need to enlarge the panel size to provide extra current to battery for rainy days:
    Panel power = 91 x (10 / 7) = 130W
    Considering the power loss and other factors, we give a safety factor 1.2 for this:
    So the final panel power = 130 x 1.2 = 156 ≈ 160W

  2. Appliance Current=Appliance Power / Appliance Voltage = 40 / 12 ≈ 3.3A
    Daily Current Capacity = Appliance Current x Working time = 3.3A x 8h = 26.4AH
    Battery Capacity = Daily capacity x (1+ Days of autonomy) = 26.4 x (1 + 3) = 105.6 AH
    For consideration of battery service life, at most 50% of its capacity can be discharged.
    So the battery size = 105.6 AH x 2 = 211 AH
    So we choose two pcs of 120 AH12V battery and connect in series.

  3. Controller voltage = Appliance Voltage = 12V
    Controller current = Max [appliance current, panel current] = Max[40/12,160/17.2] = 9.3A
    Safety Factor = 1.2
    So the current = 9.3 x 1.2 = 11.16A
    So a 12V15A controller is needed.