Solar panel size calculation

Before calculate the solar panel size, there are some main information you need to have in hand.
1. The energy that the load needs per day.

2. The daily average sun shine hours at the location when the system will be placed (make sure the solar panel will not be placed undershade of any buildings and trees ).

Let say a Led lamp(the load) of 12V 12W, on for 6 hours per day. The average sunshine hours is 5 hours.

The charge needed per day by the Led lamp is (12W / 12V) X 6 H = 6AH.

The size of the battery can be determine at this moment is minimum 12V 6AH. In this case the battery is 100% charge and discharge. The life span of the battery under this usage is about 1 year.

You need to have data of your solar controller which will protect the battery (most controllers have this feature). The controller will set to charge the battery to 90 to 95% and it will open circuit when the level is reached. Same for discharge, when the capacity of the battery below 10 to 5%, the controller will off the load.

For the reason you need to increase the capacity of your battery to about 8AH.
Under the situation, when raining your load will not work up to your expectation.

You could find the data of life span (number of cycles) versa the % of charge per cycle of the battery in most battery supplier's wed site.

For my solar system i use 20% charge per cycle. 5o% is the most economic when space of the battery is concern.

To calculate the size of the solar panel,
As we know the so called 12V solar panel have a rated voltage at around 17 to 18V when the solar panel will give the rated power.

What we need is 6AH generate by the solar panel daily.

So the power of the solar panel is (17.5V x 6AH) / 5H = 21W.

This is a very basic way to do the calculation, on actual case more considerations need to be added. Like the power need for the controller, inverter...