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If you are going to install a solar panel system and want to find the right amount of wattage and solar panel rating, you can do it easily by the following simple method. The solved example and explanation are as follow.

Suppose we want to power up four lights each of 15 watts and a fan of 60 watts and we need to use these 4 lights and 1 fan for 4 hours every day. So first, we will calculate total watts usage.

Required Load in Watts

PTotal= (4 x 15W) + 60W = 120 Watts.

This is our daily load per hour in watts we need to power up from solar panels. 

We Need it for 6 Hrs Daily

Now, we need a continuous power supply for 6 hours a day from solar panel to the load.

So multiply 120 Watts with 6 hours.

PDaily = 120 W x 6 Hrs = 720 Watt Hours per day.

Sunshine = 4 Hrs

Let’s say we are having a complete sunshine for 4 hours each day.

Now we divide 720W by 4 hours to get the final rating of solar panel.

Rating of Solar Panel

PHourly = 720 W / 4 Hrs = 180 W / H

Therefore, you need a 180 watt solar panel.

Key Point:

The above calculations are based on Ideal case. Therefore, it is recommended that always choose a panel slightly bigger in rating than we need. Because when solar panel charges the battery, there is wastage of power due to losses as well.