Q:

Powering 3 identical laptops using a solar panel

I have 3 identical laptops rated at 45W and 20V. The combined wattage of all 3 is 135W. I want to charge these laptops via a solar cell. Do I go for a solar cell rated 135W 20V or one rated 45W 20V? 

A:

Neither.
The output of a solar panel is not voltage regulated.  20V will only be the "nominal" voltage.  You may get more or less than that.
The wattage of the panel is quoted under ideal conditions.  That is direct sunlight onto the face of the panel, and the panel is not too hot.  In the real world, you will only occasionally get 135W out of a 135W panel.
So you need an over-sized panel (or a group of panels), and you need a voltage regulator so you actually get 20V.

