If each lamp has only one wattage, then you don't really need a dropdown box for the wattage. You could just have a cell display the appropriate wattage as soon as a lamp is selected in the lamp dropdown.
[EDIT: To provide more detail and clarification based on your comment below.]
You can use VLOOKUP() to show the wattage. The formula will look like this:
Here lamp is derived from the output of the lamp dropdown box, datarange is an array holding your lamp names in the first column and their wattages in the second column. The first column could also be used as the array that specifies your dropdown list.
Here's an example of how to do this:
The output of the dropdown (combo) box is just the position of the selected lamp in the list. That number is displayed in a cell that you specify (here it is A13). The number in A13 will change whenever a different lamp is selected in the dropdown box. To get the actual name of the lamp, we need to use the INDEX() function as shown below. The formula in D6 is:
INDEX() gets the lamp name (the 7th item in A2:A11), and
VLOOKUP() looks it up in column 1 of A2:B11, and returns the value in column 2.
From your comment, it looks like you'll need several combo boxes. They can all use the same lookup table, which can be located on another sheet or hidden, if necessary, for appearance's sake.
Setup a combo box by CTRL-clicking on it and choosing "Format Control..." In the resulting dialog box, you specify the array holding the list of items to be displayed and the cell that will hold the output, called the "Cell link".