With the number of lights constantly changing every day, I was once asked how I keep track of how many lights we have up.  No, I don't go out and count them every night.  I keep a simple spreadsheet that lists each display and how many lights are on each.  Buy changing the the number in the quantity column it multiplies itself by the number in the each colum.  It then gives a total of how many lights are there.  For example, if I were to add another 6' tree that had 200 lights on it then I would have 5 in the quantity column and the total would automatically recalculate to 1,000.

Here is how my spreadsheet looks for 2009.