Traditional incandescent Christmas tree light bulbs use about .5 watts of electricity per bulb. This is a great number to use if you need to estimate how many light strings you can run in series or are calculating the total number of watts the lights strings installed on your tree use to prevent overloading an extension cord.
Traditional C7 bulbs use in the range of 5-7 watts of energy as a rule of thumb and the larger C9 bulbs use generally in the range of 7-10 watts of power.
This is good info since you should never exceed 1000 watts maximum bulbs on a single household breaker. (Be sure to work with an electrician if you have questions regarding electrical specifications and your Christmas light installation.)
All Christmas light strings will have a power rating that will be listed in Watts. Be sure to make a note of this number when you purchase them for handy reference in the future.
Keeping a Christmas light inventory spreadsheet is a great place to keep those kinds of notes.