Before buying your Christmas lights you will need to have the right extension cords, timers and power calculations.  Given these we will be able to calculate the total amount of power you can use which may limit the total amount of Christmas Lights you can buy.  

There are a few things to note about safety before you get started with your decorating. Outdoor cords should be marked with labels that read either UL or ETL. These labels let consumers like me, and you know that the cord has been tested by a group with an independent laboratory, known as Occupational Safety and Health Administration, or the OSHA. The packaging that the cord comes in should also state whether the cord is for indoor or outdoor use. Since we are decorating outdoors, we need an outdoor cord, since these cords are weatherproof and can be used in harsh conditions like ice and snow. Not only are they water proof but the outdoor extension cords are generally thicker due to the insulation and sheathing that is used which makes them more sturdy and able to carry current over longer distances.  Another thing to get for your decorating is automatic timers. These work by turning off your lights during the night and when you aren’t at home.

The electrical outlets for outdoor use should always be equipped with the standard three-proving plugs for grounding. You should also make sure that the exterior outlets being used are protected by utilizing GFCI outlets.

When you first connect the Christmas Lights outside, you need to ensure that you have a general idea of the power that you will be using or you might need to split the total amount of wiring into multiple circuits.  You will be able to figure this out by figuring out the total watts of power that each product uses (Most every set of lighting will tell you the total wattage used) and making sure that the total is less than the ciruit can handle.  It is important to confirm whether you are using a 15 or 20 amp circuit because this definitely changes the math.  The calculation to determine watts of pwer is to multiple the circuit Amp by the Volts (Watts = Amps x Volts).  Normal U.S. and Canadian circuits are 15 Amp and 120 Volts which means a normal household circuit can hold a maximum amount of 1,800 Watts of power.  But keep in mind it isn't wise to approach the upper end threshold it would be wise to keep it around 80% of the maximum.  Many circuits and some outdoor circuits are 20Amp which provides more power.  The 20 Amp circuits still usually have 120 Volts which mean they can hold a maximum of 2,400 Watts.  

Keep in mind LED lights are much more energy efficient than incandescent lights.  I've found that the same volume of LED lights use about 1/10th of the electricity as incandescent lights.  However LED lights are much more expensive so there is a trade off that you have to figure out for yourself.  Also, keep in mind that most Holiday lighting is only kept on for 6-8 hours per night and around 45ish days so the total energy savings from LED lights vs incandescent over a full season is usually negligible.  I must reiterate the word USUALLY because Clark Griswold is still running around out there and he blows the meter off the normal expectations.    

If you want, you can run a three outlet extension cord out into the yard further to give you more options for your decorating. You should never use indoor outlets and then close the window on top of the cord. This can cause damage to your home through fire or another issue. 

You can also connect timers to the multi-outlet stakes which allow you to turn off the lights when you leave for the day and at night. Also, you will need to check your outdoor lighting timers to determine if they cap the total amount of wattage that way you don't overload the timer.  Some timers don't tell you this but most explicitly tell you the amount of power they can handle.  Decorating for Christmas has never been easier than when you are armed with the right tools and calculations to get the job done.