Most people have extension leads in their homes, using 4-way bar adaptors to increase the number of appliances that they can plug into a wall socket.
However, although there is space to plug in four appliances, this does not mean it is always safe to do so. Different electrical appliances use different amounts of power. To avoid the risk of overheating and possibly fire, you should never plug into an extension lead or socket appliances that together use more than 13 amps or 3000 watts of energy.
Use The ESC calculator to plug in some typical household appliances to see the effect on the load, and to get useful tips on how to avoid overloading your sockets.
link - http://www.esc.org.uk/public/safety-in-the-home/product-safety/overloading-sockets/
remember this when it comes to festive lighting