The reason the energy doesn't appear to be diminished is because of the way the appliances are connected through the outlet. They are essentially wired in parallel with each other, which means that each appliance still recieves 120 volts across the plug. As each appliance is plugged in, they each draw the amount of current they need due to the input resistance they have, up to the safety limitations on your breaker or fuse box. If, however they had been wired in series with each other, then the voltage would be divided between each device and you would notice what appears to be a drop. By energy, you probably mean; Watts. Wattage is determined by multiplying working voltage and amps. For example, a 100 Watt light powered by 120 Volts uses about .83 Amps. Since the resistance of the bulb does not change, if you placed two bulbs in series with each other, the voltage across each bulb would be 60 Volts and the amperage would be reduced to half or about .42. What you will notice, is the bulb does not appear to be as bright because you cut the voltage in half and reduced the current - the same 100 Watt bulb is now only using about 24 Watts, in a series circuit. Most electric motors and lights will probably not work as well with a reduction in voltage, and solid-state devices like a television have built in regulators and such that would make it not work at all if the voltage falls below a certain value. To figure out Power in Watts, multiply volts X amps. To figure resistance in Ohms, use R=V/I, where you divide amps into Volts to find its resistance. I = current in Amperes. V = Volts R = resistance in Ohms. There is a little more to this depending on how the circuit is wired and whether it is direct current or alternating current, but this should give you the basics of what to look at. For more help on series and parallel circuits, study Kirchoff's laws and how voltage and current is divided in various circuits.

There are many practical analogies between electricity and water. You can visualize water moving in pipes a little more easily than electrons and current in wires. Imagine that the wires in your walls are like 2" diameter water pipes. Each light or appliance on the circuit might be like a 1/2" pipe coming off the 2" main. As long as there are not too many 1/2" pipes coming off the main, they all get as much as they want. You can connect more 1/2" feeds than the 2" can supply provided they don't all turn on the water at the same time. So, I could plug 10 toasters into a circuit as long as I don't use them all at once. Because overloading a wire can generate enough heat to cause a fire, we put a shut-off device on the main (fuse or breaker). It is possible to draw more electricity than a source can provide and this does cause things to dim or slow down but the source coming into your house is not usually a limitation. If you were running on a generator or a battery powered inverter you would probably notice more (it would be a smaller main). I hope this helps you visualize the approximate situation.