How do electrical devices suck electricity? If the electric potential is 220V and some device needs 1500watts then how does it suck exactly that amount of electrical energy from outlet?
 A: It is not the most straightforward viewpoint to say that a device "needs 1500 watts". This is more a consequence than a condition. What happens is that you create an electric circuit by plugging in a device into the outlet. That circuit follows Ohm's law: $$V = I R$$ So for a given voltage and resistance a certain current $I$ will flow. The power is simply $$P = V I = V^{2}/R = I^2 R$$The device does not know anything, it just has a property, it's resistance $R$.
A: If it's a light bulb or heater, it's just a resistor.
First, forget that it's alternating current, just to simplify things.
Think of the power source as a really big 220 volt battery.
If it's drawing 1500 watts, divide that by 220, and that will tell you the current I in Amperes.
(That just measures how many electrons per second are flowing. An Ampere is about 6x10^23 electrons per second.)
To get the resistance R in Ohms, just divide the voltage V (220 volts) by the current I that you got above.
(An Ohm is just the number of volts it takes to push one Ampere through the resistor.)
I hope you can see that the smaller the resistance is, the bigger the current is, and when you multiply that by the voltage, you get the power.
So the way you make a bigger heater or light bulb is by giving it less electrical resistance.
If you want to go back to alternating current (AC) the power is a time-average, and it swings up and down at twice the AC frequency.
I'll let you figure out why, if you want.
A: Probably easier to think of it in terms of pressure and 'pushing'
Electrical potential (volts) are an analogue of pressure - there is a certain amount of electrical pressure trying to push current through the circuit. The resistance is what impedes the flow of current. So a high power device, like a 3500W kettle, has a low resistance and so the 220V pressure can push a lot of current through it.
If you then take the same device to a country with wussy little 110V of electrical pressure then it will only be able to push half as much current through the same resistance and so only give 1/4 as much power. Which is why they generally don't use electrical kettles in north America.
A: Circuits in devices are designed to regulate their own power consumption: when they need more power they decrease their resistance, and when they need less they increase their resistance, often breaking the circuit open when they stop needing energy at all
A: The easy devices have been covered already. More complex devices, such as PCs use smart power supplies. These power supplies need to create low DC voltages (e.g. 12V) from the input voltage (110V or 220V AC). 
A simplified model of such power supply has a small battery at 12V. This battery is continuously discharged to provide the device with the power it needs. The power supply checks the battery charge level, and when it drops too low the battery is quickly charged from the rectified (= AC turned into DC) input voltage. When the device draws more power, the power supply will recharge the battery more often. But if the battery is too small, the power supply can't keep up.  Therefore power supplies for PC's come in different Watt ratings.
