You will need to know the amps and the volts in the power source. To determine the wattage, use a simple multiplication formula. The ampere (or amps) is the amount of electricity used. Voltage measures the force or pressure of the electricity.  The number of watts is equal to amps multiplied by volts. That's it! In other words, watt=amp X volt. Sometimes you will see this formula written as W=A X V.   For example, if the current is 3 amps (3A) and the voltage is 110V, you multiply 3 by 110, to get 330W (watts). The formula is P=3A X 110V = 330 W (with P standing for power). This is why watts are sometimes called volt-amps. Circuit breakers usually have the amps written on their handles. This is the maximum amperage that circuit can take before the circuit breaker trips. You can also determine both volts and amps by looking on the labels or in the operating manuals. You can  look up common figures for standard appliances (most small appliances and lighting fixtures in homes require circuits that range from 15-20 amps and larger ones are 20 to 60. However, most counter top household appliances are rated for 120 volts and operated with 12 or less amps.  Larger appliances like ranges and clothes dryers require more power and are connected to circuits that are wired to 240 volt power and may draw 20 to 40 amps depending on a number of factors.   Household wiring is usually 120 or 240 volts in North America. You can do the multiplication formula in reverse. For example, let’s say you have an AC 24-40 power supply. This means your power supply has 24 volts and 40 watts.  The power source can supply 1.6 amps. The formula used is 40-? X 24. Thus, you would divide 40 by 24 to get 1.6.  Here’s another reason you might need to do this. Let’s say you want to figure out the watts used by a ceiling fan, and the label on the fan says that the fan uses a certain number of amps. You could then find out the typical number of volts used by a ceiling fan (by calling the manufacturer or just looking online), multiply the two...

TL;DR in Vietnamese:
Xác định công suất của nguồn điện. Xác định am-pe và vôn theo cách tương tự. Xác định công suất điện trở.