AC adapters convert electricity in alternating current from one voltage to another; for example, some electronic devices such as shavers and electric toothbrushes need an adapter to work correctly when plugged into a wall outlet. If you are looking at two AC adapters that both convert, say, 120V AC to 12V AC, then the different wattage ratings on the adapters might prove confusing.
What Is a Watt?

A watt is a unit of measurement. This particular unit measures power, or capacity to do work. Watt’s law, named after 18th century Scottish engineer James Watt, states that 1W is the amount of power in an electrical circuit where 1 amp of current flows across a difference of 1 volt.
Adapter Function

Using the equation P=V x I, where P = power in watts, V = voltage, and I = current, we can determine that an AC adapter rated at 65W with an output of 12V AC will have a maximum electrical current output of approximately 5.42 amps (12V x 5.42A = about 65W). An AC adapter with a rating of 90W and an output of 12V AC will have a maximum electrical current output of 7.5 amps (12V x 7.5A = 90W).
Therefore, an AC adapter with a higher wattage supplies more current to the connected device.
Practical Application

Since both AC adapters supply the same voltage, it may seem as if there isn’t much difference between the two. However, the amount of current the adapter can supply should play a major factor in your decision as to which one to use. For example, a device that requires 12V AC and 6A will require an AC adapter that is rated for at least that much power (72W). Trying to use a 65W AC adapter will not provide enough power, because the AC adapter will not provide enough electrical current. In this case, the 90W power supply would be the appropriate choice to power the electrical device. Similarly, since the power rating is considered to be a “maximum allowable power” indication, you can use an AC adapter to power devices that are rated at or below their power rating. If a device requires 12V and 3A (36W), you can use either the 65W or 90W power supply.
Other Factors

An AC adapter with a higher power rating will charge batteries faster. Cost may also be a factor and AC adapters with higher power ratings typically cost more to buy. AC adapters with higher power ratings also radiate more heat than adapters with lower power ratings. If excess heat is a consideration, an adapter with a higher power rating may not be suitable for the application.
Adapter Choice

While an AC adapter with a higher power rating will allow for faster battery charging and increased electrical loads, an AC adapter with a lower power rating may suffice, especially where there is a concern about excess heat or higher costs.