Electricity is most commonly calculated in derived units of power known as watts. A watt describes the amount of electrical energy used at a specific moment in time. One watt equals one joule per second. Therefore, a 60-watt light bulb uses 60 watts of electricity, or 60 joules per second.
In electrical theory, another way to define a watt is to relate it to voltage and current. In this context, a watt is the product of voltage multiplied by current. Therefore, electrical energy can be calculated in practice by measuring the voltage and current at an electrical node and multiplying the two measurements together.