Electricity Calculator

Every number this calculator produces comes from three chained relationships. First, current is derived from power and voltage as I = P / V, the rearranged form of the power law P = V x I, so a 60 W lamp on a 120 V circuit draws 0.5 A. Second, energy is power multiplied by time: a watt is one joule per second, so running 60 W for 5 hours accumulates 300 watt-hours, or 0.3 kWh. One kilowatt-hour equals exactly 3.6 megajoules. Third, utilities bill energy rather than power, so the money figure is simply kWh multiplied by your per-kWh tariff. Two cautions matter when you apply the result. Nameplate wattage is a maximum, not an average: refrigerators, air conditioners and washing machines cycle on and off, so multiply nameplate watts by an honest duty cycle before trusting a daily cost. And on alternating-current circuits the arithmetic P = V x I holds strictly only for resistive loads such as incandescent lamps, kettles and heaters. Motors, LED drivers and switch-mode power supplies draw current partly out of phase with voltage, so real power is V x I x power factor and the true current can exceed the amp figure shown here. That amperage output is still useful for circuit planning, since a 15 A branch circuit should carry no more than about 12 A of continuous load.

Charging for electricity by the kilowatt-hour was not obvious at first. When Thomas Edison opened the Pearl Street Station in Manhattan in September 1882 he had no practical meter, and early customers were billed a flat rate based on the number of lamps installed. Edison's chemical meter, which weighed zinc deposited in an electrolytic cell, was accurate but slow and awkward to read. The breakthrough came from induction metering: Galileo Ferraris described the rotating magnetic field in 1885, Oliver Shallenberger of Westinghouse patented an induction meter in 1888, and Otto Blathy of the Ganz works in Budapest produced an alternating-current watt-hour meter in the same period. The spinning aluminium disc that resulted stayed in service for more than a century. The pricing logic came from Samuel Insull, who took over Chicago Edison in 1892. Borrowing load-factor ideas from the British engineer Arthur Wright of Brighton, Insull separated the standing cost of generating capacity from the running cost of energy, then chased industrial and streetcar customers to fill idle daytime capacity. That two-part structure, a fixed charge plus a per-kilowatt-hour rate, is still the skeleton of the bill this calculator estimates. Smart meters have layered time-of-use windows and tiered blocks on top of it, but the billing unit itself has not changed.