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About Electricity Costs
What an appliance costs to run.
American homes are supplied across two 120-volt legs that combine for 240 volts, and breakers protect the wiring rather than the appliance. Household consumption is dominated by heating, cooling and water heating across most of the country.
American homes run split-phase 120/240 volt power at 60 Hz, and electricity is billed by the kilowatt-hour at a rate set by state regulators or the retail market, ranging from roughly 10 to 40 cents across the country. Wire sizing follows the National Electrical Code, and voltage drop over a long run is a real cost rather than a rounding detail.
Electricity is billed per kilowatt-hour, and the tariff often steps up once a monthly threshold is crossed, so the marginal rate exceeds the average. An appliance rated in watts draws that figure only while running at full output.
Watts times hours gives kilowatt-hours, and the bill then follows the tariff — which often steps up in price once a monthly threshold is crossed.
Voltage drop, breaker sizing and conductor ampacity are code questions before they are arithmetic ones, since the National Electrical Code fixes the limits a calculation must satisfy. Appliance ratings are usually given in watts, while billing runs in kilowatt-hours.
Electricity Cost Calculator sits with the rest of the library's everyday work, and it rests on the same household-electrical arithmetic the way a manufacturer's label or a timetable does. Its answer is only as good as the assumptions standing behind it.
The quantities it works from — power, hours used per day, price per kwh and days — are the ones a real household-electrical question actually turns on, and each carries a convention of its own, set out below.
Everyday arithmetic in the United States carries conventions that are easy to miss: dates run month before day, distances and weights follow customary units, and prices are quoted before tax. Those conventions make a surprising share of daily sums come out differently from the same question asked elsewhere.
The figures in this section follow American practice unless the subject itself is international.