How the numbers are made

The sum

Power in watts divided by 1,000 gives kilowatts. Kilowatts times hours gives kilowatt hours (kWh), the unit your meter counts. Kilowatt hours times your unit rate gives money. A 1,500 watt heater for 8 hours is 12 kWh. At 18.16 cents per kWh that is 2.18 dollars.

Three kinds of appliance need three versions of the sum, and each page says which it uses.

• Rated. Watts times hours times a duty share. Heaters, air conditioners, dehumidifiers and fridges cycle on a thermostat or a sensor, so they draw their rated power only part of the time they are switched on. The duty share is that fraction. Fans, pumps and lights run at their rating all the time, so their duty is 100%.
• Per use. Energy for one load, one boil or one shower, times uses a day. A dishwasher is rated 1,800 watts but what matters is the 1.2 kWh a cycle uses.
• Per day. Energy a day for things that are too lumpy to model by the hour, such as a hot tub whose heater runs whenever the water cools.

A month is 30.44 days and a year 365.25 days, so the monthly and yearly figures are the daily figure multiplied by those.

Where the electricity price comes from

United States. The average residential price in each state from the U.S. Energy Information Administration's Electric Power Monthly, Table 5.6.A, built from Form EIA-861M. The site uses the August 2026 table, released 23 October 2026, in which the US average is 18.16 cents per kWh. The 2026 values are preliminary estimates and the EIA revises them. The next table is due 24 November 2026. The figure is total residential revenue divided by total residential sales, so it blends every utility, tariff, delivery charge and fee in the state.

United Kingdom. The Ofgem price cap unit rate for 1 October to 31 December 2026, average for England, Scotland and Wales, paying by Direct Debit: 26.32p per kWh. Ofgem resets the cap every three months and publishes regional rates, which differ by a few pence. The cap for 1 January to 31 March 2027 is announced in late November 2026.

Both prices live in one data file each, which records the period and the release. Every page reads from that file, so there is no page with a stale rate while another has the new one.

Where the wattages come from

Typical ratings and ranges are taken from the extension publications and product data listed below, and from manufacturer specifications for current models. They are ranges, not measurements of your appliance. For fridges and freezers the site uses the average draw over the day (the number the energy label implies), not the nameplate, because the compressor runs about a third of the time. For dishwashers, washers and dryers it uses energy per cycle, because the rated power only tells you how fast the water heats.

Every calculator takes your own wattage from the label and your own rate from the bill. The published figures are the starting point, not the answer.

What the estimates cannot know

• How often your thermostat lets the element rest. Insulation, room size, outdoor temperature and the set point change the duty share, which is why it is a control, not a constant.
• Your tariff. Fixed deals, time-of-use rates, prepayment rates and the tiers some US utilities use all differ from the state or cap average.
• Standing charges and fixed monthly fees. They are paid whether the appliance runs or not, so they are left out of running costs and shown separately on the rate pages.
• The condition of the appliance. A clogged filter, a worn door seal or a frosted freezer raises run time.

Checks before every release

The engine that does the sums is shared between the build and the browser, so the number printed into the page and the number the calculator shows after you type agree to the cent. A test suite runs on every build: it checks the formulas against hand-worked examples, that all 51 state rows are present, that every appliance has its text and sources, and that no page carries an unrendered value or a machine character. The publisher is Apex Digital Solutions, and the data review date on each page is real: it is the day a person last compared the data files with the sources.

How often things change

• US prices: monthly, when the EIA publishes (usually the fourth week of the month, two months in arrears).
• UK price: quarterly, on the day Ofgem announces the next cap, and again on the day it starts.
• Appliance figures: reviewed when a page is edited, and at least yearly.
• Appliances covered: 29 today. New ones are added when there is a real question to answer, not to fill a list.