About WattPenny: How the Calculator and Data Work

What WattPenny does

Most people have no idea what their appliances cost to run, and the tools that claim to tell them are either buried in a utility's website, tied to one brand's products, or so simplified that the answer is wrong. WattPenny is a single-purpose calculator built to be fast, accurate for the United States, and honest about what it does not know.

You give it an appliance's power draw (or the kWh figure from its label), how long it runs, and your electricity rate. It returns the cost per hour, day, week, month and year, and the energy in kWh behind each number. Everything runs in your browser; nothing you type is sent to us.

How the math works

The core formula is the one your utility uses: energy in kWh = watts × hours ÷ 1,000, and cost = kWh × price per kWh. Weekly cost multiplies the daily figure by the days per week you set. Monthly cost uses an average month of 52 ÷ 12 = 4.33 weeks (30.4 days); yearly cost uses 52 weeks.

In "I know the kWh" mode the calculator converts the annual, monthly or daily kWh figure you enter to a daily figure (dividing by 365 or 30.4) and proceeds the same way. This mode exists because appliances that cycle on and off, like refrigerators and air conditioners, cannot be estimated well from a wattage.

The EV calculator uses cost per full charge = battery kWh ÷ (1 − charging loss) × rate and cost per mile = rate ÷ (1 − loss) ÷ miles per kWh, with a default 12% loss that reflects typical Level 2 charging.

Displayed numbers are rounded so they read naturally: costs under a dollar are shown in whole cents, costs under $100 to the cent, larger amounts to the dollar, and very large amounts to the nearest $5. Energy under 10 kWh is shown to one decimal place and above that as whole kWh. The rounding is applied only at display time; every calculation uses the full-precision values.

Where the data comes from

• Electricity rates. The state and national averages are the U.S. Energy Information Administration's average residential retail price from Electric Power Monthly, Table 5.6.A. The current figures are for June 2026; the national average is 18.34¢ per kWh. We update them when a new monthly table is published, typically within two weeks of release, and note the data month wherever a rate appears.
• Appliance wattages. The 83 presets are typical operating figures for mid-range models currently sold in the US, drawn from manufacturer specification sheets, the ENERGY STAR product database, U.S. Department of Energy appliance guidance and plug-in meter measurements. For appliances that cycle, the preset is an average over the stated hours and is labeled as such in the calculator and the wattage chart.
• Usage patterns. Default hours and days per week are chosen to represent common household use, not a worst case. They are starting points, and the calculator expects you to change them.
• Efficiency figures in the guides (thermostat setback savings, SEER comparisons, heat pump ratios) come from the U.S. Department of Energy and ENERGY STAR unless another source is named in the text.

Editorial standards

Every guide on this site is written and reviewed by the WattPenny editorial team, an independent publisher with a software and engineering background. We follow a short set of rules:

• Every number is sourced or computed. Rates come from the EIA. Costs in the guides are computed from the same presets and rates the calculator uses, so a figure in a guide and the figure in the tool never disagree.
• We say when something is typical rather than exact, and we tell you how to get the exact figure for your own appliance.
• We do not recommend products. When a guide mentions a category of device, such as a plug-in power meter or a heat pump water heater, it is because the category matters, not because of any brand. There are no affiliate links on this site.
• Advertising is separate from content. Ads are served by Google and clearly labeled. Advertisers have no input into what we publish.
• Corrections are welcome and dated. Each page shows when it was last updated. If you find an error, tell us via the contact page and we will fix it and note the change.

What the calculator cannot do

It estimates the energy charge for one appliance. It does not know your utility's fixed monthly fees, taxes, tiered thresholds, time-of-use schedule or demand charges, all of which affect the total bill. It cannot tell how efficiently your specific 12-year-old air conditioner is running, or how often your fridge door opens. And it assumes US conventions (120 V outlets, dollars, EIA state averages), although you can enter any rate in any currency's cents and the arithmetic still holds.

For anything where the money matters, measure the appliance and use the marginal rate from your bill. The guides explain how.

Who runs it

WattPenny is built and maintained by an independent software engineer as a small, focused utility, without a marketing team, a newsletter or an app to sell you. It is free to use and is supported by unobtrusive display advertising. You can reach us at hello@wattpenny.com or via the contact page.