Calculation method
How Our Calculators Work
This page explains the formulas, display rounding, sample assumptions, and limits used by the electricity-cost calculators and guides on Practical Calculator Guide.
Formula map
The calculators use simple arithmetic so visitors can trace a result back to the inputs they entered. These formulas are for usage-cost estimates, not full utility bill totals.
Read this table as a map of calculation boundaries. The formula matters less than choosing the row that matches the input you actually have: watts and hours, known kWh, one active hour, or a repeated daily pattern.
Scroll horizontally to see all columns.
| Use case | Formula | How to read it |
|---|---|---|
| Convert watts to kWh | kWh = watts x hours x quantity / 1000 | Best fit: a device label gives watts, and the runtime is a deliberate input. |
| Appliance daily cost | daily kWh = watts x hours per day x quantity / 1000; daily cost = daily kWh x rate per kWh | Best fit: the same daily pattern is clear enough to repeat. |
| 30-day appliance estimate | 30-day kWh = daily kWh x 30; 30-day cost = daily cost x 30 | Planning shortcut for a repeated 30-day pattern, not a billing-period claim. |
| Annual appliance estimate | annual kWh = daily kWh x selected days used; annual cost = daily cost x selected days used | Use the days that match the habit you are estimating; 365 is the default for daily use. |
| One-hour running cost | hourly kWh = watts x quantity / 1000; hourly cost = hourly kWh x rate per kWh | Best fit: a one-hour comparison before any longer-use scaling. |
| Known kWh into cost | usage cost = kWh x rate per kWh | Best fit: kWh already comes from a bill, meter, label, or earlier calculation. |
Assumption and rounding map
The site uses stable display rules so calculator results are readable. Manual arithmetic may differ by a small amount when rounded values are copied into another calculation.
Scroll horizontally to see all columns.
| Assumption | How the site uses it | Reader-facing meaning |
|---|---|---|
| 1 kW = 1000 W | Watts are divided by 1000 before kWh or cost is calculated. | Enter watts as shown on the appliance label or chosen estimate. |
| 30 days | The appliance calculator uses 30 days for its 30-day usage estimate. | This is a planning shortcut, not a claim about your billing period. |
| Days used per year | The appliance calculator starts at 365 days, but lets you enter a lower count for seasonal or occasional use. | The annual result follows the days you entered rather than assuming every appliance is used year-round. |
| Displayed cost | Most costs display to 2 decimals; small hourly costs may display to 4 decimals. | A positive cost below the visible precision is shown as less than that precision, rather than as zero. |
| Displayed energy | Energy results display to 3 decimals. | A positive value below 0.001 kWh is shown as <0.001 kWh rather than 0.000 kWh. |
| Displayed rate | The selected rate is shown at the precision entered rather than rounded to the cost display. | A result keeps its visible rate assumption, including values such as 0.153 currency units per kWh. |
| Currency | The site does not infer or convert currency. | Read cost results in the same selected unit as the rate you entered: currency units or cents/pence. |
Input-quality hierarchy
A cleaner input usually gives a cleaner estimate. Use the strongest input that fits the question you are trying to answer.
The table is not a ranking of pages to visit. It is a reminder to avoid using a weak input when a cleaner one is already available. For example, measured kWh for a matching period usually beats a label-wattage guess.
Scroll horizontally to see all columns.
| Input source | Best use | Watch out for |
|---|---|---|
| Measured kWh from a plug-in meter or suitable device monitor | One appliance or one measured setup over a chosen period. | The measured period should match the period you want to estimate. |
| Annual kWh from an appliance label or documentation | Appliances where annual energy use is provided, such as refrigerators. | Convert the annual period before reading it as monthly or daily use. |
| kWh from an electric bill | Whole-home usage or a billing-period input. | Bill kWh usually includes more than one appliance. |
| Appliance wattage label or manual | A fallback when measured kWh or label kWh is unavailable. | Nameplate watts may not equal average draw for cycling or variable-load devices. |
| Example wattage or sample rate from a guide | Calculation practice and comparison. | Replace samples with your own details before using the result for a real decision. |
Measured values versus estimates
A measured kWh value can be more useful than a wattage estimate when a device cycles on and off or changes draw during use. The measurement still has a boundary: it describes the device, time period, and conditions you measured. If those conditions change, the cost estimate can change too.
Sample wattage, runtime, kWh, and rate values on guide pages show how the calculation works. They are not appliance rating claims or utility billing values.
Method page versus measurement guide
This methodology page explains how the site treats inputs after they are chosen. It is not the step-by-step workflow for using a smart plug or plug-in monitor. Use this page to check the hierarchy of inputs, rounding, and estimate boundaries; use the measurement guide when the practical question is how long to measure and how to label the measured period.
Usage cost versus a full utility bill
Most calculator results are usage-cost estimates: kWh multiplied by a selected rate per kWh. A full utility bill can include fixed fees, taxes, delivery charges, minimum charges, credits, account adjustments, other household usage, and rate-plan details outside the calculator input.
If a bill is higher than a usage-cost estimate, use the bill-input guide when the number is unclear, or the bill-difference guide when the appliance estimate and full bill do not line up. Do that before changing appliance inputs.
Corrections
If you report a likely issue, include the page URL, the inputs used, the result or wording that looked wrong, and what you expected to happen. Reports are compared with the formula, assumptions, labels, links, and page purpose before a page is corrected or clarified.
Use the Contact page for correction reports, broken pages, accessibility concerns, or site-related questions.
Advertising and editorial separation
The site may earn revenue from advertising. Advertising helps keep the calculators publicly accessible without account creation, but ad placement does not determine calculator formulas, sample assumptions, or correction handling.