Calculation method
How Our Calculators Work
This page explains the formulas, display rounding, illustrative 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. |
| Average 30-day share | annual kWh x 30 / days in planning year; 30-day cost = that kWh x rate per kWh | Spreads the selected annual use schedule across an average 30-day period; it is not a measured month or 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.
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| 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. |
| Average 30-day period | The appliance calculator normally takes 30/365 of the selected annual schedule. It uses 30/366 only when 366 use days are entered to model use on every day of a leap year. | A seasonal or occasional schedule uses the site's standard 365-day planning convention rather than implying 30 consecutive use days. |
| 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. |
| Rounding ties | Calculations retain Decimal precision until display. Values at or above the display floor use ROUND_HALF_UP; a smaller positive value uses a <precision marker instead. | For example, an exact cost of 1.005 displays as 1.01, while 0.005 displays as <0.01 rather than zero; an earlier displayed input is not substituted back into the calculation. |
| Displayed energy | kWh results display to 3 decimals. The watts converter also shows Wh with up to 2 decimals. | A positive value below 0.001 kWh is shown as <0.001 kWh; a positive value below 0.01 Wh is shown as <0.01 Wh. Neither is reported as zero. |
| Displayed rate | The selected rate keeps its non-zero decimal places rather than following the cost display's rounding. | A result keeps its visible rate assumption, including a rate such as 0.153 per kWh in the currency used by the source. |
| Currency | The site does not infer or convert currency. | Read a currency-amount result in the same currency as the entered rate, or read a cents-or-pence result in cents or 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. |
| Illustrative wattage or 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.
Illustrative wattage, runtime, kWh, and rate values on guide pages show how the calculation works. They are not appliance rating claims or utility billing values.
How third-party dataset examples are related
Five guides contain case analyses selected from two published source series. The home-office, space-heater, and dishwasher measurement examples use different channels or intervals from the REFIT cleaned dataset. The refrigerator and air-conditioner examples use different channels or intervals from Plegma House 1. These are five page-level analyses, not five independent data collections and not measurements made by this site.
Reusing a source series is disclosed because each page asks a different boundary question: a multi-device desk load, a fixed nighttime heater window, a dishwasher activity window, a complete refrigerator day, or a bounded cooling interval. A source interval is not treated as typical household use, a product benchmark, or proof of future performance. Quiet readings around the dishwasher window help locate the selected activity; they do not establish that it contains a complete wash program.
What a zero Issues value means
In the Plegma documentation, the Issues field marks rows where monitored-appliance power sums
above the household aggregate. A zero value only says that this particular flag was not raised; it is not a
general data-quality score. For the
REFIT cleaned examples, the published Issues column is likewise used as one screening check.
In neither source does zero mean that every reading is original, every appliance is monitored, the device
assignment is certain, or the chosen time window is representative.
Method page versus measurement guide
The rules above explain how the site handles a chosen input. To choose a measurement period or check whether a one-day reading covers the appliance's operating pattern, use the measurement guide.
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
A reproducible issue report includes the page URL, the inputs used, the result or wording that looked wrong, and what you expected to happen. Include a public source URL or a redacted transcription of the relevant appliance label, meter reading, or bill. The contact form accepts text, not file attachments; omit account numbers and other personal details.
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. Whether an ad is present does not enter a calculator formula or change the arithmetic produced from the submitted inputs.