Calculator
The formula, and why it is the only one you need
Electricity is billed by the kilowatt-hour. One kWh is 1,000 watts running for one hour, or 100 watts running for ten hours. Everything else is unit conversion:
- Watts ÷ 1,000 = kilowatts. A 100 W bulb is 0.1 kW.
- Kilowatts × hours per day = kWh per day. 0.1 kW × 5 h = 0.5 kWh per day.
- kWh per day × days = kWh for the period. 0.5 × 30 = 15 kWh.
- kWh × your rate = usage cost. 15 kWh × $0.20 = $3.00.
The meter mode skips steps 1 and 2. A plug-in meter has already integrated the real, varying draw into a kWh figure, so you divide by the days it ran to get kWh per day and carry on from step 3. That is why a meter reading is more trustworthy than a nameplate for anything with a thermostat, compressor or standby mode.
Worked examples
These are the same examples as the preset buttons, calculated by the same code, version 1.1.0. Each has a stated basis. None of them is your appliance.
| Example | kWh / day | kWh / 30 days | Usage cost |
|---|---|---|---|
| 100 W bulb, 5 hours a day Round-number example from the calculation method, not a measured lamp. | 0.5 | 15 | $3.00 |
| 1,500 W space heater, 4 hours a day Nameplate rating of a typical plug-in heater running flat out. Real heaters cycle with their thermostat, so this is an upper bound. Calculation only: it is not a suggestion to put a meter or smart plug on a heater. | 6 | 180 | $36.00 |
| Meter read 2.4 kWh after 2 days Illustrative meter reading, projected to a 30-day bill. | 1.2 | 36 | $7.20 |
A 1,500 W heater at full output for four hours is the ceiling, not the typical draw; a thermostat will cycle it off. CPSC guidance is to plug space heaters directly into a wall outlet, so do not put a plug-in meter, smart plug or power strip between a heater and the wall. For a heater, estimate from the nameplate and the hours it is switched on, and treat the answer as an upper bound.
Finding your actual rate
The example rate is a placeholder. Your bill will show either a single price per kWh or several lines (supply, delivery, riders) that add up to one. The simplest honest number is your effective variable rate: add up every charge that scales with kWh and divide by the kWh on the bill. Leave out fixed customer or service charges; they do not change when you use less, which is why the calculator keeps them in a separate field.
If your tariff has tiers or time-of-use pricing, this calculator gives a flat-rate estimate. Use the rate for the tier or period the appliance mostly runs in, and treat the answer as approximate. A modeled tiered mode is on the roadmap; we would rather ship a flat estimate that says so than a tiered one that guesses.
Enter cents or dollars, whichever your bill uses; the unit selector converts once, and 20 cents gives exactly the same answer as $0.20.
Appliances that cycle on and off
A refrigerator's label might say 150 W, but it is not drawing 150 W around the clock: the compressor runs only part of the time and defrost heaters add short bursts. Multiplying the nameplate by 24 hours would overstate the cost. The same applies to dehumidifiers, window air conditioners, well pumps and anything with a thermostat.
For these, use the meter mode with a reading taken over at least a few full days. The exception is a space heater: it also cycles, but it belongs directly in a wall outlet, so estimate it from the nameplate and the hours it is on rather than metering it. The calculator will warn you if you enter a high wattage for 20 or more hours a day, or if you project a very short measurement across a long period. If you have not measured yet, the plug-in monitor guide explains which type of meter fits which appliance, and when borrowing one is the sensible option.
Do not add a separate standby figure to a measured kWh total. If the meter was on the whole time, standby is already in the number.
What this calculator cannot tell you
- Whether your bill went up because of this appliance. It gives a cost for one load at one rate. Compare it with the change in kWh on your bill, not the change in dollars; the high-bill diagnostic walks through that.
- Annual savings from replacing something. That needs a measured baseline, a realistic figure for the replacement, and the purchase cost. A payback module is planned; we will not fake one with a percentage.
- Anything about gas, oil or propane. This is electricity only. If your heat or hot water uses another fuel, the electric bill is the wrong place to look for those costs.
- Hardwired or 240 V appliances. You can still use the watts mode with the nameplate if the appliance runs steadily, but a plug-in meter cannot measure them.
Sources and evidence
What we are relying on
- Energy cost for a steady load = watts ÷ 1,000 × hours per day × days × your rate in dollars per kWh.
- Arithmetic method, not a measurement. Appliances that cycle on and off need measured kWh or actual runtime, not nameplate watts × 24 hours.
- CPSC guidance says to plug electric space heaters directly into a wall outlet and keep them clear of combustible materials.
- Verified from the manufacturer or official document.
What this page does not establish
- The example rate of $0.20 per kWh is illustrative and is not a national average or any utility's tariff.
- Results are flat-rate estimates; tiered, time-of-use and demand tariffs are not modeled.
- Nothing here is a measurement of a specific appliance model.
Sources
- Estimating appliance and home electronic energy use, Virginia Cooperative Extension. Calculation method; read 2026-09-07.
- Keep warm and safe this winter: tips for using generators, furnaces and space heaters, U.S. Consumer Product Safety Commission. Official guidance; read 2026-09-07.
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