Cost and CO₂ for an appliance from power, hours, days, and tariff.
kWh = power(W) × hours / 1000. Cost = kWh × price. Emissions vary 10× by country grid; check ADEME (FR), Ember, or EPA eGRID for local factors.
Every electricity bill answers a single arithmetic question — kilowatt-hours consumed times euros per kilowatt-hour — but the consumer can rarely break the total down to the appliance level. Is the new heat pump really cheaper to run than the old radiator? Does leaving the desktop on overnight cost €5 a month or €50? Does a heated towel rail justify its bill? The math is dead simple — power in watts, hours per day, days per period, tariff in €/kWh — but doing it on the back of a receipt is awkward, and the unit conversion (W to kW, scaling to a yearly figure) is exactly the kind of thing people get wrong. This calculator turns the four inputs into the cost over a chosen period, the projected yearly cost, the daily energy use, and the corresponding CO₂ emissions in kilograms — the same set of figures that a smart-meter dashboard would show, but available before purchase rather than after a year of use.
Energy used (kWh) = power (W) × hours / 1000. Energy per period = energy_per_day × days. Yearly energy = energy_per_day × 365. Cost = energy × price (€/kWh). CO₂ = energy × CO2_factor (g/kWh) / 1000 in kilograms. The CO₂ factor depends entirely on the country's electricity grid mix: France with its nuclear-dominant grid sits near 60 g/kWh (one of the lowest in Europe); Spain at about 150; Germany at 380 (still partly coal/gas); the United States average at 380; Poland over 700 (heavy coal); Norway and Iceland under 30 (hydro and geothermal). Off-grid solar is below 50 once amortised over panel lifetime. The calculator exposes the CO₂ factor as a free input so users can plug in their utility's published number — France's RTE, the U.S. EPA's eGRID database, the German Umweltbundesamt all publish region-specific factors annually. Tariff likewise varies wildly: French regulated rate (Tarif Bleu, 2024) is €0.21–€0.27/kWh peak, €0.16/kWh off-peak; German household rate is €0.32–€0.42; U.S. residential averages $0.16/kWh but ranges from $0.10 in Washington state to $0.40 in Hawaii.
Five inputs: power in watts (the appliance's nameplate rating, found on the back), hours of use per day, the period in days for the cost calculation (default 30 for a monthly bill), the electricity price per kWh, and the CO₂ factor per kWh. Defaults — 1 500 W, 3 hours/day, 30 days, €0.21/kWh, 60 g/kWh — represent a typical electric heater used as an auxiliary in a French apartment for a winter month. The result panel shows the cost over the chosen period as the headline, the yearly cost projected from the daily use, the daily energy in kWh, and the CO₂ emitted over the period and over a full year. Plug in different appliance scenarios: a 1 500 W kettle for 0.1 hours/day vs an 80 W laptop for 8 hours/day — the kettle wins on power but loses on duration; the laptop's daily energy is roughly the same, despite the 20× power difference.
A 1 500 W oil-filled radiator used 3 hours per day for a 30-day winter month, French Tarif Bleu at €0.21/kWh, 60 g CO₂/kWh: daily energy = 1 500 × 3 / 1 000 = 4.5 kWh. Monthly = 135 kWh. Yearly = 1 642 kWh. Monthly cost = 135 × 0.21 = €28.35. Yearly cost (if used at this rate for 365 days, which it isn't — heating is seasonal) = €344.92. Daily CO₂ = 0.27 kg. Monthly CO₂ = 8.1 kg. Yearly CO₂ = 98.5 kg. Compare to a heat pump at 800 W effective consumption (4× COP, drawing 200 W from the wall but delivering 800 W of heat) for the same 3 hours/day: daily energy = 0.6 kWh, monthly cost = €3.78 — an 87 % cost reduction at the same heat output. Or a 9 W LED bulb for 5 hours/day: daily energy = 0.045 kWh, monthly = €0.28 — the entire month of LED lighting costs less than one hour of the radiator. The calculator makes such comparisons trivially repeatable.
First, confusing the appliance's nameplate watts with its actual draw. A heat pump's nameplate may read 5 000 W (peak draw at startup) but average 800 W in steady-state. A computer power supply rated 750 W actually consumes 50–150 W in normal use; the 750 W is the maximum capacity, not the operating point. The right input is the typical operating wattage, often listed in the energy efficiency label or measurable with a plug-in meter. Second, mixing instantaneous and continuous power. A water heater with a 2 400 W heating element doesn't draw 2 400 W continuously — it cycles on and off to maintain temperature, with a duty cycle of 10–30 % over a day. The right input is the time-averaged watts (which the energy label gives as kWh/year, divisible by 8 760 h/year for the average watts). Third, scaling a winter or summer figure to a full year. Heating is October–April; cooling is June–September; using daily data from one season to project annual cost overestimates by 2 to 3×. Fourth, ignoring standby consumption. A device on standby draws 1–10 W; a household of 30 such devices wastes 50–300 kWh/year, or €10–€60. The calculator handles this when the user enters the standby wattage and 24 h/day. Fifth, applying the average grid CO₂ factor to nighttime consumption. Many grids are coal-heavy at night and renewable-heavy during the day; the marginal factor for a load shifted to off-peak can be higher than the time-averaged grid mix, making nighttime EV charging less green than expected on coal-heavy grids.
Electricity tariffs come in many shapes. Time-of-use (Tarif Heures Pleines/Heures Creuses in France, similar variants worldwide) charges different rates during peak and off-peak hours, typically with an 8-hour off-peak window at night. The calculator handles a single average tariff; for time-of-use, run it twice and weight by hours. Tempo (also French) has three colours (red/white/blue) at very different rates depending on grid stress, designed to incentivise demand response. Variable-rate plans track wholesale market prices hour by hour; some German and Spanish providers expose this directly. Solar self-consumption: if the appliance runs during the day and the household has rooftop solar, the marginal cost of operation is 0 (after panel amortisation) and the marginal CO₂ is near 0. Phantom load (also called vampire load): the cumulative standby of all the always-on devices in a typical household — typically 5–15 % of total bill. Per-appliance comparison context: for a back-of-envelope rule, multiply nameplate-watts by hours-of-use to get watt-hours, divide by 1 000 to get kWh, multiply by 0.20 to get euros — the calculator surfaces the same arithmetic with editable defaults and CO₂ context.