The physical relationship
Energy equals rated watts × average load ÷ 1,000 × running hours. Monthly and annual energy use use the chosen number of days; cost is energy multiplied by the entered tariff. The average-load field matters because a refrigerator, thermostat-controlled heater, and cycling motor are not continuously at their nameplate power.
A checked case
A 1,500 W heater running at an estimated 60% average output for 3.5 hours on each of 30 days uses 1,500 × 0.60 ÷ 1,000 × 3.5 × 30 = 94.5 kWh. At 0.30 per kWh, that period costs 28.35. Repeating the same daily pattern for 365 days produces about 1,149.75 kWh and 344.93 at that tariff.
Useful planning use
It helps compare a schedule before changing a habit, turn a plug-meter reading into a longer period, or separate a device's likely contribution from an entire household bill.
Failure modes in this measurement
Nameplate watts are often a maximum, not a measured average. Minutes must be converted to hours: 30 minutes is 0.5 hour, not 30 hours. Standby power, tiered tariffs, fixed charges, demand charges, and seasonal operating changes are outside a single-rate calculation. Entering a cost per month in the tariff box instead of a cost per kWh also produces a plausible but meaningless answer.
What makes this quantity different
The air-conditioner page estimates cooling capacity that a room may require. This page begins with an electrical input already known from a label or meter and estimates energy and cost. It does not predict whether a room will be comfortable or prescribe an electricity contract.
Field check before acting
For a cycling appliance, use a plug-in meter over several ordinary days, divide measured kWh by the observation hours, and use that average load in the form. Compare the result with a bill only after removing fixed fees and other loads. A reading during defrost, a heat wave, or an unusual workload is evidence about that period, not a permanent efficiency rating.
Limits of the model
The cost output is an arithmetic estimate using the tariff supplied by the visitor. It does not include taxes, export credits, or contract conditions, and it is not a utility bill.