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Electricity consumption

An appliance’s power and usage time in kilowatt-hours and cost: per day, over the period, and as a yearly projection.

Parameters

24 for an always-on appliance (fridge, router, standby).
30 for a month, 365 for a year.
On your bill or contract.

Result

Cost over the period

6

Calculation
E = P × t = 100 W × 10 h / 1000 = 1 kWh per day
Energy over the period
30 kWh
Energy per day
1 kWh
Yearly projection
365 kWh
Cost per year
73
Note

The appliance uses 30 kWh over the period, that is 6; at this pace, 73 per year.

Calculation, assumptions and sources

OverviewHow much an appliance uses and what it costs: E = P × t, the power in watts times the hours of use, in kilowatt-hours then in money at the contract’s price per kWh.

How much an appliance uses and what it costs: E = P × t, the power in watts times the hours of use, in kilowatt-hours then in money at the contract’s price per kWh. Per day, over the chosen period and as a yearly projection; beyond 24 hours per day, the calculation is refused.

Method & formulasE (kWh) = P (W) × t (h) / 1000
E (kWh) = P (W) × t (h) / 1000cost = E × price per kWhyearly projection = E per day × 365

One hundred watts for ten hours make one kilowatt-hour. The cost is this energy multiplied by the price per kWh.

Kilowatt-hour (kWh)
the energy unit of the bill, one kilowatt for one hour, that is 3.6 megajoules.
Nameplate power
the maximum the appliance can draw, written on its nameplate. An appliance that cycles or modulates uses less on average.
Standby
the small permanent power of an “off” appliance, over the 8760 hours of the year: 5 W around the clock make 44 kWh a year.
Subscribed power
the “kVA” of the subscription, a ceiling on instantaneous power; it does not measure consumption.
Validity domainThe calculation assumes a constant power during the declared hours: exact for a resistor or a lamp, optimistic for anything that cycles or modulates.

The calculation assumes a constant power during the declared hours: exact for a resistor or a lamp, optimistic for anything that cycles or modulates. No tariff is preset; prices vary by country, by contract and, with dynamic pricing, by hour. Off-peak hours are handled as two calculations, one per tariff.

Reading the resultFor appliances that cycle (refrigeration, thermostated heating, oven), the nameplate overestimates consumption; the energy label gives kWh/year measured in standard conditions, a plug-in power meter the real consumption.

For appliances that cycle (refrigeration, thermostated heating, oven), the nameplate overestimates consumption; the energy label gives kWh/year measured in standard conditions, a plug-in power meter the real consumption. For constant-power appliances (kettle, hair dryer, lighting), the nameplate and this calculation agree.