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Kilowatts and Kilowatt-Hours


The electricity bill is an account of spent kilowatt-hours, and a kilowatt-hour is a unit for measuring electrical energy. It is equivalent to the energy consumed by a device the size of one kilowatt in one hour. Kilowatt is the speed at which it consumes energy.

Introduction


The unit kW means kilowatt and kWh means kilowatt-hours. Kilowatt-hours (kWh) is a unit of energy, and a consumer pays the electricity bill based on the kWh consumption (Fig. 1). It can be difficult to distinguish kW from kWh, but first of all,

1 kW = 1 000 W

much like 1 kilometer = 1 000 meters. However, watts are more abstract than meters.

Electricity Meter
Fig. 1 Old electricity meter. The thin metal disc in the window rotates in the arrow's direction. Number of rotations indicates energy consumption. Modern meters blink and release electrical pulses which are monitored by computer.


Energy (kWh)


An electricity meter measures kilowatt hours in units of kWh. That is, a quantity of kilowatts is multiplied by a number of hours. Households report their consumption to the electricity company, the company reads it off the meter, or the company measures it automatically by means of a smart meter.

On average, a Dane uses app. 1 400 kWh electricity per year (excluding heating). If the price is app. two crowns (DKK) per kWh including VAT, this sums to annual costs of app. 2 800 DKK a year.

The more appliances that are switched on, the faster the meter counts. The longer an appliance is on, the more the meter turns. If no appliances are turned on, the meter stays still. Electrical consumption depends on time in use, which is why kilowatt hours (kWh) is the measure of kilowatt (kW) multiplied by the number of hours (h). Smaller or larger unit measures can be applied according to preference (Table 1).

Power (kW)


The size of an electrical engine is measured in kilowatt, written as kW. This is the power used by the engine, as well as its consumption of electricity. Power is energy per unit of time. If an engine is constantly running, and uses a number of kilowatt hours (kWh) and we divide this by the time in use (h), we get the power (kW). A wide variety of appliances and engines exist (Table 2). Both small and large units of measurement are useful here, and reduce the need for decimals.


Example. Vaccuum cleaner.

A vacuum cleaner with a power of 1.8 kW (or 1 800 W) which runs for half an hour will use

energy = power * time = 1.8 kW * 0.5 h = 0.9 kWh


The meters' wheel in the image in Fig.1 will have moved 0.9 units.



Other examples

  • One kWh corresponds to an electrical heater of 1 000 W heating for an hour. If the heater is on for two hours, total energy used is 2 kWh.
  • One liter of heating oil corresponds to 10 kWh energy. If the oil-based furnace is 10 kW, it can burn for one hour using one liter of oil.
  • A human will typically consume 2000 calories in one day, equivalent to 0.0023 kWh. This is equivalent to a 100 watt light bulb shining for 24 hours.


Analogy


The electricity meter is similar to the speedometer in a car in Fig. 2.

  • Power corresponds to the speed dial
  • Energy corresponds to the distance counter


Energy is power accumulated over time, as distance is speed accumulated over time.

img/wiki_up/1024px-VW_Passat_Tacho_cut.png
Fig. 2. Speedometer from a car (photo: Wikimedia commons).


Table 1. Units of energy

Name Symbol Magnitude
watt-hour Wh 1
kilowatt-hour kWh 1 000 Wh
megawatt-hour MWh 1 000 kWh
gigawatt-hour GWh 1 000 MWh


Table 2. Sources of power

Quantity Example
1 W an LED light bulb
10 W an energy saving light bulb
100 W an incandescent light bulb
1 kW an electrical heater, small vacuum cleaner, solar panel
10 kW a large heating pump, a stove
100 kW a car engine
1 MW one of Samsø's 11 land-based windmills
10 MW four ocean-based windmills
100 MW engine on the container ship, Emma Mærsk
1 GW Asnæsværket (Danish Power Plant; district heating unit excluded)


Created by jj. Last Modification: Saturday 23 November 2013 12:49:38 CET by jj.