What quantity consists of a unit of 1kWh

Kilowatt hour

Lexicon> letter K> kilowatt hour

Acronym: kWh

Definition: the amount of energy that is converted within one hour at an output of 1 kW

English: kilowatt hour

Categories: units, electrical energy, heating and cooling

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 03/06/2010; last change: 04.06.2020

URL: https://www.energie-lexikon.info/kilowattstunde.html

A Kilowatt hour (kWh) is the amount of energy that is converted within one hour with an (active) power of one kilowatt (1 kW). So it's one kilowattmultiplied with an hour, not a “kilowatt per hour”. (See the section on common mistakes below.) One kilowatt hour equals 3,600,000 joules = 3.6 MJ. The “h” in “kWh” comes from English hour = Hour.

Accordingly, there are megawatt hours (1 MWh = 1000 kWh), gigawatt hours (1 GWh = 1000 MWh = 1 million kWh) and terawatt hours (1 TWh = 109 kWh = 1 billion kWh). On the other hand, there are watt hours (Wh) and watt seconds (Ws = J) for small amounts of energy.

The kilowatt hour is used in particular (but not only) for the quantification of electrical energy quantities (electrical work) and therefore appears on electricity bills based on the reading of electricity meters. Private consumers in Germany pay around 0.30 € per kilowatt hour, often significantly less in the night tariff. In contrast to large-scale consumers, these costs stem only to a small extent from electricity generation. (See also the article on electricity tariffs.)

You can also specify other amounts of energy (e.g. amounts of heat) in kilowatt hours; For example, the primary energy demand of houses is usually expressed in kWh per square meter of heated area and year (kWh / (m2 a)) specified. Often the purchase of natural gas is also billed in kilowatt hours with reference to the calorific value, i.e. the amount of heat that can be extracted from the gas. For the customer, the amount of energy drawn is more relevant than the volume of the gas, especially since the specific calorific value can vary significantly depending on the gas quality (L or H gas). One kilowatt hour of heat from natural gas is far cheaper than one kilowatt hour of electrical energy. For example, private customers in Germany pay z. Currently around € 0.06 (as of 2020, without base price), i.e. around five times less than for electricity.

Sample calculations

Some examples of calculating with kilowatt hours:

  • An energy-saving lamp with a power consumption of 20 W can be operated with 1 kWh for a period of 1000 Wh / 20 W = 50 h (50 hours). A fan heater with 2000 W heating capacity consumes one kilowatt hour within half an hour. An energy-efficient refrigerator can thus be used, for. B. operated for three to four days.
  • A two-person household in a single-family house needs an annual average of around 10 kWh of electrical energy per day, i.e. 3650 kWh per year if it covers its heating requirements with gas or oil heating, i.e. not electrically. The average drawn power is then 10 kWh / 24 h = 417 W. (If you are currently cooking electrically, the current consumption can be several kW, so it can be much higher.) The same household in an apartment building should need considerably less because the consumption increases for the heating system, outdoor lighting, etc. is distributed over several parties or billed separately anyway.
  • A well-insulated hot water storage tank loses 50 W of heat to its surroundings when it is warm. This corresponds to 50 W · 24 h = 1200 Wh = 1.2 kWh of heat per day. If the storage tank is kept warm via an electric immersion heater, this corresponds to the same amount of electrical energy (in addition to the expenditure for hot water consumption). With a heat pump, the consumption of electrical energy would be reduced by the coefficient of performance. B. three or four times smaller.
  • Heating 1000 liters of water (specific heat capacity 4.19 kJ / (kg K)) by 40 Kelvin requires a heat quantity of 4.19 kJ / (kg K) 1000 kg 40 K = 168 MJ = 46.6 kWh. For comparison: to raise this amount of water on earth by one meter, you only need 9.81 m / s2 1000 kg 1 m = 9.81 kJ = 0.00273 kWh of mechanical energy.
  • If a medium-sized car needs a drive power of 10 kW to drive at 100 km / h, this corresponds to mechanical energy of 10 kWh per 100 km. With an efficiency of the engine of 20% you need 50 kWh in the form of gasoline, which corresponds to approx. 5.5 l.
  • A large power plant with an output of 1 GW generates 1 GW · 24 h = 24 GWh = 24 million kWh per day. With uninterrupted operation, 8760 GWh = 8.76 TWh = 8.76 billion kilowatt hours would be generated per year. More realistic are e.g. B. 7500 full load hours per year, which results in a production of 7500 GWh.
  • Germany's gross electricity consumption is around 600 TWh per year. This corresponds to an average output of 600 TWh / 8760 h = 68.5 GW.
  • A standard AA battery with 1.5 V and a capacity of 2.8 Ah stores an amount of energy of 1.5 V · 2.8 Ah = 4.2 Wh = 0.0042 kWh. If you buy these in bulk for 20 ct per battery, the energy price is around 48 € / kWh, i.e. over 100 times more than electricity from the grid. With brand name batteries or button cells, the kilowatt hour can be much more expensive. For this reason alone, battery-operated devices usually have to make do with very small amounts of energy.

Common mistakes

One kilowatt hour is Not one kilowatt per hour, but a kilowatt that is delivered or consumed for an hour!

Since the concept of the kilowatt hour is not widely understood, certain errors, for example in newspaper articles, are common. One of them is to understand kilowatt hours as “kilowatts per hour”: “The furnace needs 2 kilowatts per hour.” It would be correct that it needs 2 kilowatts, i.e. 2 kilojoules per second or 2 kilowatt hours per hour. The kilowatt hours are obtained by multiplying power (in kilowatts) and times (in hours), not dividing them: In five hours (5 h) the stove needs 2 kW × 5 h = 10 kWh and not about 2 kW / 5 h = 0.4 kW / h. Of course, longer operating times must lead to larger amounts of energy.

It also happens that, for example, the capacity of a battery storage system is specified in kilowatts. In reality, what is probably meant is the amount of energy that can be stored in kilowatt hours. However, it could also mean the maximum power that can be drawn (actually in kilowatts) or the maximum charging power.

CO2-Emissions per kilowatt hour

In addition to the costs, it is often also of interest how much2-Emissions are associated with the provision of one kilowatt hour. For example, an electricity provider should be able to express this value in units of g / kWh (grams of CO2 per kilowatt hour) (also on the electricity bill). The German electricity mix amounts to approx. 474 g / kWh (as of 2018, with a significant downward trend), while it is over 1000 g / kWh for electricity from lignite power plants and almost zero for green electricity.

Such information is also of interest for heating. With a modern gas heating (with condensing boiler) you get a good 200 g / kWh, with an oil heating approx. 270 g / kWh. Electric heat from an electric heater is between almost zero and approx. 1000 g / kWh, depending on the electricity provider; with an electric heat pump is lower by the coefficient of performance or the annual coefficient of performance (e.g. by a factor of 3 to 5). An electric heat pump is just as harmful to the climate as an oil or gas heating system when operated with electricity from coal, but considerably better with the German electricity mix or even green electricity.

Caution: The g / kWh can affect the fuel consumption or the CO2 Respectively!

CO is also sometimes found in combustion engines2-Information in g / kWh, based on the drive energy output. The specific fuel consumption can also be specified in g / kWh, which of course results in correspondingly lower numerical values ​​because 1 kg of fuel produces several kilograms of CO2 results (e.g. approx. 3 kg for gasoline). Such values ​​depend heavily on the operating conditions, e.g. full load or part load, because the efficiency varies accordingly. A modern gasoline engine (Atkinson engine, as it is used in some vehicles with hybrid drive) can best achieve a specific fuel consumption of approx. 200 g / kWh, corresponding to approx. 600 g / kWh in terms of CO2. In practical driving, however, you are much higher because the utilization is usually not optimal.

Questions and comments from readers

11.05.2018

I don't understand why kilowatts per hour is wrong.

If something needs 2 KW per hour and I use it for 5 hours, I get 2 KW × 5 h = 10 KWh. Isn't it right?

The explanation now says something about division, but there is no division at pro.

Answer from the author:

The first statement would be corrected as follows:

If something 2kWh per hour and I use it for 5 hours, I generate (a total amount of energy of) 2kW × 5 h = 10kWh.

So: Kilowatt does not designate an amount of energy, but an output, i.e. H. Energy per time.

“Pro” does mean a division. For example, the power in the above example is 2 kWh per hour = 2 kWh / h = 2 kW. Here “pro” corresponds to “/”. Conversely, however, 2 kWh = 2 kW × 1 h, i.e. kilowatt hour = kilowatt times Hour.

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See also: watts, joules, electricity tariff, electricity bill
as well as other items in the categories of units, electrical energy, heating and cooling

Understand everything?


Question: Which of the following statements are correct?

Correct answer: (b)


Question: What is the output of a consumer who consumes a kilowatt hour within a quarter of an hour?

Correct answer: (c)


See also our energy quiz!