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Glossary · Energy industry & energy transition

Electrical work (electrical energy)

Electrical Energy Energy power Kilowatt hour electricity consumption Billing

Electrical work is the amount of energy that is transferred, converted or used by electrical power during a given period of time. It does not describe how heavily a device or installation loads the grid at a particular moment. It indicates how much energy accumulates over a period. With constant power the rule is simple: work equals power times time (W = P · t). If the power fluctuates, the instantaneous values are summed over time, which mathematically is an integral. A device with an electrical power of 1 kilowatt that is operated for one hour absorbs electrical work of 1 kilowatt-hour.

Units and conversion

In Germany, the joule (J) is the statutory unit for energy, work and quantity of heat. The watt (W) is the statutory unit for power and energy flow. One joule equals one watt-second. In the energy industry, however, calculations are almost always made in kilowatt-hours (kWh). It is the product of the power unit kilowatt and the time unit hour. Since one hour has 3,600 seconds, the following applies: 1 Wh = 3,600 J and 1 kWh = 3,600,000 J = 3.6 MJ. This conversion is exact, not rounded.

Larger quantities are stated in megawatt-hours (1 MWh = 1,000 kWh), gigawatt-hours (1 GWh = 1,000 MWh) or terawatt-hours (1 TWh = 1,000 GWh). Only one prefix is permitted per unit. A "kilo-kilowatt-hour" therefore does not exist; the megawatt-hour is used instead. The kilowatt-hour has become established because it offers an intuitive order of magnitude for households, businesses, power plants, storage facilities and electricity trading.

Work is not power

Electrical work is often confused with electrical power, and this has practical consequences. Power is measured in watts, kilowatts or megawatts. It describes the instantaneous energy flow, comparable to the flow rate of a water tap. Work is measured in watt-hours, kilowatt-hours or megawatt-hours. It describes the accumulated amount of energy, that is, the water that has collected in the bucket. An example: a kettle with 2 kW of power that runs for five minutes absorbs about 0.17 kWh. A device that draws only 100 W on average but is in operation for ten hours reaches 1 kWh, which is roughly six times as much. The kettle briefly demands high power. The continuously running device accumulates more energy over time.

This distinction is central for the grid. Lines, transformers, fuses, inverters and grid connections are designed according to power, because they have to carry a certain current at a certain point in time. A low annual consumption therefore says little about what peak power a connection briefly requires. Conversely, an installation with a high connected load may cause little electrical work if it only runs rarely.

Measurement: active work and reactive work

An electricity meter measures electrical work by summing the energy flow over time. For households, active work is generally what is billed. This is the part of the electrical energy that is actually converted into heat, light, motion or chemical energy. Computers and other electronics, too, ultimately release the energy they absorb almost entirely as heat. Meters with two measuring directions record withdrawal and feed-in separately for installations with feed-in.

In alternating-current grids there is additionally reactive power. Its statutory unit is the var. Over time it yields reactive work, stated for example in kvarh. Reactive energy is temporarily stored in magnetic and electric fields, for example of motors, transformers or capacitors. It then oscillates back and forth between the grid and the consumer without being converted into useful energy. Nevertheless, it burdens lines and equipment with additional current. For households it is usually not charged separately. For larger customers with appropriate metering, grid operators in some cases charge fees for reactive work if it exceeds a defined share. The exact rules are set out in the price sheets and terms of the respective grid operator.

Energy charge, demand charge and billing in Germany

On electricity bills, electrical work appears as consumption in kWh. The energy charge (Arbeitspreis) states the amount due per kilowatt-hour. Depending on the customer group, there are also base charges, fees for metering point operation, taxes, levies and surcharges. For network charges, the structure in Germany is currently governed by the Electricity Network Charges Ordinance (StromNEV). Under § 17, the network charge per withdrawal point in principle consists of an annual demand charge in euros per kilowatt and an energy charge in cents per kilowatt-hour. The energy-based charge results from the energy charge multiplied by the electrical work withdrawn in the billing year. The demand-based portion depends on the annual peak demand of the withdrawal.

A special rule applies to withdrawal points in the low-voltage grid with annual withdrawal of up to 100,000 kWh and pure energy metering. Here, instead of a demand charge and an energy charge, a single energy charge is set, supplemented where applicable by a monthly base charge. This typically concerns households and small businesses. For large consumers with demand metering, by contrast, peak power matters in addition to the amount of energy. Electricity costs therefore do not arise from kilowatt-hours alone.

On the current legal status: the StromNEV expires at the end of 31 December 2028. The Bundesnetzagentur (Federal Network Agency) is working on a successor regulation, the Allgemeine Netzentgeltsystematik Strom (AgNes, general electricity network charge system), which is to be applied from 2029. According to the authority's published considerations, households in the low-voltage grid are to continue paying a base charge in euros per year and an energy charge in cents per kWh. More far-reaching changes are under discussion for other customer groups. The details depend on the final determination.

Distinction from electricity consumption, final energy and primary energy

In everyday use, electrical work and electricity consumption are usually treated as the same. That is fine when it comes to the measured amount of energy at a connection. Physically, however, electrical energy is not "consumed" but converted into other forms of energy. The kilowatt-hours quantify this converted amount.

Final energy is the energy that reaches the user, such as electricity, gas, heating oil, district heating or fuel. Electrical work is only the electrical part of it. It differs even more clearly from primary energy, that is, energy before any conversion, such as in coal, natural gas, uranium, wind, sun or biomass. In fossil-based electricity generation, the primary energy used is considerably higher than the electrical work generated, because waste heat arises in the power plant. For wind and solar power, the result depends strongly on the accounting convention chosen. Part of the energy is also lost on the way to the customer, the grid losses.

In electrification, this distinction becomes important. If a building switches from a gas heating system to a heat pump, electricity consumption rises. The final energy demand for heat can nevertheless fall, because the heat pump uses ambient heat and provides several kilowatt-hours of heat from one kilowatt-hour of electrical work. More electrical work drawn therefore does not in itself prove higher total energy consumption. One has to name the system boundary: is the question about electricity, final energy, primary energy, costs, emissions or grid load?

Why timing matters

A kilowatt-hour contains no information about when it flowed. For grid operation, however, timing is decisive, because generation and consumption must be balanced at every moment. Two households with the same annual work can have very different power profiles over time. Such a profile is called a load profile. Someone who uses a lot of electricity in hours of high wind and solar feed-in burdens procurement and the grid differently from someone who draws mainly in tight evening hours. Consumption that is shifted into times of high renewable generation can reduce curtailment. Consumption in tight hours, by contrast, increases the need for dispatchable generation, storage or imports.

With storage, the limit of looking at kWh alone shows up especially clearly. A storage unit is described by two figures: the capacity in kWh, that is, how much electrical work it can absorb, and the charging and discharging power in kW, that is, how fast this happens. For supply over several hours, capacity counts. To shave a grid peak, power counts. Something similar applies to flexibility: heat pumps, electric cars, cold stores or industrial processes can shift electrical work in time. The amount of energy often stays the same, but the load profile changes. The value of this shift results from the amount of energy, power, time window, controllability, and comfort or production limits.

Typical misinterpretations

  • Few kWh do not automatically mean low grid load. A rarely used device with very high power, such as an electric instantaneous water heater or a fast-charging station, can stress local grid components more than a steady continuous consumer with higher annual work.

  • Not every kWh has the same economic value. In a household tariff with a fixed energy charge, it appears so. On the wholesale market, however, prices fluctuate hourly or every quarter of an hour, and grid costs depend strongly on peak utilisation and capacity provision. Flat energy charges can mask these differences.

  • Generated and delivered work are not physically the same. Electricity flows through the grid according to physical laws, not along contracts. Balancing groups, guarantees of origin and supply contracts allocate energy quantities commercially. The work remains measurable nonetheless; only its allocation is technical, balance-based or contractual depending on the perspective.

Electrical work is a fundamental quantity of the electricity system but does not describe electricity use completely. It quantifies the amount of energy generated, transmitted, stored or billed. For technical design, power is additionally needed; for security of supply, availability over time; for cost questions, the tariff and network charge rules; and for climate balances, the generation mix. The kilowatt-hour is precise as long as it remains clear which question it answers and which it leaves open.

Sources

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Edited by eCMS on 11.11.2026 19:43

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.