Active power, reactive power and apparent power differ
Your electricity bill lists “reactive energy”, although your usual consumption is already billed in kWh. What does this mean? Three terms help explain it:
- Active power in kW: It is converted, for example, as mechanical work of a motor or as heat.
- Reactive power in kvar: Motors and transformers require magnetic fields; capacitors store energy in electric fields. During the periodic build-up and decay of these fields, energy oscillates between the system and the grid. It performs no usable work in this process, but it does place a load on the electrical system.
- Apparent power in kVA: It describes the total stress for which lines, transformers and switchgear must be designed.
Active and reactive power cannot simply be added. With sinusoidal current and voltage, the power triangle describes their relationship to apparent power. If electronic loads distort the current waveform, specialist planning must also consider harmonics: signal components at multiples of the mains frequency. Janitza explains the technical principles of reactive power compensation.
Why reactive energy may appear on the bill
Reactive energy is derived from reactive power over time and is expressed in kvarh. For example, a constant reactive power of 10 kvar over five hours corresponds to 50 kvarh. Simply summing measurements without considering their time frame would not provide an accurate energy quantity. Whether reactive energy is charged and what exemption limits apply depends on the agreed-upon terms and the specific price list.
The exemption limit may, for example, refer to the active energy consumed in kWh during the same period. The decisive factor is the specific rule on your price list; a percentage adopted from another contract may be incorrect.
- Does the item belong to the same delivery point and the same billing period?
- Does the bill show inductive or capacitive reactive energy?
- What reactive-energy allowance, reference quantity and price are agreed?
- Does the calculation match the measurement and the current price list?
The article on checking the commercial electricity bill helps to separate this item from the energy charge, capacity charge and other charges.
An example of the commercial examination
Assume that after the contractual exemption limit, 8,000 kvarh remain billable. At a purely hypothetical price of €0.015 per kvarh, this results in €120. This price is a calculation assumption. For your review, enter the quantity billable according to your contract and the price applicable therein. Then compare whether the costs shown match this calculation.
Reactive power compensation does not automatically reduce the active energy consumption, billed in kWh, by the same proportion. Economic efficiency and effectiveness must be assessed separately.
Know the load behavior before compensation.
A power compensation system provides the required reactive power as close as possible to the consumer, and can thus relieve the upstream grid. Before selecting one, it must be determined when and to what extent the demand arises. Are motors running continuously, are there shift changes, or has a major system been replaced? An existing compensation system may also be incorrectly configured or require maintenance. A snapshot is often insufficient in cases of highly variable operation.
Electrical planning assesses measurements, the existing system, harmonics, and the network operator's requirements. Independent work on the control cabinet is not a suitable way to reduce costs. For the business decision, investment, ongoing costs, and actually avoidable charges are compared.
Classifying technology and procurement together
Reactive energy, maximum active power and total electricity supplied are different quantities. The guide to peak loads and demand charges covers the second of these. If modernisation changes your grid import profile, update your next energy enquiry as well.
Discuss your bill and energy procurement needs with OPTUM. Have the bill showing reactive energy, the applicable tariff sheet and available readings ready. This helps identify outstanding billing questions; any necessary technical investigation must be carried out by qualified electrical specialists.
