Electricity, energy and interfaces – briefly explained

Active energy

The energy actually converted into work, heat or light. Measured in kilowatt hours (kWh) and the quantity practically every energy bill is based on.

Electricity and energy

Reactive energy

Energy that swings back and forth between the grid and the load without doing work. It arises from coils and capacitors, is measured in kvarh and is charged by suppliers above a threshold.

Electricity and energy

Apparent energy

The geometric sum of active and reactive energy, measured in kVAh. It determines how heavily cables and transformers are loaded.

Electricity and energy

Active power

Instantaneous power flow that does work, in kilowatts (kW). The peak values of active power determine the demand charges.

Electricity and energy

Reactive power

Oscillating power with no working component, in kvar. It loads the grid but does not show up on an active energy meter – which is why it is recorded separately.

Electricity and energy

Apparent power

The product of voltage and current, in kVA. Transformers and supply cables are sized by it.

Electricity and energy

Power factor

The ratio of active to apparent power, also cos φ. A poor power factor means high reactive power and can trigger additional costs.

Electricity and energy

cos φ

The common designation for the displacement factor between current and voltage. With a purely resistive load it is 1, with motors considerably lower.

Electricity and energy

Harmonics

Multiples of the mains frequency, produced by non-linear loads such as frequency converters and power supplies. They load conductors and distort simple measuring methods.

Electricity and energy

Load peak

The highest power value within a billing period, usually as a quarter-hour average. Often the most expensive single item on the electricity bill.

Electricity and energy

Load profile

The sequence of power values over time, usually in quarter-hour steps. It shows when energy was drawn – the question a meter reading does not answer.

Electricity and energy

Base load

The consumption that is always present, including at night and at weekends. A high base load is often the first finding of a load profile analysis.

Electricity and energy

Peak load

A briefly high power demand, for instance when plant starts up. It can often be reduced by spreading things out in time.

Electricity and energy

Load management

Deliberately shifting or limiting loads in order to avoid load peaks. It requires measured values in near real time.

Electricity and energy

Self-consumption

The share of self-generated energy consumed directly on site rather than fed in. A central quantity for photovoltaic installations.

Electricity and energy

Self-consumption association

An association of several parties in a property who use jointly generated energy. It requires metering for each party.

Electricity and energy

Tenant electricity

A model in which tenants draw locally generated electricity directly. It needs billable metering per unit.

Electricity and energy

Feed-in

Delivery of energy to the grid, for instance from a photovoltaic installation. It is measured separately from consumption and usually remunerated differently.

Electricity and energy

Consumption

Drawing energy from the grid. Meters for self-consumption models have to record consumption and delivery separately.

Electricity and energy

Tariff

A time-dependent price for energy, such as peak and off-peak tariff. A meter with several tariff registers records them separately.

Electricity and energy

Meter constant

The factor converting meter pulses into energy units, often given as pulses per kWh. Important with S0 recording.

Electricity and energy

Current transformers

A component that steps high currents down to a measurable value. The transformer ratio has to be set correctly in the meter.

Electricity and energy

Transformer ratio

The ratio between the primary and secondary current of a current transformer, for instance 200/5 A. A wrongly set ratio produces a systematic measuring error.

Electricity and energy

Direct measurement

Measurement without current transformers, in which the whole current flows through the meter. Possible only up to certain currents.

Electricity and energy

Four-quadrant measurement

Recording active and reactive energy in both directions. A prerequisite for installations that draw and feed in at the same time.

Electricity and energy

Submetering

Measuring partial consumption behind the main meter, for instance per tenant, machine or part of a building. The basis for consumption-based billing.

Metering and billing

Third-party quantity allocation

Separating out quantities of energy passed on to third parties. Regulatorily relevant in Germany and tied to requirements on metering.

Metering and billing

MID

The European Measuring Instruments Directive. An MID-compliant meter may be used for billing in the EU; module B+D designates the approval procedure.

Metering and billing

Verification law

The legal framework for measuring instruments in commercial dealings. Meters used for billing have to comply with verification law.

Metering and billing

PTB-A 20.1

The requirements document of the German national metrology institute (PTB) for electricity meters. Proof of compliance with verification law.

Metering and billing

PTB-A 50.7

Requirements for measuring instruments in third-party quantity allocation and energy management.

Metering and billing

Accuracy class

A statement of the permissible measuring deviation, such as class B for active energy or class 2 for reactive energy. Lower is more accurate.

Metering and billing

ISO 50001

The standard for energy management systems. It requires continuous recording, analysis and reporting of energy data.

Metering and billing

Energy management system

A systematic process for recording, assessing and improving energy use. Software is only one part of it.

Metering and billing

Metering concept

The decision on which quantities are recorded at which points and what for. Mistakes in the metering concept can hardly be corrected later.

Metering and billing

Measuring point

The place in the installation where measurement takes place – independently of the specific device. Naming should follow the measuring point, not the device.

Metering and billing

Meter reading

The cumulative energy value of a meter. It answers the question of quantity, not the question of when.

Metering and billing

Load profile analysis

Analysis of power values over time, in order to make peaks, base load and anomalies visible.

Metering and billing

Utility cost statement

Allocation of consumption costs to units. With submetering, based on actual consumption rather than on a floor-area key.

Metering and billing

Sankey diagram

A flow diagram whose arrow widths correspond to quantities. It makes energy flows and their proportions graspable at a glance.

Metering and billing

M-Bus

Meter-Bus, the European standard for remote reading of consumption meters to EN 13757. A two-wire line, polarity-independent, for electricity, water, gas and heat.

M-Bus

EN 13757

The series of standards for communication by consumption meters. Part 2 describes the physical layer, part 3 the application layer.

M-Bus

Unit load

The quiescent current draw of an M-Bus node, around 1.5 mA. The sum of all unit loads limits the number of devices on a master.

M-Bus

M-Bus master

The device that feeds the bus and polls the meters. It also handles level conversion and short-circuit monitoring.

M-Bus

Level converter

The assembly that provides the bus voltage and converts signal levels. Integrated in the EMU M-Center, otherwise a separate device.

M-Bus

Primary address

A short M-Bus address from 1 to 250, set on the device. Fast, but error-prone if assigned twice.

M-Bus

Secondary address

The eight-digit identification number of a meter, combined with manufacturer, version and medium. Unique from the factory.

M-Bus

Meter discovery

Automatically polling all possible addresses in order to find connected devices. It saves days on larger installations.

M-Bus

Telegram

A request-and-answer sequence on the M-Bus. The meter delivers its complete data record in response.

M-Bus

Baud rate

Transmission speed in bits per second. On M-Bus, 300, 2400 and 9600 Bd are usual, in practice mostly 2400.

M-Bus

Loop resistance

The total resistance of the bus line there and back. On M-Bus limited to 90 Ω.

M-Bus

Wireless M-Bus

The radio variant of M-Bus to EN 13757-4, widely used on water and heat meters with no cable connection.

M-Bus

RS-485

An electrical interface standard for differential two-wire transmission in half-duplex operation. Robust against interference, but not polarity-independent.

RS-485 and Modbus

Differential transmission

Signal transmission as the voltage difference between two conductors. Interference hits both conductors equally and cancels out.

RS-485 and Modbus

Half duplex

Transmitting and receiving over the same pair of conductors, but never at the same time.

RS-485 and Modbus

Termination resistor

A 120 Ω resistor at both physical ends of the bus, which prevents reflections. Only at the ends, not on every device.

RS-485 and Modbus

Bias resistor

Biasing resistors that define the idle level of the bus, so that no node interprets noise as data.

RS-485 and Modbus

Unit load

The load measure of an RS-485 node. Classically a segment tolerates 32 unit loads; modern drivers draw fractions of that.

RS-485 and Modbus

Stub

A branch from the main line to a device. On RS-485 keep it as short as possible, because branches cause reflections.

RS-485 and Modbus

Line topology

Continuous wiring from one end of the bus to the other. The only topology recommended for RS-485.

RS-485 and Modbus

Modbus

An open protocol for communication between automation devices. It works with 16-bit registers and master-slave operation.

RS-485 and Modbus

Modbus RTU

The binary Modbus variant over serial lines, usually RS-485. It uses CRC16 and a silent interval for frame detection.

RS-485 and Modbus

Modbus TCP

Modbus over Ethernet, standard port 502. Instead of a CRC, every telegram carries an MBAP header; several clients can read at the same time.

RS-485 and Modbus

Function code

The number indicating the operation wanted. For meters, mainly 3 (holding register) and 4 (input register) are relevant.

RS-485 and Modbus

Holding register

A readable and writable 16-bit register in the Modbus data model.

RS-485 and Modbus

Input register

A read-only 16-bit register, typical for measured values.

RS-485 and Modbus

CRC16

The checksum securing Modbus RTU telegrams. Faulty telegrams are discarded.

RS-485 and Modbus

Silent interval

A gap of at least 3.5 character times that marks the start and end of a telegram in Modbus RTU.

RS-485 and Modbus

TCP/IP

The protocol family on which Ethernet networks and the internet are built. The basis for Modbus TCP, MQTT, BACnet IP and OPC UA.

TCP/IP and networking

IP address

The unique address of a device on the network. Measuring devices need a stable address, either static or by DHCP reservation.

TCP/IP and networking

DHCP reservation

Assigning a fixed IP address to a MAC address. It combines the convenience of DHCP with the stability of fixed addresses.

TCP/IP and networking

MAC address

The hardware address of a network interface. It serves for unique identification on the local network.

TCP/IP and networking

VLAN

A logically separated network segment on shared infrastructure. Metering usually gets a VLAN of its own.

TCP/IP and networking

MBAP

The Modbus Application Protocol header in Modbus TCP, with transaction number, protocol identifier, length and unit identifier.

TCP/IP and networking

Unit identifier

A field in the MBAP header that, on gateways, addresses the serial device behind it.

TCP/IP and networking

Port 502

The standard port for Modbus TCP.

TCP/IP and networking

NTP

The protocol for time synchronisation. Without synchronised clocks, load profiles from several devices cannot sensibly be compared.

TCP/IP and networking

BACnet IP

The building automation protocol over IP. It is object-oriented: every measured value is an object in its own right.

TCP/IP and networking

BBMD

BACnet Broadcast Management Device. It relays BACnet broadcasts across subnet boundaries.

TCP/IP and networking

OPC UA

A platform-independent industrial standard for data exchange with an integrated information model and security concept.

TCP/IP and networking

MQTT

A lightweight publish-subscribe protocol for telemetry data. Devices publish values, interested parties subscribe to them.

MQTT

MQTT broker

The intermediary between senders and receivers. All messages run through it; sender and receiver do not know each other.

MQTT

Publish

Publishing a message under a topic at the broker.

MQTT

Subscribe

Subscribing to a topic in order to receive the messages published there.

MQTT

Topic

The hierarchical path under which messages are published. Structure and naming determine how usable the data is later.

MQTT

Wildcard

A placeholder when subscribing: + for exactly one level, # for all levels below.

MQTT

QoS

Quality of Service, the delivery guarantee. 0 sends without confirmation, 1 guarantees at least one delivery, 2 exactly one.

MQTT

Retain

The flag with which the broker keeps the last value per topic. Newly connected subscribers see data immediately.

MQTT

Last will

The message the broker publishes when a device’s connection breaks unexpectedly. It makes failures visible.

MQTT

TLS

Encryption of the transport connection. Mandatory as soon as measured data leaves your own network.

MQTT

LoRa

A radio modulation method using spread-spectrum technology. It trades data rate for range and immunity to interference.

LoRa and LoRaWAN

LoRaWAN

The network protocol over LoRa, with gateways, a network server, encryption and a join procedure.

LoRa and LoRaWAN

Spreading factor

A parameter from SF7 to SF12 that weighs range against transmission time. Higher means further, but slower.

LoRa and LoRaWAN

EU868

The licence-free frequency band around 868 MHz, used for LoRaWAN in Europe.

LoRa and LoRaWAN

Duty cycle

The statutory limit on transmission time per device and hour in the licence-free band. It determines the possible transmission interval.

LoRa and LoRaWAN

Class A

The operating mode in which a device opens two short receive windows after transmitting. The most economical of the LoRaWAN modes.

LoRa and LoRaWAN

OTAA

Over-the-air activation: joining with DevEUI, JoinEUI and AppKey, with the session keys negotiated. The recommended procedure.

LoRa and LoRaWAN

ABP

Activation by personalisation: permanently programmed session keys. Simpler, but less secure.

LoRa and LoRaWAN

Gateway

The receiving station that picks up LoRa packets and forwards them to the network server. Devices are not permanently assigned to a gateway.

LoRa and LoRaWAN

Payload decoder

The code that breaks the compactly packed payload of a packet down into named measured values. Without it the data stays unreadable.

LoRa and LoRaWAN