A gas cooled nuclear fission reactor named after the magnesium alloy used to clad the uranium fuel.

Mains Electricity
Electricity supplied to our homes from the National Grid.

Marginal abatement cost curve (MACC)
Charting tool used to plot the costs of reducing CO2 or GHG emissions against the amount of emissions reduction available from implementing energy efficient measures.

Marine Generation (Tidal And Wave)
The principle behind tidal generation is similar to wind turbines, except that instead of wind turning the turbine blades, the process uses underwater current caused by tides. One of the benefits of tidal power over wind power is the predictability of tidal currents, enabling the developers to know exactly when the turbines will be producing power. Electricity can also be generated by harnessing the energy waves. The aim is to capture the vertical movement in the water surface caused by waves and to convert that energy to electricity by turning a generator. Both tidal and wave power are in the early stage of commercial use.

Mark to Market
To revalue a position according to the prevailing market price for the remaining life of an instrument.

Market Code Sector
A classification if a site is Industrial & Commercial or Domestic.

Maximum and Minimum Annual Quantity
The maximum and minimum annual quantity of gas between which it is anticipated the consumer will use.

Maximum Annual Quantity (MAQ)
Total quantity of gas to be delivered to the customer sites during the contract year. Usually defined in a Take or Pay clause.

Maximum Available Capacity
Measured in kilo-volt-amps, the maximum quantity of electricity which can be safely used at a specific supply point, determined by the demand locally, network cabling and fusing at the supply point. Consumers pay a monthly premium for each kVA of availability at the supply point.

Maximum Daily Quantity
The maximum quantity of gas actually used.

Maximum Demand (MD)
Maximum Demand is the highest peak of usage (kWh) in any Half Hour during a calendar month or between two meter readings measured in either kW or kVA. This value is multiplied by 2 to give the MD on an hourly basis.

Maximum Demand Tariff (MD Tariff)
An electricity tariff based on the maximum demand at the supply point each month.

Maximum Import Capacity (MIC)
Maximum Import Capacity (MIC) is the level of electrical capacity contracted between your business and ESB Networks. The PSO charge on your bill is based on your contracted MIC level. The unit of measurement for MIC is the kilovolt ampere (kVA). There is a direct relationship - with financial implications - between MD and MIC, if the former exceeds the latter, the latter may need to be increased, if the latter significantly exceeds the former, the consumer may be paying for something they don't use/need.

One million BTUs, which can also be expressed as one decatherm (10 therms). MBTU is occasionally used as a standard unit of measurement for natural gas and provides a convenient basis for comparing the energy content of various grades of natural gas and other fuels. One cubic foot of natural gas produces approximately 1,000 BTUs, so 1,000 cu.ft. of gas is comparable to 1 MBTU. MBTU is occasionally expressed as MMBTU, which is intended to represent a thousand thousand BTUs.

Measurement Class
A term used to describe the way an electricity meter is read, either remotely via a communications line (half-hourly metered electricity supplies), or manually by a visiting meter reader (non-half-hourly metered electricity supplies).

Megatonne of Carbon (MtC)
One million tonnes of Carbon. Emissions of carbon dioxide are often expressed in terms of their carbon content. 1 MtC is equivalent to 3.67 million tonnes of carbon dioxide.

Megawatt (MW)
The standard measure of generating plant capacity equal to 1000 kilowatts, or one million watts. Medium to large power stations typically have a capacity in the range 500 MW to 2,000 MW.

Megawatt Hour (MWh)
The volume of energy supplied at a rate of one MW for the duration of one hour - equal to one million watt-hours of electricity.

Metal fuels
Natural uranium metal as used in a gas-cooled reactor.

The point at which the flow of electricity is actually measured. A single meter can contain one or more registers for measuring different sub-system quantities. The identifier will be up to ten characters long and consist of the make and model code, together with the meter serial number.

Meter Asset Manager (MAM)
A role that can be taken on by a number of parties who will manage a portfolio of meters on behalf of their client. They could control the meter replacement programme and arrange meter work. The MAM will act as the point of contact for a meter point and can supply all known information regarding that meter point.

Meter Asset Provider (MAP)
The party responsible for the ongoing provision of the meter installation at that meter point. Where a MAP provides the meter, the MAM may be the owner (title owner) of the meter or the MAM could lease or rent the meter from a third party.

Meter Number
A unique number identifying a specific meter wherever it is installed.

Meter Operator (MOp)
The company responsible for installing and maintaining electricity meters. Half-hourly metered clients must appoint a MOp or the supplier will appoint one by default..

Meter Point Reference Number (MPR or MPRN)
A unique 10 digit identification number (it may appear to have less if it starts with one or more zeros) allocated to an electricity meter at a business or domestic supply point.

Meter Read Frequency
Gas suppliers are only obliged to read a meter once every two years and for this reason the majority of gas supply bills will be based on estimations.

Meter Registration System Operator (MRSO)
Meter Registration System Operator (MRSO) is a "ringfenced" function within ESB Networks responsible for the Change of Supplier process and the processing/aggregation of meter data required to support Trading & Settlement in the competitive electricity market.

Meter Serial Number
The number stamped on the front of the meter. This changes when meter is exchanged.

Methane (CH4)
A colourless, odourless, flammable gas and the main constituent of natural gas. It has been identified as a greenhouse gas.

CHP (as above), but in very small scale, typically below 5kW electrical output, (e.g. in the residential and commercial sectors). It is likely to operate in place of a domestic central heating boiler.

The small-scale generation of energy, for example solar panels or domestic wind turbines. These are often referred to as generation from renewable sources at a domestic or small community level.

Million Cubic Meters (MCM)
1 MCM is approximately 360,000 therms.

See MBtu.

Month Ahead
The next full calendar month.