Glossary of M&T terms

Activity-based targeting

The process of estimating expected consumption volumes by reference to production throughput, prevailing weather and other “driving factors”. Comparison between expected and actual consumption reveals randomly-occurring accidental avoidable waste.

Avoided energy consumption:

The difference, reckoned over any chosen time interval, between the quantity of energy used and what would have been used at historical levels of energy efficiency (i.e. before any active energy-saving measures had been implemented). See also performance deficit.

Benchmarking

Identifying the things that enable good performance, either by critically comparing the performance of similar installations, or (if no comparable cases exist) identifying past periods of better performance.

Base temperature:

In the calculation of degree days, the outside air temperature at which a building would require no artificial heating or cooling. In the UK a base temperature of 15.5^oC is common for heating assessment.

Control limit:

A tolerance band on the deviance chart indicating the level of deviance which is considered to be significant.

Cusum:

Cumulative sum of the difference between actual and expected consumption in successive monitoring intervals. Usually presented as a chart whose most important property is that it manifests a horizontal trend as long as consumption remains close to expected values. (more)

Degree days:

A measure of how cold (or hot) the weather has been (relative to a stated base temperature) measured over a regular monitoring interval, usually weekly or monthly.

Deviance chart:

A time-series chart showing the difference between actual and expected consumption in each successive monitoring period. May have control limits superimposed. Precursor to the cusum chart which shows the cumulative sum of deviance values. (sample)

Driving factor:

An independently-measurable factor (e.g. production throughput, mileage, degree-day value) that determines the required quantity of energy or other consumable resource.

Expected consumption

Theoretical quantity of energy, water, etc., against which actual consumption can be gauged. Can be calculated in various ways ranging from precedent (same period the year before) to rigorous mathematical modelling from first principles, but most commonly calculated using a simple empirical straight-line relationship between past consumption and corresponding values of an appropriate driving factor. More...

Fixed demand

The “base load” consumption that is incurred regardless of prevailing weather, production output, etc.; as distinct from the variable component of demand.

Gross production

The favoured measure of production activity in energy-intensive manufacturing, as distinct from net or saleable production. Reflects the fact that it may take as much energy to make unsaleable product as saleable. In thorough implementations, it may be necessary to record gross throughput at each significant stage in a process, to recognise the fact that product may either be diverted to scrap between stages, or else held in buffer storage.

Historical baseline

The characteristic performance of a building, vehicle, or manufacturing process, when first assessed at the outset of an energy management programme.

Interval of assessment

The period between exception reports; commonly weekly for industrial plants but often monthly for dispersed estates of buildings. Daily, per-shift and even “real-time” assessments may be worthwhile in some circumstances. Returns of consumption and related driving factors must at least be synchronised with the assessment interval (although more frequent measurements may be of some diagnostic value).

The ratio between energy (etc) used, and its presumed driving factor. A very weak and unreliable method of reporting, suitable for high-level management presentations but usually of no value for active management control.

League table:

A report consisting of a list of items ranked in order of significance, for example according to the gross quantity or cost of energy used. Overspend league table: a list of consumption streams ranked in descending order of the apparent excess cost relative to a computed target. (sample)

Limit, control

Margin of error allowed in the estimation of expected consumption and used to indicate deviations from target that are significant compared with normal variability.

Moving annual total

A method of reporting in which the most recent 12 months (or 52 weeks) of consumption are stated, regardless of the time of year. Applied to budget tracking, provides a more stable estimate of end-of-year outturn than can be obtained by projecting from results for the year to date.

Norm chart:

A time-series chart in which actual consumption volumes are coplotted with the corresponding expected values (inferred from the driving-factor values). (sample)

Overspend league table

Key weekly (or daily, etc) reporting technique in which monitored streams of consumption are listed in descending order of their apparent unaccountable excess costs. Provides a rational view of where best – if anywhere -- to direct investigations and remedial action. Conveniently accommodates any number of streams, whether of energy, water, or other resources, in a single concise summary that requires no specialist knowledge to produce or interpret.

Performance characteristic line:

A line, usually straight and diagonal, superimposed on the scatter diagram of consumption volume versus driving factor. Represents the idealised relationship between the two and enables the expected consumption to be estimated when the value of the driving factor is known. The PCL can be set to show the target, in which case it will occupy the lowest justifiable position on the chart; or the standard, in which case it represents current average performance and would be used for forward budget estimation; or the historical baseline representing average performance in the base year. (sample)

Performance deficit:

The difference, reckoned over any chosen time interval, between actual consumption and what should have been used (i.e. at optimum efficiency with no avoidable losses). Enables changes in energy performance to be tracked without necessarily using specific energy consumption ratios, which are often misleading. See also avoided energy consumption.

Precedent-based targeting

Targeting method in which, usually, monthly consumption is gauged against the same month a year before. Weak because it assumes (a) that conditions were indeed comparable in the precedent month and (b) that no waste had occurred which would inflate the target for the period being assessed. Precedent-based targets can also be applied to half-hourly or other high-frequency data, usually by defining a profile “template” on the basis of historical performance.

Quiescent threshold

A simple exception-reporting method for high-frequency data in which of out-of-hours consumption is monitored to ensure that it stays below some chosen level. Higher-than-expected consumption often indicates items left running unnecessarily.

Regression analysis:

A statistical technique for determining the constant and coefficients in a multi-variate targeting model of the form:
E = k₀ + k₁.D₁ + k₂.D₂ + ... + k_n.D_n (more)

Risk of undetected loss

A formal method of evaluating the cost-effectiveness of expenditure on additional metering. Consumption is disaggregated according to where it is used, and differing percentage losses are assumed according to the nature of the application. Systems with low load factors are presumed to have more scope for undetected waste than those which have to operate continuously close to their maximum rating.

Scattergram:

An x-y plot of consumption versus driving factor, both measured at regular intervals (typically weekly or monthly). (sample)

Standard:

Current average performance characteristic (any wasteful use included). Usually used to project budgets by reference to future expected activity levels and weather.

Stream

A measurable flow of energy, water, etc,: typically that taken through an individual submeter but would also include consumptions arrived at by difference (between a main meter and its downstream submeters, say) or by adding two or more flows (such as the oil and gas used in a dual-fuel boiler). A stream need not necessarily be metered: it could be computed from changes in stock level, or estimated from a proxy measure such as hours run. Some practitioners treat driving factors as “streams” as well.

Target :

A performance characteristic Used to gauge performance in the short term. Calculated from the corresponding driving factor values, usually using a model which assesses performance against the best achieved in the past. (more)

Variable component of demand

That portion of demand that varies in direct proportion to the relevant driving factor, as distinct from the fixed (i.e. purely time-related) component. X-Y scatter

Graph in which a stream’s consumption is plotted against the relevant driving factor, say on a weekly basis, and usually with a straight line superimposed to represent the achievable target. A “standard” performance line or the “historical baseline” can also be superimposed, the former being used for budget projection.

Year to date

An inferior method of reporting in which consumption etc are reckoned from the start of the accounting year, discarding information from the year before.