SMEasure is about giving you the tools you need to successfully manage the energy use of the buildings your business occupies. You may be new to building energy management, but with a few explanations of the basics and getting stuck in with reading the meters means you can be up and running in no time.

Key terms we will be introducing you to are: degree days, performance line, control charts and cusum analysis. Don't worry if these terms feel somewhat confusing at first, because as you begin to record meter readings and look at the results information should start to fall into place.

For the majority of buildings heating is the significant source of energy use. The heating use of a building is determined by the outside building temperatures (i.e. weather), building fabric, heating system and occupant behaviour.

To properly monitor, benchmark and target building heating consumption requires taking account of outside temperatures by doing what is known in the energy field as degree-day analysis. Degree-day analysis is used to both detect changes in a building's energy performance and to quantify successes in good energy management. This analysis will help you:

1. Identify trends in your building's energy performance

2. Get an indication of any significant changes caused by building operation

3. Help with budgeting energy costs

SMEasure will automatically do the degree-day analysis for the buildings you are energy monitoring thereby making it easy to improve energy efficiency.

The information below is a synopsis of a very useful document prepared by the Carbon Trust on using degree days for energy management which we recommend reading in full.

Learning about degree day analysis for building energy management

What is a heating degree day?

It is a measure of the severity and duration of cold. The colder the weather in a given month, the larger the degree-day value will be for that month. A heating day is the difference between a reference temperature (i.e. the 'base' temperature) and the outside temperature.

What does base temperature mean?

This is the outside temperature below which a building needs to be heated. A building to some extent will be self-heating (i.e. thermal gains because lighting, IT equipment and people, for instance, give off some heat. Therefore, a base temperature will typically be lower than the internal temperature. The building base temperature is crucial for doing rigorous building energy consumption analysis using degree days.

15.5°C is generally used as the base temperature. However, examples of variations are hospitals that tend to have a higher base temperature or warehouses that tend to have a lower base temperature.

For SMEasure we have calculated degree days for 77 weather stations around the UK based on a full range of building base temperatures available. For SMEasure to do degree day analysis for each of your buildings we ask you to locate the nearest weather station (using the map provided) as well as specify the base temperature used for the building.

Data collection for degree-day analysis

The first step in degree day analysis is collect energy use data. We recommend you collect meter reading data on a weekly basis (if not weekly then definitely monthly) - as you will get a richer energy analysis of what is going on in your buildings and be able to spot any issues quickly. It will be beneficial to make meter reading part of the weekly work tasks - i.e. first thing on a Monday morning.

Graphing energy consumption

Once you have recorded a series of meter readings into SMEasure, it will be possible for SMEasure to create a scatter plot graph of the building's space-heating energy use - an example of which is illustrated in figure 3 from the Carbon Trust report. Monthly energy consumption for the building is along the Y-axis and monthly degree days are along the X-axis.

Creating a performance line

The graph of space-heating energy consumption against degree days should show a linear relationship, which is expressed as a performance line. This line should be straight. The slope of the line will be determined by the best statistical fit for the data plotted.

The performance line represents how much energy the building can be expected to use for a given number of degree days. The steepness of the line (i.e. the slope) is a measure of how much extra fuel will need to be consumed for an increase in degree days - in other words the rate the building loses heat. The steeper (more vertical) the line the less likely the building is to be energy efficient and the flatter the line (more horizontal) the more likely the building is energy efficient.

The point where the line intercepts the Y-axis gives an indication of non- weather related energy used, for example, hot water, humidification, cooking etc. This means energy use above this point will be weather-related energy demand.

A well run building should have an energy usage which is proportional to the number of degree days for the same time period.

The value of the performance line equation

The equation for the performance line can be used to predict the future energy use of the building. Energy use that deviates from the performance line means that something has changed in the way the building uses energy. Deviation may be because there is a change in building use; of control settings; installation of energy efficiency measures or malfunctioning equipment. The performance line therefore provides a benchmark for identifying and quantifying these deviations.

A control chart

Once the equation for your performance line has been identified SMEasure will create a control chart. A control chart is a visual display of how the actual energy use varies from predicted energy use of the building (i.e. the performance line). To calculate the control chart involves subtracting predicted energy consumption (the equation of the performance line) from the actual consumption for each month.

The convention for determining the difference is always actual minus predicted. This means that if actual energy use is lower than the predicted for a given month then the energy saving will show as a negative value on the chart.

With a control chart it is possible to set boundary limits to be able to quickly spot energy use is outside a reasonable range.

Cusum analysis

Cusum is the technical term referring to the cumulative sum of the difference between actual and predicted energy use. The power of cusum analysis is that by comparing ongoing energy consumption data with the historic performance line a trend indicating change can rapidly be seen. The cusum can be plotted making visible the overridding trends in the building's energy use so it is easy to monitor building energy performance.

If energy savings are being made in your building then the slope of the cumum analysis will be going downwards. However, if the slope of the cusum analysis is going upwards then this is an indication there has been an increase in energy use over time.