How do I interpret my building's energy performance line?

A well run building should have an energy usage which is proportional to the number of degree days for same time period. To start to learn how well your building is running look at your performance chart - this is the first chart you see when you click the analysis tab in the building tab of smeasure). This chart is a scatter plot of weekly degree days (x-axis) to energy use (y-axis). If you hover over each point you can see the date and energy use.

The blue line drawn on this chart is called the performance line - it is best fitting line through the points plotted. The slope of the line (i.e. the angle it is drawn from the x-axis) indicates the rate of energy loss from the building. The steeper the slope of the line going outwards along the x-axis the greater the level of energy loss for the number of degree days . A steep performance line is likely to be an indicate that the building fabric is not very air tight and this will mean: 1) the heating system is working hard to heat the building to the thermal comfort setting especially if it cold outside; and 2) a lot of the heated air inside the building is escaping to the outside. The more flat your performance line the better the energy performance of your building.

The performance chart should be set to be plotting the energy use that is related to your heating. For example, if your building uses gas for heating (as is the case for most building in the UK) your building profile should be set up this way, but if your building uses electricity for heating then electricity would be stated as your energy source for heating (which can be the case for offices and retail).

A building's energy use that tracts well with degree days should not have wild scatter. The points should fall closely along the performance line. The R value in the top righthand corner of the chart is confirmation of how closely your building's energy use is tracting degree days. The closer the R value is to 1.0 the stronger the correlation of energy use to degree days.

The point where the performance line intercepts the y-axis indicates the energy use which is independent of heating such as that for hot water heating or cooking or appliances.

Example 1: Office using gas for heating

This chart shows that energy use is tracting temperature reasonably well. The slope of the performance line is quite steep so this indicates the office building is not that efficient - meaning there is quite a lot of heat loss as temperatures get colder (i.e. more degree days). Also there is some quite wild scatter meaning there is likely to be occupant behaviour in the building causing variation in the heat demand - this could be setting the heating for longer hours or turning off when the office is not in use. The intercept through the y-axis is the baseload demand for water - you can see most of the gas demand for the building is for heating. This building probably needs to make improvements to the building fabric to increase the thermal insulation so that there is less heat loss. If this is done then the slope of the performance line should become more flat.

Example 2: Office using electricity for heating

This chart shows that energy use is also tracting temperature reasonably well. The slope of the performance line is more shallow then example 1 so this office building has a lower rate of heat loss as temperatures get colder (i.e. more degree days). For this building there is not great scatter of data points - the weekly plot points fall closer to the line. This is an indication the build's energy use is well managed. The baseload electricity demand is notable because electricity will be used for lights, applicances (e.g. computers for the 50 people workings at this office) and hot water. The weekly baseload for electricity demand not for space heating is about 1300 kWh - indicated by the point the performance line intercepts the y-axis.