Why Does an Empty Office Building Consume So Much Energy at the Weekend?
An office building continued to consume a surprisingly large amount of energy at weekends, even though no work was taking place in the building.
By analysing data from a submetering system with nearly 500 metering points, OMS24 Zrt. broke down the weekend consumption into individual consumer groups.
The analysis revealed that air handling units left running accounted for almost half of the building’s weekend energy use.
After adjusting their operation, the share attributable to the air handling units fell from 48% to 14%. According to the analysis, this change resulted in energy savings of 9,900 kWh per day.
The starting point
We analysed the energy consumption of a large office building.
On weekdays, significant consumption was naturally expected: employees were present, building services were operating, lighting was in use and other electrical equipment was running.
However, when examining the 15-minute consumption profile, something stood out:
the building’s energy use did not fall as much at weekends as would have been expected for an unoccupied building.
Yet no work was taking place in the office building at weekends.
The question was therefore simple:
What is consuming energy when almost nobody is in the building?
What did we measure?
In this case, a major advantage was that the building already had a submetering system with nearly 500 metering points.
This meant we could see not only the building’s total consumption, but also break the energy use down into different consumer groups.
We were therefore able to look behind the weekend base load.
We could see not only how much energy was being consumed, but also where it was being used.
This is exactly the difference between what a main meter can show and what a well-designed submetering system can reveal.
What did submetering reveal?
The first chart shows the office building’s 15-minute electricity consumption profile.
A significant base load remains visible even during the weekend.
However, this chart alone does not show what is causing it.
Reducing weekend building energy consumption with submetering – OMS24 Zrt.
A detailed breakdown of the consumption data provided the answer.
Reducing weekend building energy consumption with submetering – OMS24 Zrt.
The energy breakdown before the intervention clearly shows that air handling units were responsible for almost half of the total weekend consumption.
The chart after the intervention shows a very different consumption profile:
the share attributable to the air handling units fell from 48% to 14%.
Reducing weekend building energy consumption with submetering – OMS24 Zrt.
What was causing the unnecessary consumption?
The detailed consumption breakdown showed that the air handling systems remained in operation at weekends.
They were therefore consuming a significant amount of energy even when the building was no longer occupied for normal business activities.
The problem was not faulty equipment.
Nor was it that the air handling units inherently consumed too much energy.
They were simply operating at times when they were not needed.
This is a typical example of a system that may be functioning correctly from a technical perspective, while still operating inefficiently from an energy perspective.
What did we change?
Based on the measurement data, the operating settings of the air handling units were adjusted so that their operating hours were better aligned with the building’s actual occupancy and use.
No new air handling units were required.
No major building-services investment was needed.
The existing system simply needed to be operated in line with actual demand.
After the changes were made, the submetering system also allowed us to verify their effect immediately.
The result
The share of weekend consumption attributable to the air handling units fell from:
48% → 14%
This represents a reduction of 34 percentage points.
Based on the post-intervention measurements, the achieved energy saving was:
9,900 kWh/day
The result was therefore not achieved through the higher efficiency of new equipment.
It came from operating the existing system more appropriately.
What does this case show?
When analysing a building’s energy consumption, it is worth asking not only:
“What consumes the most energy?”
but also:
“What is consuming energy when it should not be?”
The power demand of an air handling unit may be completely justified while the building is occupied.
But if the same system continues to operate in an empty office building at the weekend, that same energy use can become unnecessary consumption.
Submetering does not only reveal the largest consumers. It also shows when they operate and what share of total consumption they represent.
This information can then support well-founded, specific operational decisions.
Do you know how much energy your building consumes when it is empty?
Submetering data can help separate weekend, night-time and other unnecessary consumption and show where operational changes may deliver meaningful savings.