Four fans in a parking garage
In a parking garage in Oslo, four ventilation units were scheduled to run eight hours a day. The building management system (BMS) showed them on that schedule, and the BMS vendor had reported an earlier problem with them as fixed. Vibration sensors on the fans showed all four running around the clock.
Each unit drew 12.25 kW. Sixteen extra hours a day on four units is 784 kWh a day, or about 286,000 kWh a year. The owner put the cost at 158,000 kr a year, at the electricity price they were paying (note: the self-reported kWh cost of 0,5 per hour is extremely low. A more realistic baseline is around double (1 -1,3 kr per kWh), but check your own local prices before doing this exercise.
The sum behind that figure has three inputs, and anyone can do it for their own building. This article goes through the sum, what to adjust in it, and what it leaves out.
The rule: about 6,400 kr per kW per year
The cost of a unit that runs when it should be off is its fan power, times the extra hours, times the electricity price.
Cost per year = fan power in kW × extra hours per day × 365 × price per kWh
We use 16 extra hours a day and 1.10 kr per kWh, which is an all-in price. Sixteen hours a day is 5,840 hours a year. At 1.10 kr that comes to 6,424 kr for every kW of fan power, which we round to 6,400. Per day it is about 18 kr per kW.
Fan power | Cost per day | Cost per year |
|---|---|---|
8 kW | 141 kr | about 51,000 kr |
12 kW | 211 kr | about 77,000 kr |
20 kW | 352 kr | about 128,000 kr |
The 16 hours come from an ordinary working week. A unit set to run from 07:00 to 17:00 on weekdays should run 50 hours a week. Left on, it runs 168. The difference is 118 hours a week, which is just under 17 hours a day.
Adjusting the sum to your building
Power. Use what the fans draw in operation, supply and exhaust together. The figure on the motor plate is the maximum. A fan on a frequency converter that runs below full speed draws a good deal less, because fan power falls with the cube of the speed. At 80 % speed a fan draws about half of what it draws at full speed.
Hours. Count the hours between what the schedule says and round-the-clock running. A building used ten hours a day on weekdays has about 17 extra hours a day. A building in use from early morning to late evening all week has far fewer.
Some units are found running at low speed at night when they were meant to be off. We saw this where a service provider had set low speed in place of off in every building with the same type of controller. After it was corrected, the customer’s energy monitoring showed 25 to 30 % lower electricity use per day.
Price. Use your own all-in price per kWh. The garage owner above paid about half of 1.10 kr, which is why four 12 kW units came to 158,000 kr. At 1.10 kr the same fault costs about 315,000 kr a year. In another currency the rule is 5,840 hours times your price. At EUR 0.20 per kWh that is about EUR 1,170 per kW per year.
We have a calculator at calculator.soundsensing.com that does the sum with your own numbers.
What the sum leaves out
The rule counts only the electricity the fans use. Two other costs come on top.
The first is wear. A unit on a ten-hour weekday schedule that is left on runs 3.4 times the hours it was meant to. Bearings and belts wear with running hours, and filters fill up with the air that passes through them, so all of them need replacing sooner. In an office building in Luxembourg where nine units ran outside their schedule, the cost the customer confirmed included wear at about 18 % on top of the electricity.
The second is heating and cooling. A unit that runs on a winter night brings in cold air that has to be heated. Heat recovery takes back most of that heat, and the rest is paid for as district heating or electricity. We leave it out of the rule because it differs too much from one building to the next.
For both reasons the 6,400 kr rule gives a figure on the low side.
How long the fault lasts
A 10 kW unit left on costs about 175 kr a day, 1,200 kr a week and 64,000 kr a year. The yearly figure is the one that matters, because these faults last.
Two kindergartens owned by a public property owner in Norway had ventilation running day and night from 2015. Together they wasted about 120,000 kr a year at 1.10 kr per kWh. The fault was found after nine years, which puts the total above a million kr. In that time the buildings had state of the art Energy Maintenance System, and more than twenty service visits. We have written a separate article on why the monitoring and the technicians miss it.
After installing on the fans, two things happen:
We have a status meeting with the customer, manually going through all the schedules. Usually, we weed out a couple of wayward units already at this stage.
Secondly, our machine learning algorithms will spend 30 days in learning mode, after which we start sending automatic alarms.
When we put a value on a fault caught that way, we count four months of avoided cost. That is a fairly cautious assumption next to faults that ran for nine years.
Questions people ask
How much does a ventilation unit running 24/7 cost?
About 6,400 kr per kW of fan power per year, at 1.10 kr per kWh and 16 extra hours a day. An 8 kW unit comes to about 51,000 kr a year and a 20 kW unit to about 128,000 kr.
How do I calculate the energy wasted by an air handling unit running outside its schedule?
Use our calculator! (calculator.soundsensing.com) Or if you’re a DIY person, you can multiply the fan power in kW by the extra hours per day, by 365, and by your price per kWh. Use the power the fans draw in operation, supply and exhaust together.
Why 16 extra hours a day?
A unit scheduled from 07:00 to 17:00 on weekdays should run 50 hours a week. Left on, it runs 168. That is 118 extra hours a week, or just under 17 a day.
What is the rule in Euros?
Just swap the electricity price for your local price. This varies, but is usually between 0.15-0.20 EUR per kWh. Sixteen extra hours a day is 5,840 hours a year. Multiply that by the fan power and your own price per kWh. At EUR 0.20 per kWh it is about EUR 1,170 per kW per year.
Is electricity the whole cost?
No. The unit also wears faster, and in winter the air it brings in has to be heated. In one case the customer put wear at about 18 % on top of the electricity.
About Soundsensing
Soundsensing provides AI-based predictive maintenance for HVAC in commercial buildings in the Nordics, Benelux, Germany and France. Vibration sensors on fans, pumps and motors detect units running outside schedule and components wearing toward failure, before the building management system or the service technician sees it. As of 2026 it monitors around 4,000 assets in roughly 500 buildings. Soundsensing leads the research project BeHoVent with SINTEF, Interfil, OBOS Eiendom and Eiendomsspar, funded by the Research Council of Norway, on replacing fixed service intervals on ventilation units with data-driven maintenance.
Ole Johan A. Bjerke is co-founder and CEO of Soundsensing. Written November 2026.