Work, power, energy, and efficiency. We hear these terms all the time, often used interchangeably.
but what do they really mean when we’re talking about buildings?
While they might seem like basic physics concepts, understanding these definitions is crucial when comparing different building systems, evaluating efficiency, and making buildings — you guessed it — an itty bit better.
This blog post will break it all down for you. Let’s dive in!
OR watch the video that goes along with this post!
What is Work?
According to Britannica, work is:
“The measure of energy transfer that occurs when an object is moved over a distance by an external force, at least part of which is applied in the direction of the displacement.”
In simpler terms:
Work happens when something moves over a distance.
We often think of something physical — like moving a box across the floor — but in buildings, the “object” could be:
Chemical energy → atoms in a fluid
Electrical energy → electrons moving
Thermal energy → air particles transferring heat
It’s important to remember that work doesn’t always mean something you can see or touch. It includes all the small, often invisible, interactions happening inside a building system.
What is Power?
According to Britannica, power is:
“The quantity of force required to cause a unit displacement.”
It can also be described as: Power is the rate at which work is performed.
Another way of saying this:
Power measures how quickly work is being done.
For example, if you increase the rate of work (move something faster or with more force), you’re increasing the power involved.
What is Energy?
According to Britannica, energy is:
“The capacity for doing work.”
And remember the golden rule from physics class: Energy cannot be created or destroyed.
The 2 States of Energy:
Potential energy → stored, waiting to be used
Kinetic energy → active, in motion
The 6 Forms of Energy:
- Electrical
- Chemical
- Radiant
- Mechanical
- Thermal
- Nuclear
What do the states and forms of energy have to do with each other?
Since energy can neither be created nor destroyed, energy can only be transferred from one form to another. This is done by moving between potential and kinetic energy through the different forms, kinetic and potential.
Example:
Take a toy car with batteries.
The batteries sitting on the table → potential chemical energy
When turned on, the car moves → kinetic mechanical energy
When charging, the plug provides kinetic electrical energy to refill the battery’s chemical energy
Some energy is always lost as heat — no process is 100% efficient
What is Efficiency?
According to Britannica, efficiency is:
“The ratio of useful work performed by a machine or process to the total energy put in.”
Efficiency = (Work Out) / (Energy In)
Building Systems
Why System Boundaries Matter
When discussing energy efficiency, it’s important to define system boundaries.
Example:
You might think the toy car’s boundary is just the batteries and wheels. But zoom out:
Where does the electrical energy come from? → the power grid
Where does the grid get energy? → power plants, fossil fuels, or renewables
In building systems, you can analyze efficiency piece by piece or across the entire chain, depending on your goals.
Building Systems and Their Interactions
Here’s a snapshot of key building systems:
Systems typically analyzed for efficiency:
- Building envelope (insulation, walls)
- HVAC (heating, ventilation, air conditioning)
- Electrical systems
- Plumbing systems
- Lighting systems
- Building automation + control systems
- Onsite renewable energy systems
- Water treatment + distribution
- Refrigeration (for commercial/industrial)
Other essential (but less efficiency-focused) systems:
Fire protection + life safety
Elevators + escalators
Communication + security systems
How Building Systems Affect Each Other
Lighting → Heat loads
Upgrading from fluorescent to LED reduces heat produced by lighting. In winter, that means the heating system must work slightly harder to maintain indoor temperatures — a factor considered in detailed energy analyses.
Insulation → HVAC efficiency
Adding insulation reduces heat transfer through the building envelope, making it easier (and cheaper) for your HVAC system to maintain comfortable indoor conditions.
Ready to go Deeper?
If you found this helpful, make sure to check out Part 2, where I explain the common efficiency acronyms you’ll encounter in HVAC and building systems.
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Thanks for reading. See you next time!