Heat Pumps 101: The Difference Between Energy & Power

This is the first in a series of Heat Pump 101 videos from instructor and author, John Siegenthaler, P.E.. You can enroll in John’s Heat Pump 101 course for free to learn the very basics of heat pumps in less than an hour.

Transcript, “The Difference Between Energy & Power”:

Let’s look at some basic concepts from thermodynamics. These are concepts that are very important to understand in order to grasp how heat pumps work and to see how these concepts are extended in later modules.

We’ll start off with the difference between the concepts of energy and power. These words, energy and power, are often loosely used in casual conversation.  Here are some examples: 

“Look at my power bill from the local utility company.” Sounds like a normal sentence, and many of us understand what the person saying this is trying to convey, but from a technical standpoint this sentence is an inaccurate.

Here’s another one: “How much power does a light bulb use every hour?” Or, “My generator lacks the energy to keep all my appliances going.”

Again, from a technical standpoint these sentences really are meaningless. They’re misrepresenting the concepts of power and energy.

So let’s look at what energy is.

Thermodynamically, energy is defined as the ability to do work.  Energy is a concept that we deal with every day, all of us, and yet we don’t see energy. Has anybody reading this ever seen a BTU or a kilowatt hour? You can see the results of what that energy has done, but you can’t directly see the energy.

Energy exists in different forms.

Thermal energy (or heat), electrical energy, mechanical and chemical energy are the most common forms that are constant present in our lives.  We’re gonna talk a lot about thermal energy in the context of heat pumps.

Nuclear energy would be another form of energy. It’s used in large scale systems such as utility power plants, and submarines, but it’s not something we directly work with on an everyday basis.

A very important concept with energy is that it can be converted from one form to another.

For example, electrical energy can be supplied to run the motor in the compressor of a heat pump.   That motor will convert the electrical energy to mechanical energy to move the scroll or the piston inside that compressor, both of which change the pressure and temperature of the refrigerant as it flows through the compressor. Ultimately the energy that comes out of the compressor is a form of thermal energy.

Another important concept is that energy cannot be created or destroyed.

We have to account for all the energy that enters a process. For example, we can’t send 10 units of energy into a process and only be able to account for nine units coming out.

What happened to that other unit of energy? It can’t just disappear. This simple concept (e.g., what goes in has to come out), forms the basis of what are called energy balance equations. And those equations are the basis of of many different engineering processes and methods of analysis, including the performance of any heat pump. 

Another way to think about energy is it is an amount. It is not a rate.

The amount of energy present in any process or device can be expressed in many different units. A common one that are used to describe the operation of thermal system is BTU, which stands for British Thermal Unit.  Others include kilowatt• hours, which means kilowatts times hours. The • between the two words means multiplication.

In the metric system Joules would be a unit of energy. In mechanical systems, foot•pounds is a unit of energy. If you buy natural gas you’ll likely see that the energy you purchased is expressed in therms.  A therm is defined as 100,000 BTU.

Now let’s contrast energy with power.

Power is the rate of energy transfer. As with energy, power can be expressed in many different units.  In thermal systems, one of the most frequently used units is BTUs per hour (abbreviated as BTU/hr). If we compare BTU/hr as a rate, to BTU as an amount, you can see the difference.

Other units of power included watts or kilowatts.  Although we often associate these units with electricity, they can also be used to express the rate of energy output of devices such as boilers or  heat pumps.  In Europe and Asia, heat pumps are often specified by their thermal kilowatt output.

Mechanical power is often expressed as foot•pounds per second (ft•lb/sec), or as horsepower. One horsepower equals 550 ft•lb/sec.

Any unit of power can be converted to another equivalent unit of power. For example, we could express the rate of heat loss from a building in units of horsepower, rather than BTU/hr.  Although this is possible it’s not customary.  Many HVAC professionals could quickly envision a typical house with a design heat loss rate of 50,000 BTU/hr, but very few of any of them would immediately ‘picture’ the same building if it was solely described as having a design heat loss rate of 19.65 horsepower.

So to summarize thus far, energy and power are two different, but related concepts. Energy is an amount, and power is a rate.

The relationship between energy and power can also be expressed as:

Energy is power times time.

This relationship is analogous to that between distance and speed. 

Distance is speed times time.

For example, if a car is moving steadily at 50 miles per hour, and it does so for two hours, we multiply 50 times 2, to get the distance travelled as 100 miles.

Here’s an example involving a heat pump. Let’s say we have a heat pump that has an electrical power input of 4,000 watts, and it sustains that condition for 2.5 hours. How much electrical energy was transferred to the heat pump in that process? It’s a simple calculation. We have a power of 4,000 watts sustained for 2.5 hours, so multiplying power times time yields energy: 4000 watts x 2.5 hours = 10,000 watt•hours.  If we divide 10,000 watt•hours by 1000, we convert from watt•hours to 10 kilowatt• hours of energy.

The concepts of energy and power are an important part of the ‘physics’ that underlies any technical discussion of heat pumps.  We’ll rely on them extensively in subsequent portions of this course.

The next video in this series is Thermodynamic Basics, which we’ll cover in the next post. If you want to see that video now or jump ahead to the others, you can jump into John Siegenthaler’s free Heat Pump 101 course.