And they have different units of measurement!
This is no brainer, right? and we all know that power is different from energy. However, I still find it significant to write about this fact because I still come across some articles, white papers and even software interfaces in which the power and energy terms (and particularly) their units of measurements are totally misused. Energy in physics is defined as the quantitative property that must be transferred to an object to perform work on, or to heat, the object (Energy-Wikipedia). Energy is the potential/capacity of a system to do some work. On the other hand, power is the ratio of doing work in unit time (power = work/time). Power is the time rate of energy and it has no direction, no temporal span and it is just a scalar quantity.
When it comes to building performance modeling, energy seems to be more straightforward term. We have the space cooling energy vs. heating energy, there is the energy consumed by the electric lights, heat rejection equipment or the appliances. We can easily aggregate energy by time (since it has the temporal span unlike the power term). That’s why we have the building energy use intensity (EUI) of predicted energy use intensity (pEUI) metric over a year of building operation (actual or simulated) normalized by the useable floor area. The most common units of measurement for the energy term is “kWh” (in SI) or kBTU (in IP). Based on this, the units of EUI will be kWh/m^2/yr or kBTU/ft^2/yr. “Yr” term here signifies the temporal dimension which is a single year. What I see is that while people are pretty confident about the definition and the units of energy in BPM, they start to get confused about the power term during building performance presentations and interpretations. How do we use the “Power” term in BPM? Power is usually associated with heating and cooling loads of a building (we can expand it to ventilation loads, lighting loads, IT systems loads, etc.) which should be handled by mechanical and electrical systems and equipment to keep the indoor environment within acceptable levels of human habitability.
ASHRAE HOF* defines heating and cooling loads as the rates of energy (this is where you get rid of time dimension) input (for heating) or removal (for cooling) required to maintain an indoor environment at a desired temperature and humidity condition. From engineering perspective, the concept of peak load (peak heating, cooling loads) is quite significant since all indoor climate control systems and their equipment (i.e., HVAC systems) are designed, sized, and controlled based on the magnitude and temporal profile of thee peak loads. It is so important that there are an entire chapters in ASHRAE HOF that discusses heating and cooling load calculations for non-residential and residential buildings (Chapter 17 and Chapter 18). It is apparent that there is close relationship between the building (peak) loads and the size and capacity of the mechanical system. We have the space loads and the system loads and the system sizes as a consequent of the interactions. Just think about the peak loads as the weights to haul with your truck from one place to another. You will be selecting a specific engine power for truck based on the maximum weight you are planning to haul. This is your HVAC system size. And then with all the weights loaded (or sometimes less than your maximum carrying capacity) you are driving the engine for some amount of time for a certain distance and then you are consuming gas the amount of which will indicate your energy consumption for this task (this is the heating, cooling energy consumption over a given time period).
What is the unit of measurement for the building loads? This time we need to get rid of the time dimension by division so we end up with kW (in SI) and BTU/h (in IP). We can also normalize the term by the useable floor area and then start talking about the building load density/intensity with the unit of measurement as kW/m^2 (in SI) or BTU/h/ft^2 (in IP). What I see as an indication of a confusion is that some authors (of articles and program interfaces) tend to use kWh and BTU when they are referring or representing data about building loads (space heating or cooling loads). kWh and BTU are energy terms but not power (or load) terms. At one instance, I even come across a simulation output for heating, cooling energy associated with kW or BTU/h. That is a similar confusion of power with energy. It doesn’t make sense to aggregate the power term over time and come up with a total power (you can aggregate power requirements of different components though). That’s why the peak loading is an instantaneous event that take place at some point in time. However, we can plot building loads at each specific time internal and generate a loading profile. The space heating, cooling load profiles (variations over a single day or over summer months, etc.) of different building spaces highly impacts the choice and operational controls of heating and cooling systems. We can conduct a similar analysis for the building energy term and we can go ahead and sum them up for a cumulative metric of energy expenditure (which won’t be applicable for a cumulative building load or power by time).
Can we aggregate power or energy over spatial dimensions? The answer is yes we can but with some caution when it comes to the power term again. This will lead itself to another lesson for the building performance modeling where I will be discussing coincident and non-coincident space/room loads, the difference between room loads and system loads.
ASHRAE HOF: ASHRAE Handbook of Fundamentals 2017
Chapter 17: Residential Cooling and Heating Load Calculations (page 17.1)
Chapter 18: Non-Residential Cooling and Heating Load Calculations (page 18.1)
Omer T. Karaguzel, PhD
Comments