More Information on the Site and Statistics
About the Stats
What is Demand?
Demand, a key concept in energy management, refers to the amount of energy needed per unit of time to do some work; it can be thought of as the amount of power required for a device or system to do its job. This concept is the same whether considering the electrical demand of a single light, or the energy demands of large HVAC systems.
Consider a lightbulb rated at 60 watts. The "watt" measurement refers to the bulb's demand, and in this case the rating means that the bulb needs 60 units of energy every second to light up. If you turned on a lamp with 3 of these bulbs, your electrical demand would increase by 180 watts (3 bulbs * 60 watts per bulb = 180 watts).
As devices and HVAC systems start, stop, or need more power to function, a building's energy demand changes. In your home, every time you turn a television on or off, start the microwave, or flip a light switch, your demand for electricity goes up or down. Across campus, as the weather changes buildings need more or less energy to meet heating and cooling needs.
This means demand changes from moment to moment, similar to temperature. So, this site uses line graphs to illustrate a building's demand over time. However, a building's current energy demand is displayed as a gauge, or as a bar chart when comparing the demand of multiple buildings.
What is Consumption?
Consumption, another key term in energy management, refers to how much energy is actually used (consumed) over a certain period of time. When you pay bills for residential electricity, the charge is usually based on your monthly consumption: how much electricity you actually used over the course of the month.
As devices and systems demand energy over time, energy consumption grows and grows. Say you turn on a lamp at home with two 50 watt bulbs (and pretend you aren't using any other electricity). If you leave this lamp on for 1 hour, your electrical consumption would be 100 watt-hours (2 bulbs * 50 watts per bulb * 1 hour = 100 watt-hours). If you leave it on for 2 and a half hours, your electrical consumption would be 250 watt-hours (2 bulbs * 50 watts per bulb * 2.5 hours = 250 watt-hours).
It is important to remember that consumption is a measurement taken over a period of time, similar to how you might track how much sugar you eat. Asking "How much sugar am I eating right this second?" is not as useful, because that varies from moment to moment; instead you ask: "How much sugar have I consumed so far today? How much did I consume yesterday?" On this site, consumption is illustrated with bar charts comparing how much energy each building used over different time periods and in various forms.
How does demand relate to consumption?
Demand and consumption are two of the most fundamental concepts when examining energy usage. They are related but distinct, and knowing their similarities and differences is valuable.
One way to think of it is that demand over a period of time results in consumption. A water faucet is a good example to demonstrate this relationship:
- Imagine the faucet on your kitchen sink lets out 2 gallons of water every minute on the highest setting. If you turn the faucet on full-blast, the demand for water would be 2 gallons per minute (GPM). However, how much water you actually consume depends on how long you let the faucet run. If the water runs at this rate for 1 minute you consume 2 gallons, if it runs for 2 minutes you consume 4 gallons, for 10 minutes 20 gallons, and so on. While the faucet is on full blast the demand for water stays the same (2 GPM), but how much water is actually used up (the consumption) increases the longer the faucet runs.
- An important feature of this relationship is that higher demand results in higher consumption. Consider a different faucet that lets out 6 gallons of water every minute; if you let this faucet run for 3 minutes, you will consume 18 gallons of water. Compare that to the first faucet: it would need to run for 9 minutes to consume the same amount. Because your demand with the second faucet (6 GPM) is 3 times your demand with the first faucet (2 GPM), it consumes water 3 times as quickly. If you let both faucets run for the same amount of time, the second one would consume triple the amount of water.
Put another way, demand is the rate of energy consumption. Residential customers are usually billed for utilities based solely on consumption, but for large institutions such as UNMC and Nebraska Medicine demand also plays a roll. While low demand over a long period of time can result in the same consumption as high demand over a short period of time, it’s cheaper to have lower demand over a longer period, because this places less stress on the systems involved.
What is EUI?
EUI (Energy Use Intensity) is a benchmark developed as part of the Energy Star standards. It provides a quick metric for judging a building's overall energy efficiency.
Determining a building's EUI requires two parts:
- First, the building's total energy consumption must be calculated, using the following steps:
- On campus, buildings consume energy in three main forms: electricity, steam (for heating), and chilled water (for cooling). When calculating energy consumption, different units of measurement are used for the different types. (See below for more information on the measurements.)
- These different units of measurement can all be converted to one, standard measure of energy: the British Thermal Unit (BTU).
- Once the consumption measurements for electricity, heating, and cooling are converted to BTU's, they can be added together to determine a building’s total energy consumption.
- Next, that grand total is divided by the total square footage of the building (the total amount of space in the building available for use). This results in a measurement of BTU’s per square foot.
In other words, it tells the total amount of energy a building needs to operate per square foot of usable space. This is helpful because very large buildings naturally take more energy than smaller buildings (Buffett Cancer Center compared to Truhlsen Eye Institute). Dividing by the square footage allows us to "level the playing field" and compare energy efficiency across buildings regardless of size.
About the Measurements
What types of energy do buildings use?
In order to keep a building up and running, to meet the power requirements of the occupants and maintain a comfortable environment, it needs a constant supply of energy. Depending on what work the energy needs to do, it may be delivered in different forms. For example, your home uses electricity to run appliances, devices, and probably an air conditioner for cooling. Presuming you use a furnace for heating, that energy comes most commonly in the form of natural gas because it is better suited to the task.
For an entire campus with many large buildings, such as that shared by UNMC and Nebraska Medicine, it would be inefficient and hazardous to simply use larger versions of residential heating and cooling systems. Our buildings use energy delivered in three main forms:
- Electricity is used to power the vast variety of appliances, from lights to computers to research equipment.
- Steam is used to provide buildings with efficient, even heating.
- Chilled water is used for cooling buildings on hot days.
Because steam and chilled water can be generated in a central location and delivered across campus on-demand, they offer far more efficiencies than equipping every building with its own A/C unit or furnace. It also allows for the implementation of many features, such as heat recovery devices, designed to push energy efficiency even farther. UNMC and Nebraska Medicine still purchase electricity from OPPD. Below, you can find out more about how we measure usage of these types of energy.
How is electricity measured?
Electricity is a familiar form of energy, one that touches every part of modern life. Lighting, computers, vital sign monitors, MRI machines, freezers, and even the coffee maker all contribute to how much electricity a building uses.
The basic unit for measuring electrical demand is the watt, which represents how much energy something needs per second to function. A demand of 1 watt means something needs 1 unit of energy every second; a demand of 5,000 watts means it needs 5,000 units of energy per second. (A "unit of energy" in this case is the joule, pronounced "jool".)
Other common measurements include the milliwatt (0.001 watts), the kilowatt (1,000 watts), and the megawatt (1,000,000 watts). Because large buildings need so much electricity, their demand is typically shown in kilowatts (abbreviated kW); a demand of 5,000 watts is equal to 5 kW.
Examples of electrical demand of some appliances (how much power they need to operate):
- 60 watt incandescent bulb: 60 watts
- CFL bulb, 60 watt equivalent: 18 watts
- Microwave oven: 1,200 watts (1.2 kW)
- MRI machine: 40,000-50,000 watts at peak operation (40-50 kW)
For measuring electrical consumption, the basic unit is the watt-hour. It's calculated by multiplying demand by an amount of time: a demand of 1 watt for 1 hour results in a consumption of 1 watt-hour, 50 watts for 2 hours results in a consumption of 100 watt-hours. In the real world, the demand of a building is constantly changing, so electrical meters are used to accurately track and record consumption.
Since large buildings use so much electricity, their consumption is typically shown in kilowatt-hours (1,000 watt-hours), abbreviated kWh. As a frame of reference, according to the U.S. Energy Information Administration, "In 2015, the average annual electricity consumption for a U.S. residential utility customer was 10,812 kilowatt-hours (kWh), an average of 901 kWh per month". In contrast, the Durham Research Center consumed over 770,000 kWh of electricity in January of 2017 alone.
How is heating measured?
On campus, many of the buildings use steam for heating. In a central boiler, water is heated and turned into steam; in other words, energy is transferred to the water. The steam is then piped to buildings or rooms, where that energy is radiated out as heat, keeping the location warm. This cools off the steam, so it goes back to the boiler to start the process over again.
In the field of steam heating, an important term is "pound of steam"; this refers to the amount of steam created by boiling one pound (about one pint) of water. The amount of energy held by one pound of steam depends on how hot it is.
The steam we use is heated to about 350 degrees Fahrenheit. Using some other calculations, we can determine that one pound of our steam contains about 1,193 units of energy. (A "unit of energy" in this case is the British Thermal Unit.)
Heating demand is measured in pounds per hour (abbreviated lbs/hr); this measurement expresses how much energy (in the form of steam) is needed per hour to keep a building warm. A demand of 2 lbs/hr means 2 pounds of steam, or 2,386 (1,193 * 2) units of energy, are needed per hour to keep the building warm. This number is small for the sake of example; large buildings routinely demand thousands of pounds per hour.
Heating consumption is measured in pounds (abbreviated lbs). As with other consumption measurements, this is calculated by multiplying demand by a period of time. If you have a demand of 10 lbs/hr for 2 hours, the consumption for that period of time would be 20 lbs. It took 23,860 units of energy (20 * 1,193) to keep the building warm over that 2 hour period. Again, these numbers are deliberately small; the Durham Research Center routinely consumes 100,000 - 200,000 pounds per day during winter.
How is cooling measured?
For cooling, most buildings on campus use a chilled water system. Water is cooled in large central chillers to about 40 degrees Fahrenheit, then pumped to buildings; once there, air handling units run air over pipes containing the chilled water, which cools the air (the air is "conditioned"). The conditioned air can then be delivered to rooms in the building, while the water returns to the chiller to start the process over.
Terminology from the beginning of refrigeration, when cooling was achieved with large blocks of ice, is still used today. As refrigeration technology advanced, people had a hard time grasping how much cooling new systems could provide, so they started rating these new systems by "tons of refrigeration". A system rated at 1 ton of refrigeration provided the same amount of cooling as 1 ton (2,000 pounds) of ice that melted over the course of 24 hours.
Today, cooling demand is still measured in tons. In order to melt 1 ton of ice in 24 hours, you need to supply 12,000 units of energy per hour, so a building demanding 1 ton of cooling needs 12,000 units of energy per hour to keep the indoor temperature comfortable. If the demand increased to 5 tons, it would need 60,000 units of energy per hour to keep cool (5 * 12,000 = 60,000). (A "unit of energy" in this case is the British Thermal Unit.)
Cooling consumption, measured in ton-hours, expresses how much energy it took to keep a building cool over a certain period of time. Like other consumption measurements, it's calculated by multiplying demand by a certain amount of time. If a building demands 2 tons of cooling for 1 hour, its consumption for that hour would be 2 ton-hours; if it demands 10 tons for 4 hours, its consumption would be 40 ton-hours for that time period.