Building Energy Loads
Energy "loads" are how much energy your building needs. These demands can be provided by electricity, fuel, or by passive means. A building's energy loads depend on both its site and program.Thermal loads, such as heating and cooling to keep people comfortable, come from the external environment (like sun, wind or weather) and internal operations (like heat generated by people and equipment).
Lighting load is the energy used to power electric lights; plug load is the amount of electricity used for other equipment like computers. These loads are determined by the building’s intended use, its occupancy, and its scheduling. In short: its program.
Use Energy Loads to Your Advantage
By understanding the building’s thermal loads and its intended use, you can more effectively use energy from the sun and wind to passively heat, cool and ventilate your building, light your building, and design efficient HVAC systems. You can even generate energy on-site using things that would otherwise be energy loads.Internal Loads
Internal loads are the activities and energy sources that demand energy inside the building. There are several kinds of internal loads.Lighting loads are the energy used to power electric lights; plug loads are the electricity used for other equipment, like computers. These loads are determined by the building’s intended use. When deciding which products to use, look at third-party quantitative reviews, or read the maximum power use listed on product specification sheets (average power use data is usually not available because it can vary greatly by usage.)
Thermal loads, such as heating and cooling, come from the external environment (like sun, wind or weather) and internal operations (like heat generated by people and equipment). These loads need to be managed to keep the building comfortable.
The thermal load of lighting and equipment is generally equal to their energy use: when a light fixture converts a watt-hour of electricity into photons, those photons bounce around the room until they get absorbed, turning their light energy into heat energy. Likewise, all the electrical energy that the lighting fixture did not turn into photons (due to inefficiency) turns directly into heat energy. The same is true of equipment: electrical energy used to move mechanical parts is transformed into heat via friction, energy used to power electronics turns into waste heat, etc.
The thermal load of people depends on the number of people and their activity level. It can be as little as 70-80 watts for an adult sleeping to over 1,000 watts for an athlete engaging in intense exercise.
| Activity | Watts |
| Sitting | 100 |
| Standing at ease / Conversation | 130 |
| Eating meal | 130 |
| Strolling | 160 |
| Housekeeping | 175 |
| Heavy work (e.g. carpentry) | 270 |
| Fast walking / Hiking | 400 |
| Long distance running | 1,000 |
| Sprinting | 1,600 |
Thermal loads from people doing different activities1
External Loads (Envelope Loads)
External heating and cooling loads come from the weather. The sun may heat up the building too much, or the outside air may be too cold or too humid. Since all these effects reach into the building through the outside envelope, they are also called "envelope loads". See climate considerations and heat transfer basics for more information about the outside environment heating or cooling the inside environment.Internal vs. External Loads
Massing and the building program also help determine how important internal heat loads are compared to external loads from sun and wind. Densely populated buildings with high activity and/or energy-intensive equipment are generally dominated by internal loads, while sparsely populated buildings with little activity or equipment are generally dominated by external loads.Energy Intensiveness
When comparing buildings, people not only talk about total energy demands, but also talk about "energy intensiveness". Energy intensiveness is simply energy demand per unit area of the building's floorplan, usually in square meters or square feet. This allows you to compare the energy demand of buildings that are different sizes, so you can see which performs better.Site Energy vs. Source Energy
Energy intensiveness only considers the amount of electricity and heat that is used on-site ("secondary" or "site" energy). It does not consider the fuel consumed to generate that heat or electricity. This "primary" or "source" energy can be generated on-site or at a power plant far away.When measuring energy used to provide thermal or visual comfort, site energy is the most useful measurement. But when measuring total energy usage to determine environmental impacts, the source energy is the most accurate measurement.
The electricity that you use on your site, may use much more energy upstream.
1kW of site electricity requires 3.3 kW of source energy in the United States.
Sometimes low on-site energy use actually causes more energy use upstream. For example, 2 kW of natural gas burned on-site for heat might seem worse than 1 kW of electricity used on-site to provide the same heating with a heat pump. However, 1 kW of site electricity from the average US electrical grid is equal to 3.3 kW of source energy, because of inefficiencies in power plants that burn fuel for electricity, and because of small losses in transmission lines. So in fact the 2 kW of natural gas burned on site is better for heating.1kW of site electricity requires 3.3 kW of source energy in the United States.
1 kW of site electricity from a solar panel on the building's
roof is equal to 1 kW of source energy,
because the solar panel itself is the source.Autodesk Sustainability Workshop
roof is equal to 1 kW of source energy,
because the solar panel itself is the source.Autodesk Sustainability Workshop
