Define internal loads (occupants, lighting, equipment)

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Heat gains from lighting, equipment, and occupants. (Source: IBPSA-USA BEM Workshop)
Example lighting schedules for an office building model (data source: COMNET)

Internal loads refer to features in a building that generate heat (or act as heat sinks). These may come from occupants (people), lighting, or equipment ("plug loads"). In the case of lighting and plug loads, these loads also consume energy. Accurate inputs of the internal loads will lead to better estimation of end-use energy consumption, and loads that must be offset by heating and cooling (HVAC) equipment.

The input process to define internal loads are typically in the form of peak values, such as installed lighting power, and fractional schedules that represent the fraction of that peak that occurs at each timestep. In other words, the peak is when all the lights are on, all equipment running, and the space is fully occupied. The schedules represent hour-by-hour changes, such as turning off some lights or people leaving for lunch. Note that weekday and weekend schedules will often differ significantly in many building types.

Typical input properties

The following are common inputs related to internal heat gains.

  • Occupants
    • Peak number of occupants (occupancy density), typically entered as ft2/person
    • Occupancy schedule, a schedule indicating the fraction of the peak number of people that is present at each timestep
  • Lighting
    Daylight dimming - control options
    • Peak lighting power, typically entered in W/ft2
    • Lighting schedule
    • Automatic daylighting control, a capability of many simulation programs.
  • Plug loads
    • Plug load peak power, typically entered in W/ft2
    • Plug load schedule
  • Other internal loads
    • Cooking equipment energy consumption and heat gain
    • Refrigerators

Choosing Appropriate Inputs

Evolution of ASHRAE Standard 90.1 Lighting Power Allowance for an Office Building from 1975 to 2019

Things to keep in mind when choosing internal loads inputs:

  • An important consideration when choosing inputs for a simulation model is that these internal heat gain inputs will typically be lower than the corresponding inputs used for worst-case cooling load calculations. The goal of a simple box model is usually to estimate annual energy consumption, and the appropriate inputs for internal heat gain are those that represent typical loads, not worst case loads.
  • Some default schedules for receptacle power have unrealistically low values for off-hours, typically night and weekend. Monitoring studies show a significant fraction of receptacle loads do not shut off, and nighttime values of 20% to 40% are typical.[1][2] A realistic schedule is important if an accurate energy consumption estimate is desired.
  • Typical interior lighting power has dropped significantly over the years as lighting technology has improved. The adjacent figure shows the change in lighting power allowed for an office building by ASHRAE Standard 90.1 over time. Check to see that your lighting power inputs are up to date.
  • Nameplate values for equipment power such as copiers or computers are usually much higher than the actual energy demand. Find data sources such as tables in the ASHRAE Fundamentals Handbook,[3] which are based on measured energy demand.
  • Cooking equipment energy does not necessarily all end up as heat in the space if it is located under an exhaust hood.

Assumptions for early stage modeling

At any stage of energy model development, some assumptions are needed about likely space usage, internal heat gains and infiltration. At the simple box modeling stage, many of these assumptions will require educated guesses, and this section provides some guidance on choosing appropriate values for occupant density, lighting power, plug load power and other potential sources of internal heat gain.

There are several potential sources of information for model inputs. Simulation software often provides default values, which may depend on building type. When defaults are used, the values should be recorded and provided to the design team for review. Energy codes are also a common source for lighting power inputs; the lighting power density allowance for the relevant building type can be a good choice if the lighting system has not yet been designed. Other reference sources also provide typical values for internal gain inputs and are discussed below.

Comparative analysis vs. EUI estimation

When one of the goals of the simple box modeling exercise is to estimate EUI or energy performance compared to a benchmark, then extra attention to internal load inputs is warranted, because lighting, plug loads and other process loads are often a big part of the total energy consumption. On the other hand, if the goal is to compare relative performance of design alternatives, then using default values for internal loads may be a reasonable choice.

Sources for default internal gain assumptions

As noted above, there are a number of sources that provide information about internal loads that can be used to verify or replace simulation software default inputs.

Read more here: Sources for default internal gain assumptions

Detailed design input data

Detailed design model lighting inputs should represent the project's actual lighting design as specified on the lighting drawings for design development and construction documents phases. Compliance models should use inputs that represent the final design. Occupancy and equipment power can be fine-tuned at this stage, but some assumptions may still be needed even during the detailed design phases.

Lighting system inputs

The drawing set includes detailed lighting design plans as part of the electrical drawings.

The lighting floorplan overlays a view of the ceiling onto the architectural floorplan. It shows the locations of the luminaires and usually includes labels that identify the luminaire types. You will often also see sensors used for lighting control noted on the plans.

Labeling and naming conventions will vary depending on the firm, but the example images and accompanying descriptions provided here give a good indication of typical practices.

In addition to a lighting plan, the electrical drawing set will also include a luminaire schedule, or lighting fixture schedule. Here you can look up the labels shown on the lighting plans and find more information about the luminaires. The most important for our energy models is the input power per luminaire.

By counting the number of luminaires in each zone (in the lighting plans) and multiplying by the luminaire power per fixture, you can calculate the total lighting power for each zone in the model. Often, zones will have more than one luminaire type, so be sure to get accurate counts of each luminaire.

Equipment (plug load) power inputs

Even for a fully designed building, plug loads can be difficult to estimate. This is due to the variability of how occupants will use a given space. One office may have a single desktop computer, another may have multiple monitors, printers, air purifiers, etc. It is very difficult to know what occupants will plug in to the receptacles!

A helpful first step is to discuss with the design team or owner what the expected use of each space will be. If there is a company policy to use certain types of computers or other equipment, then this can be used to create better estimates. If you do get good information about what type of equipment is likely to be plugged in in each space, then you can look up the power requirements of each item.

Refer to this page for resources to look up equipment power assumptions: Sources for default internal gain assumptions

Occupancy inputs

Similar to plug loads, occupancy is also difficult to estimate. Some offices may have a single occupant, others may have multiple, even if they're the same size. It's best to discuss with the design team or owner to see if they have any insight into how the rooms will be occupied.

In the case of spaces with fixed seating (e.g. an auditorium) you can count the number of seats to determine the peak occupancy.

Refer to this page for resources to look up occupancy assumptions: Sources for default internal gain assumptions

Schedule data

Schedules that define how the loads vary over the course of a day/week/year are often based on assumptions because it is difficult to know precisely how the building will be used and how the occupancy levels will vary over time.

However, in detailed design phases, it often makes sense to fine-tune the "default" schedules that are available in software tools, or defined in lookup tables (e.g. in an energy code or standard). For example, the default schedule for occupancy for an office may assume that a building is occupied between 9:00am and 5:00pm, but if the owner plans to have longer work hours, or operate in shifts, then the schedule would need to be adjusted to account for the planned use.

Refer to this page for resources to look up schedule assumptions: Sources for default internal gain assumptions


  3. "2021 ASHRAE Handbook—Fundamentals".
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