3.4.5 Receptacle and Process Loads

Applicability

All building projects except multi-family residential

Definition

Receptacle power includes computers, monitors, printers, copiers, vending machines, residential size refrigerators, personal space heaters and other equipment loads normally served through conventional electrical receptacles. Small servers are included with receptacle power, but large data centers are generally modeled separately. As a rule-of-thumb, if the servers are in a room with a dedicated HVAC system, they should be modeled separately. Receptacle power does not include task lighting, equipment used for HVAC purposes, open or closed refrigeration cases, walk-in freezers and refrigerators, elevators, and commercial cooking equipment. These loads should be modeled separately.  See Sections 6.4.6, 6.4.7 and 6.4.8 for guidelines and requirements on how to model these process loads. Receptacle power is specified at the space level and is modified by the receptacle schedule. Values in the default receptacle schedules for each hour approach but do not exceed 1.0 (see Appendix C).

Receptacle power is generally considered an unregulated load and is treated as a neutral dependent input; no credit has been offered for savings. Identical assumptions are made for both the baseline building and the proposed design.

Offering credit for receptacle loads is difficult due to their temporal nature and because information is not always available on what equipment will go into the building at the time the plans are being reviewed. Tenants also have the ability to plug and unplug devices or switch them out for different equipment, adding to the difficulty of assigning credit for promised energy efficiency.

COMNET provides a procedure for crediting plug load reductions for building owners and/or managers who are willing to make a long-term commitment to purchase ENERGY STAR equipment. 

Units

Total power (W) for the space or power density (W/ft²)

Input Restrictions

Receptacle loads in the proposed design may be calculated in one of three ways:. The first method is neutral independent, the second method is neutral dependent, and the third method treats equipment that result in receptacle loads as an asset and credit may be taken for power reductions from surveyed devices. The three methods are as follows:

1. The COMNET recommended defaults from Appendix B may be used, in which case the same values are used for the baseline building and there is no credit for reductions in the proposed design.

2. If detailed information is known, the receptacle power can be calculated using Equations 3.4.5-1 and 3.4.5-2. In this instance, the energy analyst must be able to estimate the number of personal computers, the number of printers and the number of other equipment in the space, as listed in Table 3.4.5-1. If detailed information is not available, then Method 1 must be used. With Method 2, “Baseline” power from Table 3.4.5-1 shall be used for both the baseline building and the proposed design.

3. If detailed information is known and the owner/property manager is willing to make a long term commitment to require ENERGY STAR equipment throughout the building, then Method 3 may be used. Method 3 is the same as Method 2, except that the proposed design may use reduced equipment power for surveyed devices from Table 3.4.5-1 based on the ‘Length of ENERGY STAR Commitment’.

Baseline Rules

With Methods 1 and 2, the receptacle power in the baseline building shall be the same as the proposed design. With Method 3, the ‘Baseline’ equipment values from Table 3.4.5-1 are used with Equations 6.4.5-1 and 6.4.5-2 to determine the equipment power for the baseline building.

Requirements for Long Term Commitment

One of the largest hurdles is establishing accountability for receptacle load reductions; savings must be verified and credible. A commitment to future good behavior must  be documented appropriately, either in leasing language, within corporate or governmental resolutions, or in tenant manuals to ensure that ENERGY STAR equipment will be used not only initially but also for future replacements. The inability to verify long term commitments is one of the obstacles to offering credits for plug load reductions, since the equipment that makes up receptacle loads is short lived and generally replaced within a 3-5 year timeframe.

Equations for Estimating Receptacle Loads (Methods 2 and 3)

This procedure provides a means for more accurately estimating plug loads (Method 2) and for taking credit for energy reductions when the owner is willing to initially install all ENERGY STAR equipment and make a long term commitment to purchase ENERGY STAR equipment for replacements and additions (Method 3). COMNET Methods 2 and 3 are based on procedures developed by NREL¹ which estimate receptacle and process power density based on a count of computers, printers and other equipment in the space (surveyed equipment). The procedure is shown in Equations 3.4.5-1 and 3.4.5-2.

(Equation 3.4.5-1)

$$P=\left ( C_{sd}\cdot PD_{sd}+PD{_{misc}} \right )$$

where

P           is the estimated power density for the space in W/ft².

PD_sd     is an estimate of receptacle power from personal computers, monitors, servers, printers and other “surveyed devices” determined from Equation (3.4.5-2). Units are W/ft².

PD_misc  is an estimate from Appendix B of miscellaneous receptacle power for equipment not specifically accounted for in PD_sd. This value depends on the occupancy of the space.

C_sd,      is an adjustment coefficient from Appendix B based on the occupancy of the space. This coefficient along with PD_misc accounts for equipment that is not included among the “surveyed devices”. C_sd scales the equipment power of “surveyed devices”, while PD_misc is a constant.

d           is a diversity factor from Appendix B based on the occupancy of the space.

(Equation 3.4.5-2)

$$PD_{sd}=\frac{\begin{bmatrix}
N_{PCdt}\cdot P_{PCdt}+N_{PCnb}\cdot P_{PCnb}+N_{Mon}\cdot P_{Mon}+N_{Svr}\cdot P_{Svr}+\\ 
N_{POS}\cdot P_{POS}+N_{PRNTlas}\cdot P_{PRNTlas}+N_{PRNTink}\cdot P_{PRNTink}+\\N_{Copy}\cdot P_{Copy}+N_{Fax}\cdot P_{Fax}+N_{Refrig}\cdot P_{Refrog}+N_{Vend}\cdot P_{Vend}
\\ 
\end{bmatrix}}{Area}$$

where

N_xx        is the number of devices in the proposed design for the “xx” surveyed device in question. See Table 3.4.5-1 for a list of surveyed devices to be included.

P_xx        is the nominal mean power from Table 3.4.5-1 for the “xx” surveyed device in question.

Area      is the area of the space in ft².

Credit is limited to equipment listed in Table 3.4.5-1 where the ENERGY STAR program applies, including PCs, monitors, copiers, laser and inkjet printers, vending machines, and refrigerators. No credit is offered for equipment not listed in Table 3.4.5-1.

1. Griffith, B, et. al., Methodology for Modeling Building Energy Performance across the Commercial Sector, Technical Report, (NREL/TP-550-41956, March 2008, Appendix C, Section C.14. Note that elevators and escalators have been removed from the NREL equation, since they are treated separately.

Applicability

Multi-family residential building projects

Definition

For multi-family projects, receptacle power includes the refrigerator, cooking equipment, clothes washer, clothes dryer and other major appliances. Receptacle power is specified at the space level and is modified by the receptacle schedule. Values in the default receptacle schedules for each hour approach but do not exceed 1.0 (see Appendix C).

Much receptacle power in residences is considered unregulated load and is modeled as a neutral independent input; no credit is offered for savings. Identical assumptions are made for both the baseline building and the proposed design. However for refrigerators and clothes washers, the modeling assumptions for the proposed building can differ from the baseline building when ENERGY STAR complying equipment is used in the proposed design. 

Units

Total power (W) for the space or power density (W/ft²)

Input Restrictions

Receptacle loads in the proposed design shall be calculated on the basis of: the number of dwelling units, the number of bedrooms, the floor area, and the use of ENERGY STAR appliances. Credit for ENERGY STAR appliances applies only to refrigerators and clothes washers. Other equipment energy use is neutral independent. Lobbies, corridors and other common spaces shall determine plug and process electrical loads using the general procedure for nonresidential buildings.  

Baseline Rules

The receptacle power in the baseline building shall be determined using the same method (see below) as the proposed design, but conventional equipment shall be assumed for the refrigerator and clothes washer. 

Procedure for Estimating Receptacle Power

Receptacle loads in the proposed design may be calculated in one of two ways:1 . The first simplified method may be used when each dwelling unit has its own clothes washer and dryer and all of the dwelling units use either electricity or gas for cooking and clothes drying. The second method is more flexible and must be used when some dwelling units have in-unit washers and dryers and some do not, or when some of the dwelling units use gas while others use electricity. The two methods are as follows:

Method One

The annual electric energy for dwelling units in the multi-family building is given by equation 3.4.5-3 below:

(Equation 3.4.5-3)

$$ kWh_{total} = NmbrDU \cdot a + NmbrBR \cdot b + FlrArea \cdot c $$

where

kWh_total      the total annual electrical receptacle energy for the multi-family building (kWh/y)

NmbrDU     number of dwelling units in the multi-family building

NmbrBR     number of bedrooms in the multi-family building (efficiency apartments count as one)

FlrArea       finished floor area for all dwelling units. 

a, b, and c  coefficients taken from Table 3.4.5-2

Table 3.4.5-2: Annual Residential Energy Use - Method One

Note: Use values in parenthesis when all the dwelling units have ENERGY STAR refrigerators and clothes washers.

Once kWh_total is determined, it is converted to power by dividing by the number of full-time equivalent hours used in the receptacle schedule and by adjusting units. This step is shown in equation 3.4.5-4. . 

(Equation 3.4.5-4)

$$Power (W/ft2) = \frac{kWh_{total} \cdot 1,000}{FTEhours}$$

where

FTEhours   full-time equivalent hours used in the receptacle schedule. The default value from Appendix C is 5,840 hours (hours)

Method Two

Method Two can be used under any conditions, but is required when some or all of the clothes washers and/or dryers are located in common areas or when some of the dwelling units have gas for cooking and or drying clothes while other units use electricity. 

The annual electricity use is given in equations 3.4.5-5 through 3.4.5-10:

(Equation 3.4.5-5)

$$kWh_{total}=kWh_{ref}+kWh_{cths}+kWh_{cook}+kWh_{dry}+kWh_{misc}$$

(Equation 3.4.5-6)

$$kWh_{ref} = NmbrDU_{E*} \cdot 423 + NmbrDU_{conv} \cdot 529$$

(Equation 3.4.5-7)

$$\begin{align} kWh_{du,cths} = & NmbrCW_{du,E*} \cdot 57 + NmbrCW_{du,conv} \cdot 81 + \\ & NmbrCW_{com,E*} \cdot 138 + NmbrCW_{com,conv} \cdot 196 \end{align} $$

(Equation 3.4.5-8)

$$kWh_{cook} = NmbrDU_{EC} \cdot 604$$

(Equation 3.4.5-9)

$$\begin{align} kWh_{dry} = & NmbrDry_{du,elec} \cdot 418 + NmbrBR_{du,elec} \cdot 139 + \\ & NmbrDry_{com,elec} \cdot 1013 + NmbrBR_{com,elec} \cdot 337 + \\ & NmbrDry_{du,gas} \cdot 38 + NmbrBR_{du,gas} \cdot 12.7+ \\ & NmbrDry_{com,gas} \cdot 92.1 + NmbrBR_{com,gas} \cdot 30.8 \end{align}$$

(Equation 3.4.5-10)

$$kWh_{misc} = FlrArea \cdot 1.05$$

where

kWh_total      the total annual electrical receptacle energy for the multi-family building (kWh/y)

kWh_ref        the annual refrigerator energy for the multi-family building (kWh/y)

kWh_cths      the annual clothes washer energy for the multi-family building (kWh/y)

kWh_cook     the annual cooking energy for the multi-family building (kWh/y)

kWh_dry       the annual clothes dryer energy for the multi-family building (kWh/y)

kWh_misc      the annual miscellaneous energy for the multi-family building (kWh/y)

NmbrDU_E*   number of dwelling units with ENERGY STAR refrigerator

NmbrDU_conv   number of dwelling units with conventional refrigerator

NmbrCW_du,E*   number of ENERGY STAR clothes washers located in dwelling units

NmbrCW_com,E*  number of ENERGY STAR clothes washers located in common areas

NmbrCW_du,conv  number of conventional clothes washers located in dwelling units

NmbrCW_com,conv   number of conventional clothes washers located in common areas

NmbrDU_EC  number of dwelling units with electric cooking

NmbrDry_du,elec   number of electric clothes dryers in dwelling units

NmbrDry_com,elec   number of electric clothes dryers in common areas

NmbrDry_du,gas   number of gas clothes dryers in dwelling units

NmbrDry_du,gas   number of gas dryers in common areas

NmbrBR_du,elec   number of bedrooms served by electric clothes dryers in dwelling units

NmbrBR_com,elec   number of bedrooms served by electric clothes dryers in common areas

NmbrBR_du,gas   number of bedrooms served by gas clothes dryers in dwelling units

NmbrBR_com,gas   number of bedrooms served by gas clothes dryers in common areas

FlrArea     finished floor area for all dwelling units. 

Once kWh_total is calculated, it is converted to power by dividing by the full-time equivalent (FTE) hours in the schedule of operation and adjusting the units. See equation 3.4.5-4.  

Applicability

All projects

Definition

Schedule for receptacle power loads used to adjust the intensity on an hourly basis to reflect time-dependent patterns of usage.

Units

Data structure: schedule, fraction

Input Restrictions

The default schedule is taken from Tables 1 through 11 of Appendix C.

Baseline Rules

Schedules for the baseline building shall be identical to the proposed design.