3.3 Net Source Energy Criterion

In addition to heating/cooling performance requirements (Section 3.1), the overall performance, including all other energy enduses, must be addressed in the design. The metric used for this criterion is net source energy use. The overall energy limit is based on source energy because that is a good measure of the impact of the building’s operation has on society. That impact consists mostly of CO2 emissions but the source energy metric differs from carbon-scoring in that it includes the impact of nuclear power as well.

There are three certification “tiers” available. These differ with respect to stringency and in how renewable energy is regarded as offsetting source energy use.

3.3.1 PHIUS+ Core

The PHIUS+ Core path emphasizes on-site conservation and efficiency and allows for meeting the Net Source Energy target on-site.

3.3.1.1 Residential Buildings: Low-Rise and High-Rise

Buildings considered residential include single-family detached housing, single-family attached housing, multifamily buildings, excepting hotels and motels. The net source energy (SE) limit for all residential buildings is 5500 kWh/yr/person. For purposes of the SE limit, the number of persons, the design occupancy, is taken to be the number of bedrooms+1, on a per unit or unit-by-unit basis (e.g., four 2-bedroom units have a design occupancy of 12, not 9.)

3.3.1.2 Non-Residential Buildings

The annual source energy limit for non-residential buildings is 11.1 kWh per square foot of interior conditioned floor area (iCFA), equivalent to 38 kBTU/ft2 .yr or 120 kWh/m2 .yr. See Section 4.4.1.4 for the iCFA calculation rules. For commercial buildings with process loads, PHIUS will determine the source energy allowance on a case-by-case basis. See Section 3.3.1.

3.3.1.3 Renewables Credits

Source Energy demand is calculated as the annual usage net of renewable energy that is produced and used directly on-site. Renewable energy production that is exported from the site is not credited.

Solar thermal: 

• An active solar thermal system is regarded as reducing the source energy demand associated with water heating and, if so configured, space heating as well. The system is not regarded as reducing the space heat demand itself, but rather the source energy use needed to provide the heating.

Solar PV and other renewable electricity: 

• An estimate of coincident production-and-use of energy from renewable energy systems (such as PV) may be included in the calculation similarly to the way solar thermal systems are treated, that is, the limit applies to source energy consumption net of that generation. 
• NREL's PVWatts calculator may be used to calculate annual PV array output. 
• The amount of source energy that may be offset by a renewable electricity source depends on the utilization factor. This can be calculated by determining the coincident production-and-use fraction of the generation. If only using PV without battery storage, PHIUS' pre-defined utilization curves may be used to determine the utilization factor, and are built into WUFI Passive. 
• If using more than one renewable electricity generation technology on-site, electrical battery storage, or for off-grid buildings.

3.3.2 PHIUS+ 2018

In PHIUS+ 2018, the source energy criterion has a view toward zero. The source energy limit is not set based on cost optimization, but rather on the ‘fair share’ of carbon emissions allowed for each sector. To limit global warming and avoid many harmful impacts on society, emissions must go to zero overall and the energy system must go to 100% renewable. But, there are many options for meeting the target. This criterion aims to answer questions like:

When has the building done all it can with both conservation and on-site renewables and must look to its energy suppliers for clean/renewable energy?

The answer is there is no cap on total source energy use as long as the predicted ‘annual net source energy’ use meets the target. This ‘net’ source energy use is the remaining source energy use, after what is offset by qualified renewable energy measures, on-site and off-site. Any combination of conservation measures and qualifying renewables can be used to meet the net source energy criterion. This target is intended to taper to zero in years to come, with downward revisions on a three-year cycle, the next coming in 2021. The zero year is not yet decided.

This varies from PHIUS+ Core, where off-site renewable energy is not accounted for, and only a fraction of on-site renewable energy can be counted to offset source energy use (exports do not count). However, in tapering the source energy limit to zero, it is unproductive to put the entire burden on the building and its on-site production potential. Doing so may push past the point of diminishing returns in energy conservation, or prohibit projects with constrained sites from ever achieving this goal. At some point, the building has conserved and generated all it can, and the focus will shift to cleaning up the energy supply. More details can be found in Section 3.3.2.3.

3.3.2.1 Residential Buildings: Low-Rise and High-Rise

Buildings considered residential include single-family detached housing, single-family attached housing, multifamily buildings, excepting hotels and motels. The net source energy (SE) limit for all residential buildings is 3840 kWh/yr/person. For purposes of the SE limit, the number of persons, the design occupancy, is taken to be the number of bedrooms+1, on a per unit or unit-by-unit basis (e.g., four 2-bedroom units have a design occupancy of 12, not 9.)

3.3.2.2 Non-Residential Buildings

The annual source energy limit for non-residential buildings is 10.2 kWh per square foot of interior conditioned floor area (iCFA), equivalent to 34.8 kBTU/ft2 .yr or 110 kWh/m2 .yr. See Section 4.4.1.4 for the iCFA calculation rules. For commercial buildings with process loads, PHIUS will determine the source energy allowance on a case-by-case basis.

3.3.2.3 Renewables Credits

Source Energy demand is calculated as the annual usage net of all on-site renewable energy production, and net of off-site renewable energy procured (subject to the offset weighting factors listed below).

Solar thermal: 

• An active solar thermal system is regarded as reducing the source energy demand associated with water heating and, if so configured, space heating as well. The system is not regarded as reducing the space heat demand itself, but rather the source energy use needed to provide the heating.

Solar PV and other renewable electricity: 

• All of the predicted annual onsite-renewable electricity generation is now regarded as offsetting source energy use, not just the fraction used right away or stored and used on-site (as in PHIUS+ Core).
• • On-site renewable electricity generation counts toward source net zero with the same source energy multiplier for electricity, i.e. 2.8. In other words, with the source energy factor for grid electricity at 2.8, every kWh electric generated on site is considered to neutralize 2.8 kWh at the source. 
• NREL's PVWatts calculator may be used to calculate annual PV array output. 
• The following arrangements for procuring off-site renewable energy are also regarded as offsetting the source energy use: 
  • Directly-owned off-site renewables. 
  • Community renewable energy. 
  • Virtual Power Purchase Agreements. 
  • Green-E Certified Renewable Energy Certificates (RECs), discounted 80%, that is, each 1 kWh purchased offsets 0.2 kWh.

• The building owner must present an actual contract for procurement of renewable energy sufficient to meet the source energy target (at time of certification) for 20 years. 
• Projects that are off-grid for indoor water supply and for wastewater treatment have a process load allowance of 800 kWh/p.yr for residential projects, 14 kWh/kgal for non-residential. That allowance is cut in half that if a project is off-grid for water supply only or wastewater treatment only.

3.3.3 PHIUS+ Source Zero

PHIUS+ Source Zero pushes past the target set for PHIUS+ 2018. To meet PHIUS+ Source Zero, the building must generate or procure as much renewable energy as it uses, on an annual, source-energy basis.

For an all- electric building, the same on-site PV array (or PPA, etc) that gives site zero also gives source zero. It is different if the building uses other fuels - for example if the building made heavy use of biomass for heating, the overall source/site ratio of its usage might be only 1.5, while the source/site ratio of the PV is 2.8. Therefore on that building the PV array size for source zero would be smaller than the array size for site zero, because of the additional source-reduction strategy.

To determine if a project meets this certification, determine if the total estimated annual renewable energy production will offset the annual source energy.

There are no process load allowances for non-residential buildings under PHIUS+ Source Zero, i.e. the net source energy use must equal zero with process loads included in the model.

3.3.3.1 Renewables Credits

See Section 3.3.2.3 Renewables Credits.

3.3.4 Non-Residential Building

Process Loads For non-residential/commercial buildings with process loads, PHIUS will determine the source energy allowance on a case-by-case basis. The process load allowance is added to the base net source energy use allowance specified in 3.3.1.2 and 3.3.2.2, that is, it applies to PHIUS+ Core and PHIUS+ 2018 tiers, but does not apply to PHIUS+ Source Zero.

Process loads: If there is a service being provided to a customer or client in the building, this is considered a process load. Whether it's the customer or client using the energy (ex: arcade, training center), or if the energy is used on their behalf (ex: restaurant), both situations qualify as process loads. 

• ASHRAE 90.1-2010 Process Energy Definition: Energy consumed in support of a manufacturing, industrial, or commercial process other than conditioning spaces and maintaining comfort and amenities for the occupants of a building.

3.3.5 Mixed-Use Buildings

The residential SE limit also applies if the building has common spaces and conditioned spaces that are not dwelling units, but that primarily serve the residents. Nonresident occupants of such common spaces (staff) are not included in the occupant count for determining the source energy allowance. (Nor do they determine lighting and miscellaneous energy use of the common spaces, as explained in Section 6.6.1.1).

If there are non-residential spaces designed to mainly serve non-resident customers/clientele, an additional PE allowance may be calculated based on the iCFA of those spaces. The nonresident occupants of such spaces, the staff and customers, are not included in the occupant count for determining the per-person portion of the source energy allowance.

Certification staff may require separate modeling of the residential and non-residential parts of a mixed-use building.

The mixed-use protocol (see Section 1.5.5) is written to avoid the calculation of the source energy in the unfinished/undefined spaces - there is no source energy allowance for the unfinished parts and no source energy impact associated with the internal gains, so the details of how the gain is generated are not pertinent.

3.3.6 Tip: Determining the Equivalent Source Energy Allowance in kBTU/ft2.yr

To determine the PHIUS+ source energy target per square foot: Calculate the design occupancy as the number of bedrooms plus one (unit-by-unit), multiply by the source energy allowance per person, convert (3.412 kBTU = 1 kWh), and divide by the iCFA.

Example: 4-bedroom house, 2000sf iCFA

Source Energy Limit (PHIUS+ 2018): 3840 kWh/person.yr

(5 persons * 3840 kWh/person/yr * 3.412 kBTU/kWh)/2000 ft2 = Source Energy Limit: 32.76 kBTU/ft2 .yr

3.3.7 Co-Generation

There are two ways to operate a combined-heat-and-power (CHP) unit – on electrical demand or on heating demand.

On the “E-priority” path, the CHP is run independently of the heating load, with the intent of producing electricity, and the heat production is a byproduct, which may or may not be usable.

On the “H-priority” path, the CHP is run to match the heating demand (hot water and/or space heat), and the electricity produced while the CHP operates is regarded as a byproduct, that is either used on site or sold to the grid.

The H-priority path in turn has two variants depending on whether the backup heat is supplied by electricity or fuel combustion. See Appendix A for more details. The calculation protocol is implemented in the PHIUS Co-Generation Source Energy Factor Calculator.

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