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PHIUS+ 2018 Source Zero

PHIUS+ Source Zero is a practical and practicable approach to advance community goals of carbon neutrality in the building sector. Source Zero extends from the PHIUS+ Standard, which was developed based on a robust analysis of building optimization to determine a sweet spot for investment in energy conservation measures. PHIUS+ provides numerical energy targets that project teams must meet but retains flexibility and creativity to design to achieve these targets while satisfying other project requirements. Achieving PHIUS+ means dramatically reducing demand. Achieving PHIUS+ Source Zero means meeting the small remainder with on-site or off-site renewable energy.


To meet PHIUS+ Source Zero, the building must generate or procure as much renewable energy as it uses, on an annual, source-energy basis. Because total energy is addressed with a source energy criterion, when it comes to adding an extra badge for “zero”, it was logical to base that on “source net zero”.

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.

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)

Off-site renewable energy procurement works in the same way as on-site, with a discount factor applied for Renewable Energy Certificates (RECs).

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:

Type Offset Factor (CRE)

On-Site Photovoltaic Array


Directly Owned Off-Site Renewable


Community Renewable Energy


Virtual Power Purchase Agreements (PPA)


Renewable Energy Certificates (RECs)


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 badge, determine if the total estimated annual renewable energy production (with a utilization factor of 1) will offset the annual source energy. That is, the net annual source energy use PEA,net is calculated in the same manner as PEA, but with CRE as shown in the table above.

Renewables Credits and Co-Generation in the Calculation of Source Energy 

The annual source energy for a building is calculated by multiplying the site energy by the fuel- dependent source energy factor, and then subtracting credit for renewable energy production multiplied by the fuel-dependent source energy factor of the fuel type it is offsetting. Generally, the total annual source energy use of the building is calculated as:



Fuel = gas, oil, coal, propane, biomass, electricity
SEA = total annual site energy use(from the building energy model)
FPE = source energy factor
TEDHW = usable onsite solar thermal energy for domestic hot water
TESH = usable onsite solar thermal energy for space heating
REA = total annual onsite renewable electricity generation
CRE = offset factor of renewable electricity generation (zero to 1)

FPE = 2.8 for electricity, 0.2 for biomass, and 1.1 for fossil fuels.

The end uses specifically include space heating, cooling, humidification, dehumidification, ventilation fans, pumps, domestic hot water heating, major appliances, lighting, elevators, miscellaneous electric loads and miscellaneous gas loads. Electric vehicle charging is specifically excluded.

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