ASHRAE 90.1 Appendix G 2019 - Understanding the Performance Cost Index (PCI)
1. Introduction
The Performance Rating Method (PRM), described in Normative Appendix G of ASHRAE Standard 90.1, provides a whole-building approach to evaluating energy efficiency. Instead of checking every wall assembly and chiller against a prescriptive checklist, the PRM uses computer simulation to compare the annual energy cost of your proposed building design against a standardised baseline building. One number in, one number out. Conceptually simple, even if the modelling effort to get there is anything but.
The primary metric produced by this method is the Performance Cost Index (PCI): a dimensionless ratio that expresses the proposed building's annual energy cost as a fraction of the baseline building's annual energy cost. Lower is better. Think of it as a fuel economy sticker for a building.
The PRM can be used for compliance with the standard, for documenting beyond-code performance (LEED certification, utility incentive programmes, impressing your client), or both. It applies to new buildings, additions, and alterations, provided the building has a mechanical system and an envelope design. If your building has neither, you've got bigger problems than Appendix G.
2. Key Concepts
2.1 Baseline Building vs. Proposed Building
Two energy models are created using the same simulation program, climate data, purchased energy rates, and schedules of operation. The proposed building is modelled as actually designed and built. The baseline building is a hypothetical variant of your design, developed following the prescriptive rules of Table G3.1. It substitutes standardised, less efficient options for envelope, HVAC, lighting, and other regulated systems while retaining your building geometry, schedules, and process loads.
The comparison is deliberately apples-to-apples. Same weather file, same rate structure, same occupancy schedules. The only variable is your design decisions.
Important change in 90.1-2016 and later: The baseline building is intentionally less stringent than the prescriptive requirements of the standard. It does not itself comply with the standard. This catches people off guard. Compliance requires achieving a specified improvement over the baseline, as defined by the Performance Cost Index target. The baseline is the starting line, not the finish line.
2.2 Regulated vs. Unregulated Energy
The baseline building's annual energy cost splits into two components:
Baseline Building Regulated Energy Cost (BBREC): Energy used by systems the code actually governs (Sections 5 through 10): HVAC, lighting, service water heating, motors, transformers, vertical transportation, refrigeration, computer-room cooling, and other regulated systems.
Baseline Building Unregulated Energy Cost (BBUEC): Everything else. Process loads, plug loads, kitchen equipment, server racks. The code doesn't set efficiency requirements for these, so it would be unreasonable to penalise you for them.
Regulated energy cost is calculated by multiplying the total energy cost by the ratio of regulated energy use to total energy use for each fuel type. Unregulated energy cost is simply the remainder. The total baseline building performance is:
3. Calculating the Performance Cost Index
The PCI itself is straightforward:
Where:
PBP = Proposed Building Performance (annual energy cost of the proposed design, including any reductions from on-site renewable energy systems)
BBP = Baseline Building Performance
A PCI of 0.72 means your building costs about 72% of what the baseline costs to operate. A PCI above 1.0 means you've somehow made the building worse than the baseline, which is a conversation nobody wants to have with the building official.
4. The Performance Cost Index Target and Building Performance Factor
4.1 Compliance Criterion
You don't just need a PCI below 1.0. You need to beat a specific target, the $\text{PCIt}$:
The target is calculated as:
This formula applies the improvement requirement only to the regulated portion of the energy cost. Unregulated energy cost passes through at its full value. The reasoning is sound: if 80% of your building's energy goes to process loads you can't touch, the standard shouldn't demand you cut your total energy bill in half.
4.2 The Building Performance Factor (BPF)
The BPF is the mechanism that makes this work. It's a value between 0.36 and 0.76, pulled from Table 4.2.1.1 of the standard, and it depends on two things: building area type and climate zone.
The BPF represents the fraction to which the regulated energy cost must be reduced. A BPF of 0.50 means the standard requires your regulated energy cost to hit 50% of the baseline value, a 50% improvement in regulated energy performance. A BPF of 0.70 is more forgiving: you only need a 30% cut.
For mixed-use buildings, the BPF is calculated as the area-weighted average of the BPFs for each building area type. For types not listed in the table, use "All Others." Which, to be fair, is a row that gets more exercise than you'd expect.
The complete BPF values from Table 4.2.1.1 of ASHRAE Standard 90.1-2019:
Table 1: Building Performance Factor (BPF), ASHRAE 90.1-2019 Table 4.2.1.1
Multifamily
0.68
0.70
0.66
0.66
0.69
0.68
0.59
0.74
0.76
0.74
0.70
0.73
0.75
0.68
0.71
0.68
0.72
Healthcare
0.60
0.60
0.58
0.54
0.56
0.55
0.55
0.55
0.54
0.54
0.57
0.52
0.54
0.57
0.52
0.57
0.57
Hotel/Motel
0.55
0.53
0.53
0.52
0.53
0.54
0.54
0.53
0.53
0.52
0.50
0.51
0.51
0.50
0.51
0.50
0.50
Office
0.52
0.57
0.50
0.56
0.53
0.56
0.48
0.51
0.52
0.49
0.51
0.51
0.49
0.52
0.51
0.49
0.51
Restaurant
0.63
0.64
0.60
0.60
0.60
0.61
0.58
0.62
0.57
0.61
0.63
0.60
0.64
0.65
0.62
0.67
0.70
Retail
0.51
0.54
0.49
0.55
0.51
0.55
0.53
0.51
0.55
0.54
0.50
0.54
0.55
0.50
0.51
0.48
0.50
School
0.39
0.47
0.38
0.43
0.38
0.42
0.40
0.37
0.40
0.38
0.36
0.40
0.36
0.36
0.37
0.36
0.37
Warehouse
0.38
0.42
0.40
0.42
0.43
0.44
0.43
0.44
0.43
0.46
0.49
0.47
0.48
0.54
0.51
0.57
0.57
All Others
0.56
0.57
0.50
0.52
0.50
0.54
0.53
0.53
0.52
0.54
0.51
0.51
0.50
0.50
0.50
0.50
0.46
Interpreting the BPF Table
Schools have the lowest BPFs (0.36–0.47). Predictable schedules, large roof areas, and favourable system configurations mean there's plenty of cost-effective savings to be had. The standard knows this, and sets the bar accordingly.
Warehouses show BPFs that increase with colder climate zones (0.38 in mild climates up to 0.57 in Climate Zone 8). Deep savings in extreme cold cost more and deliver less. The table reflects that reality.
Multifamily buildings have the highest BPFs (0.59–0.76). Individual dwelling units with distributed systems simply don't offer the same optimisation opportunities as a centralised commercial building. The standard gives them more room.
Healthcare facilities land in a moderately relaxed range (0.52–0.60). Continuous operation, high ventilation requirements, and process-intensive medical systems limit what's achievable.
5. Practical Application of the BPF
Consider a data centre where 80% of energy goes to servers (unregulated) and only 20% goes to HVAC and lighting (regulated). If the applicable BPF requires a 47% reduction in regulated energy, that reduction applies only to the 20% regulated share. That's roughly a 9–10% reduction in total energy cost. Without the BPF mechanism separating regulated from unregulated, the data centre would face an impossible whole-building target. The split is what makes this system workable across fundamentally different building types.
6. Renewable Energy Systems and Compliance
On-site renewable energy systems (typically PV panels, but potentially other types) reduce the proposed building's annual energy cost, which lowers the PCI and makes compliance easier. But there's a catch: the standard caps the compliance credit from renewables at 5% of the Baseline Building Performance.
The intent is clear. You can't slap an oversized solar array on a poorly insulated building with undersized equipment and call it compliant. The building itself has to be reasonably efficient. Solar can get you over the finish line, but it can't run the whole race.
6.1 Renewable Fraction
The renewable contribution is quantified as:
Where PBPnre is the proposed building performance excluding renewable energy cost reductions.
6.2 Compliance Rules
When the Renewable Fraction $\leq$ 0.05: The standard PCI \PCIt check applies as normal. The PCI naturally decreases as renewables offset cost.
When the Renewable Fraction $>$ 0.05: An Adjusted PCI kicks in:
The Adjusted PCI produces the same value regardless of how much additional renewable capacity you install beyond the 5% threshold. You could triple the size of your PV array and the compliance math wouldn't budge. Additional panels still save money on energy bills, of course, but they won't buy you any more compliance credit.
7. Worked Example
An office building in Climate Zone 3A produces the following simulation results:
BBUEC (Baseline Unregulated Energy Cost): $10,000
BBREC (Baseline Regulated Energy Cost): $15,000
BBP (Baseline Building Performance): $25,000
PBP (Proposed Building Performance, no renewables): $19,000
From Table 4.2.1.1, the BPF for an office in Climate Zone 3A is 0.53.
Since PCI 0.76 >PCIt 0.718, the building does not comply without renewables. Not ideal, but not uncommon. Here's what happens when you start adding PV:
Table 2: Impact of Renewable Energy System Size on Compliance
Renewable Fraction
0.02
0.04
0.05
0.08
Proposed Building Perf. (PBP)
$18,500
$18,000
$17,750
$17,000
Performance Cost Index (PCI)
0.74
0.72
0.71
0.68
Adjusted PCI
N/A
N/A
0.71
0.71
Complies? (PCIt = 0.718)
No
No
Yes
Yes
Once the renewable fraction hits 0.05, the Adjusted PCI locks at 0.71 regardless of additional PV capacity. Both the $1,250 and $2,000 systems achieve compliance, but there's no point oversizing purely for compliance reasons. The $1,000 system almost gets there; at $1,050 it would have made it.
8. Calculating Percentage Improvement
For beyond-code applications (LEED, utility incentive programmes, green building certifications), the percentage improvement is:
If the building includes renewable energy and the rating authority applies the 5% cap, use the higher of $\text{PCI}$ or the Adjusted PCI in the formula. Different rating authorities may treat renewables differently, so check the specific programme requirements before assuming.
9. Simulation Requirements
To keep the comparison honest, all simulation runs for the proposed and baseline buildings must use the same:
Simulation program
Climate data
Purchased energy rates
Schedules of operation (except adjustments needed to account for energy-efficiency features)
These rules prevent extraneous differences from contaminating the results. If you used different simulation engines, part of the cost difference would just be algorithmic disagreement. If you used different rate structures, you'd be optimising for a tariff that might not exist in five years. The standard wants a clean comparison of building design, nothing else.
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