Measuring CHP Output for PA Tier II AECs: Metering and Reporting

AEC volume is not estimated, modeled, or derived from fuel savings. It is measured.

52 Pa. Code § 75.63 provides for credit certification per MWh of electricity generated or conserved. PennAEPS describes the mechanism operationally: facility meter readings are recorded in PJM-GATS, and for each MWh of generation a corresponding AEC is issued. Monthly generation data is entered into GATS to create credits.

The operative rule is simple:

AEC volume equals the generation recorded by the certified facility meter and reported monthly to PJM-GATS.

Everything else in this article follows from that sentence.

What is not measured for AEC purposes

Fuel input is not measured for AEC purposes. Neither is thermal output, avoided boiler fuel, nor combined system efficiency.

Those measurements matter — to project economics, to Investment Tax Credit documentation under IRA § 48, to EPA CHP Partnership recognition, and to emissions reporting. They do not enter the AEC calculation. A methodology that starts from baseline fuel use and derives credits from BTU savings is measuring the right thing for the wrong purpose.

For Pennsylvania Tier II, the meter is the measurement.

Meter placement determines the number

This is where two systems with identical nameplate and identical run hours can report different AEC volumes.

• Gross generation is measured at the generator terminals — total electrical output before anything is consumed.
• Net generation is measured downstream of station service, after the parasitic loads the CHP system needs to run: fuel gas compressors, lube oil pumps, cooling fans, radiator fans, controls, and ventilation.

Parasitic load on a reciprocating engine or gas turbine CHP commonly runs 2–8% of gross output depending on configuration. Fuel gas compression is usually the largest single contributor, and systems on low-pressure gas supply sit at the high end of that range.

Whether the reported figure is gross or net depends on where the certified meter is installed. That placement is established during certification — not adjusted afterward.

If you are specifying metering for a new system, or reviewing an existing installation before an application, meter location is a decision with a recurring revenue consequence. It should be made deliberately.

Reporting to GATS

Once a facility is certified, it receives a certification number and a PJM-GATS account is established. Generation data is entered monthly.

Three things matter operationally:

• Cadence. Monthly entry is the expected rhythm. Gaps create reconciliation work and delay issuance.
• Accuracy. § 75.62(g) and § 75.64(6) list providing false or inaccurate information to the credit registry among the grounds on which qualification may be suspended or revoked, and past, current, or future credits withheld or retired. Reporting discipline is a compliance matter, not just an administrative one.
• Change notification. Those same provisions require notifying the administrator of changes affecting generation output, and of changes in ownership or aggregator. A capacity uprate, a prime mover replacement, or a change of aggregator all trigger notice.

Three cadences are easy to conflate. Reporting is how often generation data is entered into GATS — monthly. Issuance is when GATS mints the certificates, which can lag the generation month. Settlement is when an aggregator pays the generator, typically after buyers retire the credits. A generator reporting monthly may still receive quarterly payments. The three are independent, and a reference to quarterly settlement is not in conflict with monthly reporting.

Thermal load drives electrical output — indirectly

Here is where thermal measurement does connect to AEC volume, though not as a credit basis.

A thermal-following CHP plant modulates electrical output to match heat demand rather than electrical demand. When the heat load drops, the engine turns down or shuts off — and electrical generation falls with it.

For a campus or district plant sized to heating load, the seasonal pattern is severe. Winter runs near full output. Summer heating demand collapses, and with it the plant's run hours.

Adding absorption cooling changes the shape. Chilled water creates summer thermal load, the plant keeps running, and electrical output continues through months when it would otherwise have been idle.

The annual difference in the illustrative profile above is 57% versus 86% capacity factor. For a 3 MW plant:

3.0 MW × 8,760 h × 0.57 = 14,980 MWh. 3.0 MW × 8,760 h × 0.86 = 22,601 MWh.

A difference of 7,621 AECs annually — roughly $205,157 at the 2024–25 weighted average of $26/MWh.

The thermal load earns no credits. It keeps the engine running, and the engine earns the credits. For a thermal-following plant, extending thermal utilization is the most direct lever on AEC volume.

This is also why capacity factor for a thermal-following plant cannot be assumed. It has to be measured or modeled against the actual thermal load profile.

Quantification errors that cost credits

• Using nameplate capacity as though it were output. Nameplate × 8,760 assumes 100% capacity factor and continuous full-load operation. No CHP system runs that way.
• Assuming a capacity factor instead of measuring one. For a baseload industrial system the assumption may be close. For a thermal-following plant it can be wrong by 30 points or more.
• Confusing gross and net. A projection built on gross output against a meter installed downstream of station service will overstate by the parasitic load.
• Deriving credits from fuel savings. Fuel savings are a real and often larger economic benefit. They are not the AEC basis.
• Including thermal output. Thermal in MMBtu does not convert to MWh for credit purposes. Any calculation applying a thermal multiplier to electrical output is producing a number that will not match what GATS issues.
• Missing months. Unreported generation periods do not accumulate for later claim. Reporting discipline determines what gets issued.

Frequently asked questions

• Is AEC volume based on gross or net generation? It is based on what the certified meter records and reports to GATS. Whether that figure is gross or net depends on meter placement, which is established during certification.
• Does exported electricity count differently from electricity consumed onsite? No. AECs attach to metered generation. § 75.13(i) provides that a customer-generator owns the credits for the electricity it generates, absent express contractual assignment. Net metering is a separate billing mechanism and does not govern AEC issuance.
• Do I need to measure thermal output for AEC purposes? No. Thermal measurement supports project economics, ITC documentation, and emissions reporting — not AEC quantification.
• What happens if a month of generation data is not reported? Credits are issued against reported generation. Unreported periods do not accumulate for later claim, which makes monthly reporting discipline directly a revenue question.
• How accurate does the meter need to be? Meter specifications and calibration records are part of the certification record. Requirements should be confirmed with the program administrator for your specific configuration before metering is finalized.
• Can capacity factor be estimated for a system that has not been built? For a proposed system, use the engineering estimate from the feasibility study, modeled against the actual thermal load profile if the plant will operate thermal-following. For an operating system, use meter data.

Review your metering configuration

Meter placement, parasitic load treatment, and reporting cadence all determine what a CHP system actually earns. [Submit your system details](/#submission) for a measurement review.