The vocabulary behind utility clean energy programs — inventory, allocation, compliance, and reporting.

Generation inventory

The record of every MWh in a utility's physical portfolio by source and certificate status — owned generation, purchased power, PPAs, imports, and exports — regardless of whether a certificate was ever issued against it. A generation inventory includes bundled RECs/EACs (generation that has a certificate against it) and unserialized generation (generation for which no certificate has been issued).

Many systems only track the certificate inventory — what's in the registry. The generation inventory is a second, independent record. Without it, mismatches between what a utility generated and what it holds in certificates accumulate silently.

Certificate inventory

The record of RECs and EACs held by a utility, sourced from registries such as M-RETS, WREGIS, NAR, and PJM-GATS. Tracks each certificate's status: bundled (backed by specific generation from a specific source), unbundled (certificate exists but no direct generation tie), retired (used for compliance or voluntary claims), or available (held for future use).

The certificate inventory is the half of the picture most systems track. The generation inventory is the other half. They don't automatically match — reconciling them is where gaps surface.

Null power

Electricity from which the RECs or EACs have been stripped and sold to a different party. The physical generation flows to the grid, but the certificates documenting its clean energy attributes have been unbundled and transferred elsewhere — leaving electricity with no renewable or clean energy claim attached.

Null power is central to GHG Protocol Scope 2 market-based accounting. When a utility sells RECs into the market, the residual electricity that remains is null power. Residual mix emissions factors — what customers implicitly consume when they haven't purchased certificates — are calculated after removing all certificate-bearing supply from the grid mix, leaving null power and other unattributed generation as the baseline.

Unserialized generation

Generation that has occurred but has not been serialized — submitted to a registry and converted into tradeable certificates. The electricity was produced and delivered to the grid, but no EAC was ever issued against it.

Unserialized generation matters for two related reasons. First, it shapes residual mix calculations: generation with no certificate issued becomes part of the default grid supply that customers implicitly consume when they haven't purchased EACs to cover their load. Second, it creates gaps in customer attribution — when a customer's consumption isn't fully matched with purchased EACs, the unmatched portion is effectively backed by unserialized generation or residual mix supply rather than a specific clean energy source.

Residual mix

The default electricity supply that customers implicitly consume when they haven't purchased certificates to cover their load. After all certificate-bearing supply — RECs, EACs, and other attributed generation — is removed from the grid mix, what remains is the residual mix: null power, unserialized generation, and other uncertified electricity. The residual mix typically carries a higher carbon intensity than the grid average, because the cleanest generation tends to have certificates issued and sold against it.

Residual mix emissions factors are used in GHG Protocol Scope 2 market-based accounting. When a customer's consumption isn't fully matched with purchased certificates, the unmatched portion is assigned the residual mix emissions factor rather than a source-specific clean energy claim. Knowing the residual mix composition is essential for utilities that need to report what their unattributed customers are actually receiving.

Standard delivery mix

Also: standard supply service mix

The generation mix that a utility delivers to customers who are not enrolled in any green tariff or voluntary clean energy program. It's a utility-specific emissions factor — calculated from the utility's own generation portfolio and purchased power after removing supply allocated to special program customers — and can be used as a supplier-specific factor in GHG Protocol Scope 2 market-based accounting.

The standard delivery mix is often confused with the residual mix, but they're calculated differently and represent different things. The residual mix is a grid-wide or region-wide figure: all certificates sold into the market are removed from the total regional generation, and the remaining electricity forms the residual. The standard delivery mix is utility-specific: it reflects what one utility actually supplies to its unattributed customers based on its own resource mix. A utility with a high proportion of owned hydro or nuclear may have a standard delivery mix with very different carbon intensity than the regional residual mix.

Renewable energy certificate (REC)

REC

A tradeable instrument representing the clean energy attributes of one MWh of electricity generated from a qualifying renewable source — wind, solar, geothermal, biomass, and others depending on state rules. When a renewable generator produces one MWh and registers it with a tracking registry (M-RETS, WREGIS, NAR, PJM-GATS), one REC is created. The REC can be sold together with the physical electricity (bundled) or separately from it (unbundled).

Retiring a REC — removing it permanently from circulation in the registry — is how utilities and corporations make renewable energy claims. RPS compliance is demonstrated by retiring RECs in the applicable state registry. Voluntary claims by corporate buyers follow the same mechanism. Each REC can only be retired once, which is the fundamental mechanism that prevents double counting.

REC registry

A tracking system that manages the issuance, transfer, and retirement of renewable energy certificates. When a qualifying generator produces electricity, the registry creates a unique certificate — one REC per MWh — tied to that generation event. The certificate can then be transferred between accounts as it's bought and sold, and ultimately retired when used for a compliance or voluntary claim. Retirement is permanent and one-way: a retired REC can't be reactivated or transferred, which is the mechanism that prevents the same MWh from being claimed more than once.

The U.S. has several regional registries covering different geographic footprints: M-RETS (Midwest Renewable Energy Tracking System) covers the Midwest and several other states; WREGIS (Western Renewable Energy Generation Information System) covers the Western Interconnection; NAR (North American Renewables Registry) operates across multiple regions; and PJM-GATS (Generation Attribute Tracking System) covers the PJM Interconnection territory. A utility with generation or customers spanning multiple regions may need to manage certificate accounts in more than one registry simultaneously, which adds complexity to reconciliation and reporting.

Bundled REC

A REC sold together with the underlying physical electricity — the power and its clean energy attributes move to the same buyer. Bundled RECs establish a direct connection between a specific generator and a specific customer's load: the buyer knows where their electricity came from, not just that clean energy was generated somewhere on the grid.

Bundled RECs are more expensive than unbundled RECs because the buyer is purchasing both the commodity electricity and the clean energy attributes. They're common in green tariff structures, bilateral PPAs, and programs that require source-specific attribution rather than portfolio-level averaging.

Unbundled REC

A renewable energy certificate that has been separated from the underlying electricity generation it represents. The physical electricity and the certificate are sold to different parties — one receives the power, the other receives the clean energy attributes. Unbundled RECs occupy the certificate-only zone in the inventory model: a certificate exists, but the physical generation behind it is not held by the same program or customer.

Unbundled RECs are widely used for annual REC matching and RPS compliance. They're typically a less expensive and more liquid instrument than bundled RECs, but they don't establish a direct link between a specific generator and a specific load — which matters for hourly CFE matching and some voluntary program structures.

Energy attribute certificate (EAC)

EAC

A market instrument representing the clean energy attributes of one MWh of electricity generated from a renewable or alternative source. Each EAC is issued, tracked, and retired through a registry to prevent double counting. RECs (renewable energy certificates) are the most common form in the U.S., covering wind, solar, and other eligible renewables.

The EAC category is broader than RECs. It also includes zero-emission credits (ZECs, used in some states for nuclear generation) and international instruments such as I-RECs (international RECs) used in markets outside the U.S. where regional registries don't operate.

Alternative energy certificate (AEC)

AEC

A certificate representing one MWh of electricity generated from an alternative energy resource. The eligible resource set varies by state program but typically extends beyond the renewable sources that qualify for standard RECs — including resources such as large-scale hydro, waste-to-energy, coal mine methane, and demand-side management, depending on the jurisdiction. Some states organize AECs into tiers, with more strictly renewable or distributed resources in a higher tier and broader alternative resources in a lower tier, each satisfying different compliance obligations.

AECs are part of the broader EAC family. Like RECs, they are issued through a tracking registry, can be bundled or unbundled from the underlying electricity, and are retired to demonstrate compliance with a state alternative energy standard. The key distinction from RECs is the eligible resource set: a resource that qualifies for an AEC may not qualify as renewable under a state's RPS, and vice versa. Utilities operating under alternative energy standards need to track AECs alongside RECs in the same inventory and reconciliation workflow.

REC reconciliation

The process of matching generation data from utility internal systems and third-party generators (PPAs) against the certificate records held in external registries — and resolving the discrepancies between them. The two sources often don't agree: a generator may have produced MWhs that the utility's metering system recorded but that haven't yet appeared as certificates in the registry, or a registry may reflect certificates that don't tie cleanly to the generation data on file.

Mismatches accumulate for a range of reasons — timing differences between metering cycles and registry issuance, manual entry errors, PPA data that arrives in a different format than the registry expects, and generation from sources that haven't been fully onboarded into a registry. REC reconciliation surfaces these gaps before they compound: before a utility files an RPS report, before an auditor asks for documentation, and before a C&I customer asks what clean energy served their load.

Annual REC matching

The standard approach for renewable energy accounting: a customer or utility purchases enough RECs to equal total electricity consumption over the course of a year, then retires them. If a customer consumed 100,000 MWh in a year and retired 100,000 RECs, their annual electricity consumption is considered matched with renewable energy — regardless of when or where that generation actually occurred relative to their load.

Annual matching is the basis for most RPS compliance programs and the majority of corporate renewable energy claims today. It's administratively straightforward and works well for goals framed in terms of total annual renewable volume. Where it falls short is temporal and geographic precision: a utility retiring RECs from a wind farm that generated in spring doesn't establish that clean energy was actually serving customer load on a summer afternoon peak. For customers with 24/7 CFE goals, that gap matters — which is why hourly matching exists as a more precise alternative.

Hourly CFE matching

A carbon-free energy accounting method that matches clean energy supply to customer load hour by hour, rather than on an annual average basis. Annual REC matching confirms that a customer purchased enough RECs to cover their total annual consumption — but doesn't establish when or from where that clean energy came. Hourly matching provides that precision: for each hour, the clean energy supply is traced to a specific source and matched to a specific load.

Hourly matching is required for large C&I customers pursuing 24/7 CFE goals — organizations like Google and Microsoft that measure carbon-free energy on an hourly, not annual, basis. It requires an allocation engine capable of routing generation to specific customer loads by hour, and a generation record granular enough to support it.

24/7 CFE

Carbon-Free Energy

A procurement and accounting standard that matches carbon-free energy supply to customer load in every hour of the year — not just on an annual average. Under annual REC matching, a customer can purchase certificates representing generation that happened at any time during the year and claim their electricity was renewable. 24/7 CFE closes that gap: supply must be matched to load hour by hour, from sources that are actually generating carbon-free energy in that specific hour.

The 24/7 CFE standard is driven by corporate buyers with the most ambitious clean energy goals, most prominently Google (which has committed to operating on 24/7 carbon-free energy by 2030) and Microsoft, along with the broader 24/7 Carbon-Free Energy Compact. For utilities running green tariff programs that serve these customers, 24/7 CFE requires both an hourly generation record and an allocation engine capable of matching supply to load at hourly granularity.

Allocation engine

The logic layer that routes RECs and clean energy attributes from the inventory to specific programs and customers. In a utility running multiple programs simultaneously — RPS compliance, a green tariff, a bilateral deal, a 24/7 CFE program — the allocation engine is what prevents those programs from drawing on the same certificate twice. It applies each program's rules, routes supply accordingly, and produces a timestamped settlement record for each allocation.

Because the allocation engine holds the rules for every program simultaneously, it also provides visibility into REC position across the full portfolio — which programs are long, which are short, and where gaps exist before they become a compliance problem.

Double counting

Occurs when the same clean energy MWh — or its certificate — is attributed to more than one customer or program. The clearest case: a utility operates multiple voluntary green tariff programs and allocates the same REC to two different customers, each of whom receives a report claiming that generation as their clean energy supply. The certificate was retired once, but two parties made a claim against it.

Double counting isn't always intentional — it often happens when programs are managed with separate tools that don't share a ledger. Each system makes its own allocation decisions without visibility into what the other has already committed. The error surfaces when an auditor or corporate buyer asks for the underlying certificate serial numbers and finds the same ones on two different reports.

The relationship between mandatory RPS compliance and voluntary customer claims is a related but distinct question. Because RPS is retired on behalf of all customers in a service territory, customers technically receive a share of that compliance — which means the boundary between what RPS covers and what a voluntary green tariff adds on top requires careful accounting. This is one reason the GHG Protocol Scope 2 market-based method and standards like Green-e distinguish between mandatory and voluntary retirement, and why program-level attribution needs to be tracked separately from compliance-level retirement.

Green tariff

A utility rate structure that allows large customers — typically commercial and industrial (C&I) — to subscribe to specific renewable energy from identified generation sources, with the utility managing procurement, certificate tracking, and reporting on their behalf. Green tariffs are distinct from a utility's default supply in that they offer load-matched, source-specific attribution rather than bundled averages across the full supply mix.

Common green tariff structures include generation-percentage programs (the customer receives a share of a specific plant's output), fixed-MWh subscriptions, and hourly CFE programs (where supply is matched to the customer's load hour by hour). Each structure has different allocation logic and reporting requirements.

RPS compliance

Renewable Portfolio Standard

The obligation for utilities to demonstrate that a specified percentage of electricity sales came from qualifying renewable sources, documented through REC retirement in the applicable registry. Each state sets its own RPS requirements: what percentage must be renewable, which resource types qualify, how compliance is demonstrated, and what the filing deadlines are.

Utilities running voluntary clean energy programs — green tariffs, bilateral deals, 24/7 CFE — face an additional complication: the same certificate inventory funds both RPS compliance and voluntary commitments. Without an allocation engine that tracks both simultaneously, it's possible to over-allocate to voluntary programs and fall short on the compliance obligation, or vice versa.

REC banking

The practice of carrying surplus RECs from one compliance period into future periods to satisfy upcoming RPS obligations. When a utility generates or procures more renewable energy than its current-year requirement, the excess RECs can be banked rather than retired immediately — held in the registry and applied against a future year's obligation instead.

Banking rules vary significantly by state. Most states allow it but restrict how far forward a REC can be carried: a REC issued in 2024 may only be eligible for compliance through 2026 or 2027, depending on the jurisdiction's vintage rules. Some states limit which certificate vintages can satisfy a given compliance year at all. Tracking banked RECs requires retaining their issuance date and source alongside their retirement status, and reconciling that vintage data against each year's applicable rules — which varies across a multi-state utility's service territory.

Voluntary programs impose their own vintage windows, which may differ from state RPS rules. The Green-e Energy Standard — relied upon by programs such as RE100 — specifies that "a Green-e® Energy certified product may include only renewables that are generated in the calendar year in which the product is sold, the first three months of the following calendar year, or the last six months of the prior calendar year." That 21-month window (6 months prior + 12 current + 3 following) is narrower than some state RPS banking allowances. A utility supplying RECs to corporate customers making voluntary claims under RE100 or similar frameworks must ensure certificates meet Green-e's vintage window, not just the applicable state banking rule — which means tracking both sets of vintage constraints from the same certificate inventory simultaneously.

GHG Protocol Scope 2

The accounting standard that governs how organizations report indirect greenhouse gas emissions from purchased electricity. Scope 2 emissions aren't produced by the organization directly — they result from the generation of electricity the organization consumes. The GHG Protocol provides two methods for calculating them.

The location-based method uses grid average emissions factors — whatever the regional grid emits per MWh, that's what the organization reports. The market-based method allows organizations to use supplier-specific or certificate-based emissions factors instead. When a utility or corporate buyer retires RECs, EACs, or purchases supply through a green tariff, they can claim the emissions factor associated with that specific source rather than the grid average — potentially claiming zero Scope 2 emissions if fully matched with carbon-free supply.

The market-based method is what creates demand for utility clean energy programs and the attribution infrastructure behind them. Without proper certificate tracking, retired RECs, and verifiable allocation records, a Scope 2 market-based claim can't be substantiated in an audit.

FASB ASU 2026-02

Financial Accounting Standards Board Accounting Standards Update 2026-02

An accounting standard that reclassifies RECs and similar environmental attribute certificates as intangible assets on the balance sheet, rather than treating them as operating expenses or footnotes. Under ASU 2026-02, external auditors will scrutinize REC holdings on the same standard as any other balance sheet asset — requiring documented acquisition cost, attribution to specific certificate serial numbers, and a verifiable audit trail from issuance through retirement.

Effective for fiscal years beginning after December 15, 2027 for public entities, and after December 15, 2028 for all other entities. Early adoption is permitted. The practical consequence for utilities: a spreadsheet-based REC tracking system will not produce records that meet the new audit standard. The allocation decisions, certificate serial numbers, and program-level attribution need to be documented in a system that can produce an auditable record on demand.