Hourly matching without additionality has little to no impact on emissions reductions

Hourly matching only accelerates renewable energy progress if additionality is part of the standard

The current proposal for an hourly matching standard in the Greenhouse Gas Protocol Scope 2 reporting standard does not include requirements that matched energy be additional. This means that voluntary corporate clean energy buyers could claim energy attribute certificates (EACs such as RECs or GOs) from already-built clean generation resources towards reducing their carbon footprint. This will likely lead to lower amounts of clean energy on the grid and higher emissions compared to a stricter standard.

Additionality is defined as an intervention that causes an action that would not have occurred otherwise. For example, a corporation signing a voluntary PPA for solar or wind energy (the intervention) provides the financial certainty for a new clean energy project to get financed, built, and interconnected to the power grid (the caused action). This has long been understood as a critical principle of clean energy procurement necessary to cause the desired reduction in carbon emissions (Bjørn et al. 2025).

Previous studies have reported that hourly matching can reduce emissions, but almost all of them assume at least some level of additionality for the procured renewable resources. Only one work (Ricks et al. 2023) considered hourly matching without an additionality requirement, in the context of US hydrogen production, and found that without additionality “a 100% hourly matching requirement loses all of its consequential impact." 

In order to understand the impacts of an hourly matching standard as it is currently proposed, we used the PyPSA-Eur capacity expansion and dispatch model to analyze the impacts of an hourly matching standard with and without additionality requirements in the European grid in 2030. We find that a non-additional hourly matching standard has little to no impact on total system emissions.

How our analysis modeled additionality and hourly matching

Because true additionality depends on a counterfactual, it can be difficult to determine outside of models. Instead, a “new build” requirement is often used as a benchmark for additionality in renewable energy. Many groups have also proposed that for a project to be additional, there needs to be a long-term contract, which has been shown to significantly reduce risk for renewable project financing.

Meanwhile, purchasing energy from an existing project that has already been built and is already in operation is — by definition — not additional, since the action (building the generation resource) happened before the intervention (purchasing the RECs).

We modeled additionality using scenarios where only new build resources are allowed to count towards the hourly matching goals, and modeled non-additionality using scenarios where clean generation of any age can count towards the hourly matching goals (consistent with current GHGP Scope 2 proposals).

From an economic perspective, consumer demand for hourly matched RECs could increase the supply of clean energy, if the demand is high enough. In practice, we see that supply of unbundled RECs often surpasses demand, leading to low REC prices that have little impact on causing additional new renewable resources to be built.

Hourly matching proponents argue that its time-matching requirement will make REC supply scarce during certain hours, driving up the cost of RECs in those hours and leading to more investment in clean resources that generate during those times. However, this effect only occurs if the demand for RECs in those hours substantially exceeds the supply.

If unbundled RECs from existing generators are allowed (as in the current Scope 2 proposal), our modeling indicated that in the case of the European grid in 2030, a demand for clean energy attributes from 25% of all commercial and industrial (C&I) load is insufficient to cause investments that lead to significant emissions reductions.

Modeled scenarios

We also investigated the impacts of different methods of accounting for clean attributes of energy consumed from the grid.

• No Grid Contribution to Carbon Accounting: First, we consider the scenario (“No Grid”) where no attributes from the grid are counted, which replicates the formulation of hourly matching that was used in Xu et al. 2024. This scenario only counts purchased clean energy toward carbon accounting.
• Grid CFE - Imports: Next, we consider a scenario (“Grid CFE - Imports”) where clean attributes are counted proportional to the share of clean energy on the grid times the amount of energy “imported” from the grid (the gap between procured energy and total load). This is the formula initially proposed by Google and used in the study by Riepin and Brown. 
• Grid CFE - SSS: We then consider the scenario (“Grid CFE - SSS”) where consumers can count credits from their Standard Supply Service (SSS), which are clean energy credits procured by their utility and attributed to consumers proportional to their load. To our knowledge, this method of hourly matching has not been studied before, but is an element of the current standard proposed by the GHGP. We model this for corporate purchasers by allowing them to count a number of MWhs of clean energy equal to the share of carbon free energy on the grid times their total load in addition to any clean energy they procure directly. 

For all three of these scenarios, we modeled a requirement that procured resources are all new build (additional). We then modeled the scenario (“No New Build Req”) where there is no new build requirement by allowing consumers to count any amount of clean energy credits up to the total amount available on the grid.

No additionality means no impact

Without a new build requirement, we find that hourly matching has no emissions benefit for matching levels up to 90%. Even at a 100% hourly match, it has only a small benefit (4 MT) compared to the emissions caused by the load (62 MT). This is likely because there is a large amount of carbon-free energy already on the grid in Europe in 2030 without the addition of corporate procurement. If the corporate procurement is not limited to new build, buyers can take credit for the many carbon-free generation sources that already exist on the grid, including wind, solar, nuclear, and hydro. These purchases are non-additional, so they do not lead to changes in total system emissions. To achieve 100% matching, some additional resources are required, but they amount to a comparatively small amount of wind and battery storage resources. The battery storage resources are mostly dispatched to charge during times when there is excess CFE credits available and discharge during hours when there are fewer.

If a new build requirement was added, hourly matching is only impactful at high levels of hourly matching or if grid resources are not counted towards the hourly match. But these high-impact scenarios are also much higher in cost. Hourly matching that counts grid CFE either proportional to “imports” or SSS can have a significant impact on emissions if 100% hourly matching is achieved. However, below 100% matching, the emissions-reduction impact is small to non-existent. Again, this is likely caused by the high levels of CFE already on the European grid, which means corporate buyers can take credit for those existing resources and have a fairly high hourly matching score. The scenarios where no CFE from the grid is counted have a higher impact at all levels of matching, but also come at a much higher cost, exceeding €120 Billion in the 100% matching case. These high costs could be a deterrent, reducing the number of corporations willing to pursue voluntary renewable purchasing.

These results show that the specifics of how an hourly matching standard is written can lead to massive differences in reported emissions without any change in real-world grid decarbonization. The current proposed standard, which lacks a new build additionality requirement, will increase the difficulty of implementation by requiring more detailed accounting, but is likely to lead to little to no actual reduction in emissions. Adding a new build requirement to the standard could increase the impact, but it is highly dependent on how grid resources are counted towards the standard. The scenarios where impact is high come with a high cost however, which corporate buyers may not be willing to pay. This study also highlights the importance of considering all of the details when comparing a proposed standard to existing studies, as those studies may not apply to the proposed standard (as is the case here). This is the first time that hourly matching without a new build requirement (as proposed) has been studied, so previous research on hourly matching should not be used to project the impacts of the current proposed standard.

These results are part of a larger study we plan to release in the near future. If you are interested in learning more about it, or would like to access the code or data that was used to model these scenarios, please contact nat@watttime.org.

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