RPS Effectiveness

Thirty US states have RPS mandates. Renewable energy deployment has surged. But how much of that growth is attributable to the RPS versus federal tax credits, falling technology costs, and corporate PPAs? Disentangling policy effects from market forces is the central challenge of evaluating RPS effectiveness.

The evidence. Studies (Lawrence Berkeley National Lab, 2021) estimate that state RPSs drove roughly 50-60% of renewable deployment in RPS states between 2000 and 2020. Federal tax credits drove 30-40%. Falling costs and voluntary corporate demand drove the remainder. These estimates use counterfactual modeling: comparing deployment in RPS states to non-RPS states with similar resources, controlling for federal incentives.

State variation matters. Texas's RPS was non-binding (exceeded early), so its wind boom was market-driven. California's 60% by 2030 mandate is binding and drives incremental procurement beyond what economics alone would justify. Massachusetts's solar carve-out with high REC prices directly finances projects that would not otherwise be built.

Stringency is key. Non-binding targets (set below what the market would deliver anyway) have no incremental effect. Binding targets (set above market trajectory) force additional procurement and raise costs slightly. The most effective RPSs set ambitious targets that pull deployment forward, capture learning-curve benefits early, and then become non-binding as costs fall below the mandate level.

Are RPSs still needed if renewables are now cheapest?

In many states, no. Where renewables are cost-competitive without mandates, the RPS is redundant. Where grid constraints, utility resistance, or regulatory barriers slow adoption, RPSs remain the forcing function that overcomes institutional inertia.

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