Joe Biden’s long-anticipated climate plan landed this week, and it contained big promises designed to please some of the presumptive Democratic nominee for president’s key constituencies. After four years of borderline denialism and regulatory rollbacks on climate by the Trump administration, the plan seemed to excite environmental advocates past the point of reality checks. But hey, that’s why we’re here.

First, let’s parse the politics of the plan. It calls for $2 trillion of spending over the next four years on areas such as energy efficiency and mass transit, up from the $1.7 trillion Biden pledged to climate issues as a primary candidate. It also calls for a move to 100% carbon-free electricity by 2035, a change from his original net-zero goal of 2050. That new, rushed timetable is direct assimilation of ideas from the progressive wing of his party. The youth-led Sunrise Movement and Washington Governor Jay Inslee, who ran against Biden for the nomination on a climate platform, quickly gave enthusiastic endorsements.

Biden’s plan also has a big social justice component, promising to use 40% of the spending commitment on accountability and infrastructure in low-income and vulnerable communities. Those measures are likely aimed at shoring up Biden’s bona fides with younger Black voters, who polls show are less committed to his candidacy than their elders.

Presidential platforms shouldn’t be judged as policy. Still, those looking at the Biden climate plan from a more technical perspective have raised serious questions about its feasibility. Biden’s goal of clean electricity by 2035, for instance, surpasses even those of the most ambitious states, such as California and New York.

Jesse Jenkins, an energy systems engineering professor, helps lead Princeton University’s Net-Zero America Project, which has been researching the infrastructure changes needed for a future free from fossil-fuel electricity. To meet Biden’s clean electricity goal and keep up with new demand from electric vehicles and heating, based on Jenkins’s estimates, the U.S. would have to build 4 billion megawatt-hours of new clean electricity generation over the next 15 years. That’s about 2.5 times the country’s current clean energy capacity (including nuclear and hydroelectric) and roughly equal to all electric generation in the country today.