Building electrification is essential to many full-economy decarbonization pathways. Current decarbonization modeling does not incorporate fluctuations in building energy demand. U.S. monthly average total building energy consumption varies by a factor of 1.6x—lowest in May and highest in January. This is largely managed by fossil fuel systems with storage capability. Under all prototypical coefficients of performance (COPs), electrifying buildings substantially increases winter electrical demand, enough to switch the grid from summer to winter peaking. Meeting this with renewables would require a 28x increase in January wind generation, or a 303x increase in January solar, with excess generation in other months. Technologies with a COP of 6 shrink the winter peak—requiring 4.5x more generation from wind and 36x more from solar.