Fast, low-cost, and efficient energy storage technologies are urgently needed to balance the intermittence of sustainable energy sources. High-power capacitors using organic polymers offer a green and scalable answer. They require dielectrics with high permittivity (εr) and breakdown strength (EB), which bio-based poly(hydroxy urethane)s (PHUs) can provide. PHUs combine high concentrations of hydroxyl and carbamate groups, thus enhancing their εr, and a highly tunable glass transition (Tg), which dictates the regions of low dielectric losses. By reacting erythritol dicarbonate with bio-based diamines, fully bio-based PHUs were synthesized with Tg ∼ 50 °C, εr > 8, EB > 400 MV·m-1, and low losses (tan δ < 0.03). This results in energy storage performance comparable with the flagship petrochemical materials (discharge energy density, Ue > 6 J·cm-3) combined with a remarkably high discharge efficiency, with η = 85% at EB and up to 91% at 0.5 EB. These bio-based PHUs thus represent a highly promising route to green and sustainable energy storage.