In summary, we have synthesised an isocyanate-free 100%-bio-based poly(hydroxy urethane) with a glass transition temperature near 50 °C and a high density of polar groups, granting it high dielectric permittivity and low losses near room temperature. The combination of its high breakdown strength, with EB > 400 MV•m -1 , linear dielectric behaviour and low losses, enables this PHU to reach energy storage performance comparable with the state-of-the-art petrochemical polymers (discharge energy density, Ue > 6 J•cm -3 ) combined with a remarkably high discharge efficiency (η = 85%). Therefore, these bio-based PHUs represent a highly promising route to environmentally-friendly energy-storage.