Betavoltaic energy converters (i.e., β-batteries) are attractive power sources because of their potential for high energy densities (>200 MWh/kg) and long duration continuous discharge (>1 year). However, conversion efficiencies have been historically low (<3%). High efficiency devices can be achieved by matching β-radiation transport length scales with the device physics length scales. In this work, the efficiency of c-Si devices using high-energy (>1 MeV) electrons emitted from 90Sr as a power source is investigated. We propose a design for a >10% efficient betavoltaic device, which generates 1 W of power. A Varian Clinac iX is used to simulate the high-energy electrons emitted from 90Sr, and a high efficiency c-Si photovoltaic cell is used as the converter. The measured conversion efficiency is 16%. This relatively high value is attributed to proper length scale matching and the generation of secondary electrons in c-Si by the primary β-particles.