CO2 emission is more serious in recent years and automobile manufacturers are interested in developing technologies to reduce CO2 emissions. Among various environmental-technologies, the use of solar roof as an electric energy source has been studied extensively. For example, in order to reduce the cabin ambient temperature, automotive manufacturers offer the option of mounting a solar cell on the roof of the vehicle . In this paper, we introduce the semi-transparent solar cell mounted on a curved roof glass and we propose a solar energy management system to efficiently integrate the electricity generated from the solar roof into internal combustion engine (ICE) vehicles. In order to achieve a high efficiency solar system in different driving, we improve the usable power other than peak power of solar roof. Peak power or rated power is measured power (W) in standard test condition (@ 25°C, light intensity of 1000W/m2(=1Sun)). However, improved fuel efficiency cannot be measured in the US driving test cycle. Therefore, we have to establish a new test methodology to get CO2 credit based on off-cycle credit regulation from the Environmental Protection Agency (EPA) & the National Highway Traffic Safety Administration (NHTSA). We expect that the alternative methodology our solar power system will contribute to the automotive industry by reducing CO2 emissions in the near future.