This study aims to develop a reliable instrument to measure how latent motivation relates to technology use and English language proficiency during self-directed learning among Korean high school students. The study involved 175 participants who completed two main instruments: a background questionnaire and the Self-directed Technology Use Motivation in English Learning Scale (STUMELS). Exploratory factor analysis was used to discover that the instrument had a high validity and that there were six underlying variables: positive emotions towards technology use, external expectations and goal setting, social interactions, usefulness and familiarity of technology, cost value, and understanding language and cultures of English-speaking countries. The participants reported usefulness and familiarity of technology as the most motivating factor while social interactions was the least motivating one. In addition, correlation analyses show that English competence exhibited a positive correlation with motivation. This study also found that when students use technology to learn English outside of the classroom, overall English competence as well as listening skills are closely linked to the factors of social interactions, usefulness and familiarity of technology, and understanding language and cultures of English-speaking countries. This study concludes by offering suggestions to teachers on how to help their L2 learners use technology more efficiently outside of the classroom.