Also be aware that your combined elective deferral and employer contributions are not permitted to exceed your net earnings from self-employment. Net earnings are net profit minus the deductible portion of self-employment taxes. To be able to make an elective deferral of $9,000 to the solo 401(k), you'll need at least $9,684 of net profit (or a bit less if you max out the Social Security wage limit between the two jobs, reducing the deducible portion of SE taxes).
