ThomasM125's answer is close, but not entirely accurate. As ThomasM125 indicates, elective deferrals (employee contributions) are permitted to be made to a solo 401(k) but not to a SEP IRA. Both are permitted an employer contribution. However, for a self-employed individual the employer contribution is limited to 20% of net earnings, not 25%. Net earnings are net profit minus the deductible portion of self employment taxes. With a solo 401(k) the employer contribution may be further limited based on net earnings and how much in employee elective deferrals is contributed.
You can see the calculation on Part III of TurboTax's Keogh, SEP and SIMPLE Contribution Worksheet (now that, as of 2/2/2023, the display of Part III has been properly updated for 2022).
