A Solo 401(k) lets a self-employed person contribute in two roles: as "employee" (elective deferral, up to a fixed annual cap, plus a catch-up amount if you're 50+) and as "employer" (roughly 20% of net self-employment income). Combined, this often lets you shelter significantly more than a SEP IRA at the same income level.
This is a simplified estimate, not tax advice. Confirm current-year IRS limits and your exact calculation with a tax professional before contributing. See the SEP IRA vs. Solo 401(k) guide for help deciding which account fits you better.