Employer contribution (SEP or Solo 401(k)): for a sole proprietor, about 20% of net SE earnings (profit minus ½ SE tax), from the Pub 560 rate table/worksheet.
Elective deferral limit: $24,500, plus an $8,000 catch-up at 50+ ($11,250 at ages 60–63). It's shared with your hospital 403(b)/401(k). If you max out at work, you have no Solo 401(k) deferral room left, but the employer piece is still available.
Total additions limit: $72,000 per plan. The compensation limit is $360,000.
Solo 401(k): Form 5500-EZ is required once plan assets exceed $250,000 at year end, and for the final plan year.
SEP: can be opened and funded up to the return due date, including extensions.
Payroll taxes: deferrals reduce income tax, not SE or FICA tax.
A common sequence (general information, not advice)
Many people capture any W-2 match first, then build an emergency fund and pay down high-interest debt, then consider a SEP or Solo 401(k) for 1099 profit. Deferrals save income tax at your marginal rate but tie up cash. The right order depends on your situation.