IRC 415(c) limits annual additions to defined contribution plans: your elective deferrals and the employer's contributions, added together. For 2026 the limit is the lesser of $72,000 and 100% of your pay. Catch-up contributions go on top, so the most a Solo 401(k) can take is $72,000 under 50, $80,000 from 50 and $83,250 at ages 60 to 63.
The limit covers all the defined contribution plans of one employer, and of businesses under common control, together. A SEP IRA and a Solo 401(k) for the same business share one $72,000.
It does not apply to a cash balance plan, which is limited by the 415(b) limit on benefits paid out instead. That is why owners in their 40s and older can contribute several times $72,000 to a cash balance plan, and why many keep a Solo 401(k) alongside it. See cash balance plan vs Solo 401(k).