Your solo 401k has the same $18k contribution limit for 2015 that all 401k plans have, assuming you're under 50 years old.
But because you're self employed you also get access to a SEP IRA, which is also tax deductible, with a contribution limit that is 25% of your gross income (capped at $51k, just in case you're self employed and rich as hell).
So if you make 60k, you should be able to contribute 18k to your 401k and another 15k to your SEP IRA, for a total of $33k.
This would reduce your taxable income down to 27k, which is below the taxable income limit for many families. If you were a married person with two kids, for example, you'd have zero taxable income after deductions and exemptions.