Since becoming self-employed, which was not so much of a choice for me, but more a forced path, I have had to become privately insured when it comes to health insurance. I had very much hoped to avoid doing so, but it turns out that public health insurance gets very expensive if you are no longer an employee, or Angestellte(r).
You are “allowed” to join the private system only if you are self-employed, are a civil servant, or if your income is above a threshhold, right now which is at about €52,000 per year. If you are an employee and decide to remain in the public system, you are considered freiwillig (voluntarily) insured. In the public system, if you are below the threshhold, you are pflichtversichert (mandatorily publicly insured). Ruth wrote about the benefits and detriments of the two systems.
If you are self-employed and then becoming a regular employee again, you may be able to return to the public system, but usually only if you earn less than that year’s upper limit. The good thing about the public system is that they have to accept you, no matter what the state of your health. The private ones can get picky. Also, if you are publicly insured, your kids are usually covered for free. That is, unless your partner is privately insured and earns more than you. In that case, the kids have to be privately insured as well, and at a cost.
The private system bases the premiums on a couple of factors. The most important one that I am aware of is age. The older you are when you join, the more expensive the premiums are. My deductible is huge by German standards, and I feel like I am paying for the privilege of having health insurance. The broker called it catastrophe insurance. I have noticed, however, that I am treated slightly better, especially when it comes to getting into doctors like the dermatologist that we go to in Heidelberg, who seems to have very few appointments for non-privately insured people.
If I had the choice, I would be publicly insured. But if you are self-employed you basically have to pay the entire 15% of your income to the health insurance company, and it seems they don’t base that number on what you actually earn — at least until you have a tax return. It appears to be based on the highest income a person could have without being privately insured. So it is something like €700 per month. Too much for me at this point! Even if I were to earn much less, the lowest rates are based on an income of €2021 per month, and that is without long-term care insurance. And if I were sick for a longer time period, there is also no Krankengeld (sick pay).
The Techniker Krankenkasse has a few examples of how it is calculated here (in German): http://www.tk.de/tk/bei-der-tk-versichert/selbststaendige/selbststaendige/346546.