Professional indemnity and public liability divide the world between money and harm. If somebody was hurt or something was damaged, public liability. If the only loss is money and it came from your work, professional indemnity. Everything else about the two follows from that.
What each answers
Public liability: injury to a third party or damage to their property arising from your business, and the cost of defending it. Professional indemnity: financial loss caused by a negligent act, error or omission in your professional work, and the cost of defending that.
How each responds
Public liability responds to events that occur during the policy period, so it can be allowed to lapse once the activity stops. Professional indemnity responds to claims first made during the period, whenever the work was done, so it must be kept continuous long afterwards.
Why most businesses need both
Because most businesses both work where other people are and produce something somebody relies on. A consultant visits offices. A builder sometimes designs. A therapist has clients in a room. Client contracts name both for that reason.
Buying them together
As sections of one policy from one insurer, which is cheaper, simpler, and materially better when a single incident touches both. Where they sit with different insurers, tell each about the other so a contribution argument does not delay the claim.
What each costs relative to the other
For advisory businesses professional indemnity is usually the larger premium, because the losses are financial and unbounded by anything physical. For site based trades public liability usually is, because the exposure is injury on a working site. Neither ratio tells you anything about the other business, which is why comparing premiums between trades is unhelpful and comparing cover between quotes is not.
Questions people ask about difference between professional indemnity and public liability
What is the difference in one line?
Public liability covers physical harm to others; professional indemnity covers financial loss from your work.
Which lapses safely?
Public liability, once the activity stops. Professional indemnity should not, because it is claims made.
Should I buy them together?
Usually. One insurer, one certificate, and no argument when an incident touches both.