Generally speaking, a pianobar player will need the same kind of system a 4-piece band would need.
This would include:

Mixing desk. -Mixer with at least 12 input channels and built in digital reverb OR: Mixing desk with at least 14 input channels and a separate digital reverb. (note digital reverb is essential)

Monitor speaker for the artist. For example JVC 10-inch powered monitor.Amplifiers and Speakers.

Amplifiers/speakers - We recommend a split system with a crossover feeding to separate bass and top amplifiers. Total power should be around 1.5-3kw for most average sized pubs (100-200 capacity crowd). Bass cabinets are usually placed under the piano. It can often be difficult to find the right place for the top speakers. The classic position a band would use of having the speakers point away from the artists and towards the audience is not always the best configuration since many of the guests will be around the piano. We often see the main pair of speakers positioned a few metres away from the piano pointing TOWARDS the piano. It can also be a good idea to place some smaller, 'satellite' speakers around the rest of the room.

Cables. - Microphone cable and all cables for the sound system and electric piano are provided by the venue. Microphone is usually provided by the artist, but it is advisable to have a spare. Cables for any bass keyboards, drum machines etc, are provided by the artist. It is a good idea to get some extra plug sockets/power points at the piano for the drum and bass machines.

Piano - A grand piano shell is required fitted with a digital piano keyboard. See info on ‘electric refit for grand piano’