Best choice for 1 MWp PV Power plant, Central or String Inverter?

Both technologies are acceptable. On one hand the string inverter will deliver better performance, easier installation and built in string level monitoring. There is also 20-40 times as many parts as there are in central inverter system. The central inverter is going to require less capital and less O&M cost.

There are several considerations that need to be assessed before making a design choice - this has to be a techno-commercial decision (include your design Engineer in this conversation). Just to clear a premise, many central inverters now come with modular design, and therefore, partially overcome some of the traditional limitations on O&M (or full loss of production).

1. What does PV plant design looks like: Contiguous or distributed PV plant? If distributed what are the sizes of the individual plant? Is it ground mount or roof-top? What is the availability of land/real-estate where inverter/transformer can be housed/kept? What obstructions on the plant site exist that need to be overcome?
2. What are the technical requirements from module perspective: Does it need grounding? Any statistical information about the spread of module degradation (comparison within modules)?
3. What is the interconnection requirement? What voltage do you need to connect (do you even need a transformer)? I have been part of large PV installs where one can connect directly to the grid (480V or 208V etc.)
4. What is the level of monitoring required (and this in-turn goes back to contractual requirements on power plant availability? Power plant level, Array level, String level etc.
5. What is the financial success metric - ROI (Energy harvest) or Capex? What is approved by the financing institution or funding partner? Is there multi-lateral funding on this project? What is your business model (will own the asset or sell it some time later)?
6. What support is available from inverter manufacturer today and what assumptions about support have you factored in for next 20 to 25 years? Remember, there is consolidation happening in the industry. This goes from technical personnel to parts to organization etc.
7. Have you considered cost from a Total Cost perspective? The BOM with a string inverter will be much different from the BOM for a string inverter.

Going by opinions without analysis could lead to false impressions. My guess is central inverter is used if the initial cost of the project is a driving factor, especial if there is low $/kWh. If the O&M company wants to monitor and maintain a much larger fleet of equipment, then the string inverter may be a good choice. What you really need to ask is what happens after 10-years of operation? Are we going to have to rebuild one central inverter or do we need to replace a large percentage of the string inverters?