The capacity of a solar panel, solar inverter and solar battery you'll need for a 3 BHK apartment depends on your electricity usage, location, and how much you want to rely on solar power.
To power a regular 3 BHK apartment with solar system, it's a good idea to begin with a solar panel setup that has an solar inverter and battery backup (optional). If you want to reduce electricity bill then "on grid solar system" is suitable and if you want a power backup then "off grid solar system" is good for home and business. 3 to 5 kilowatt solar system is suitable for 3 BHK home or apartment.
This setup can make 15 to 25 units of electricity every day, and it can run your things like fans, lights, TV, fridge, AC and more, so you won't need to use electricity from the regular power grid. Adding a battery storage system is a good idea to store excess solar energy for use during cloudy days or at night. It's important to note that battery costs can be a significant part of your investment.
It's advisable to consult with a local solar installer who can assess your specific energy needs, available sunlight, and local incentives. They can help design a system that best suits your requirements and budget.