The most common metric we see in use is typically $/watt or Rs/watt in India. As it says, what that means is you consider the module cost, and divide it by its nameplate capacity to arrive at a number. Thus, a 500 Watt module priced at say, Rs 8000 per module will give you a RS/W cost of Rs 16 per Watt. While this measure works for smaller projects, you might want to consider many other factors for larger projects.

These range from the technology you prefer (Mono PERC or TOCon or even HJT) , to rate of degradation, thermal performance etc. With larger modules in recent years, we have also seen that these are faster and easier to install, although there is an enhanced risk of breakage as well during setup.  Similarly, how these are aligned to other key suppliers like trackers (in case you have opted for them) to even the inverters matters. So make sure to consider all these factors carefully.

Remember, cost per Watt should be your starting point, no more. Look for credible, third party validated indicators for all other key parameters, before deciding finally.