Put two 600 Wp modules side by side on a datasheet and they look identical. Put them on the same roof for a year and the meter can tell a different story. The gap is small each day, but over the years it adds up. Here is where it comes from, and what Solitech Solar publishes on its datasheets so buyers can compare.

A solar cell is rated on its own, under controlled laboratory conditions. The module that reaches your site adds glass, polymer layers, metal ribbons and a frame around that cell, and each one changes the final output. In well-built modules, this cell-to-module (CTM) gap typically takes away around 2-4% of the cell’s efficiency.

Every few weeks another solar tender closes somewhere in India and another gigawatt gets announced. It sounds like progress, and it is. But anyone who has run a plant for a few years knows the number in the announcement is not the number that pays the bills.