The differences that matter in India are how much output a module loses as it heats up, how it behaves in diffuse light and haze, and how it holds up to dust and humidity. A module chosen on peak rated output alone can deliver less over a year than one chosen on how it behaves at forty degrees in dusty air.
The rating is a benchmark, not a forecast
Every module carries a rated output measured under standard test conditions: a defined light intensity, a defined spectrum, and a cell temperature of twenty five degrees.
Those conditions exist to make modules comparable. They are not conditions an Indian site spends much of the year in. A module in open sun in May runs far above twenty five degrees, and silicon loses output as it warms.
So the useful question is not what a module is rated at. It is how much energy it will deliver, on this site, across a year. Two modules with the same rating can differ materially on that once heat, haze and dust are accounted for.
Temperature, which matters more here than almost anywhere
The temperature coefficient states how much output falls for each degree above the test temperature. It is a small number, and it is multiplied by a large temperature difference for much of the Indian year.
This is the single specification most worth comparing between candidate modules for a hot site, and it is also the one least likely to come up unprompted.
Mounting affects it too. Modules with air moving freely behind them run cooler than modules pressed against a surface, which is one reason ground mounted arrays with clearance behave differently from rooftop installations on the same site.
Light that is not direct
A clear desert sky and a hazy Indian plain deliver light differently. Dust, humidity and seasonal haze scatter sunlight, so a meaningful share of what reaches the module is diffuse rather than direct.
Module technologies differ in how well they convert diffuse light. A technology that performs strongly in direct beam conditions and less well in scattered light will underperform its datasheet on a hazy site more than the datasheet suggests.
The monsoon compounds this. Several weeks of heavy cloud are part of the annual pattern, not an exception to it, and a yield estimate built on clear sky assumptions will be optimistic in a predictable way.
Bifacial, and the condition attached to it
A bifacial module collects light on its rear face as well as its front. The gain is real and it is entirely conditional on what is behind the module.
A pale, reflective ground surface and enough mounting height for light to reach the rear produce a worthwhile gain. Dark soil, low mounting, or vegetation growing up under the array produce very little.
On a site where the land under the array is also being used, for grazing or for cropping, the reflectivity underneath changes through the season. That is worth modelling rather than assuming, because the bifacial premium only pays back if the conditions that create the gain are actually present.
Dust, and the water it implies
Dust reduces output by blocking light before it reaches the cell. How much depends on the dust, the rainfall pattern, the tilt of the modules and how often they are cleaned.
The part that gets underestimated is the connection to water. Cleaning at scale needs a supply, and on the kind of site where solar is cheapest, water is often the scarcest input. Waterless cleaning methods exist and carry their own cost and their own wear.
Deciding the cleaning regime during design, and budgeting the water for it, avoids a site that produces less than its model for reasons the model never contained. On an integrated site this connects directly to the recharge structures, because the same parcel is being asked to hold water as well as generate.
Degradation over the term
Modules lose a little output every year. Manufacturers state a first year loss and an annual rate after it, usually backed by a warranty.
Over a twenty five year term those small annual figures compound into a large difference in total energy delivered. A module with a slightly better degradation rate can be worth more over the life of a power purchase agreement than one with a slightly better starting rating.
The warranty is only as good as the entity behind it, which for a twenty five year commitment is worth as much attention as the specification.
How to compare honestly
Ask for modelled annual energy for the specific site, with the assumptions listed: ambient temperature profile, soiling loss, cleaning frequency, ground reflectivity if bifacial, and the degradation curve used.
Then compare cost per unit of energy delivered over the term rather than cost per unit of rated capacity. Those two rankings are frequently not the same, and the second one is the one that pays for the site.
See also capacity factor and hybrid renewable site in the glossary.
Related questions
- Why does a module produce less than its rating?
- Ratings are measured under standard test conditions, which include a cell temperature of twenty five degrees. A module in an Indian summer runs far hotter than that, and output falls as temperature rises. The rating is a comparison benchmark between modules, not a prediction of what a site will produce.
- What is a temperature coefficient?
- It states how much output is lost for each degree the cell rises above the test temperature. A smaller coefficient means less loss in heat. On a hot site it is one of the most consequential numbers on the datasheet and it is rarely the one quoted in a sales conversation.
- Are bifacial modules worth it?
- They collect light on the back face as well as the front, so the gain depends entirely on what is underneath and behind them. A pale, reflective surface and a mounting height that lets light reach the rear give a real gain. Dark soil, low mounting or dense vegetation beneath give very little, and the extra cost is then not recovered.
- How much does dust cost?
- Enough that cleaning schedules are a design decision rather than a maintenance detail. Losses depend on dust type, rainfall pattern and tilt, so they vary widely by site. What matters is that the cleaning regime, and the water it needs, are planned before the site is built rather than discovered afterwards.