PV Cells, Modules & Efficiency

Solar generation is built from one repeating block, scaled up in two steps. The cell is the basic unit - the silicon device of the last lesson - and, as we saw, it produces only a small voltage (roughly half a volt) and a modest current, a few watts at most. That is far too little to be useful directly, so cells are combined. Wiring cells in series (end to end) adds their voltages: string enough cells together and you reach a working voltage; this is the essence of a module - the familiar rectangular 'solar panel', typically dozens of cells laminated between a protective front (usually glass) and a weatherproof backing, framed and fitted with electrical connectors. A module is rated in watts (its power under a reference condition) and is the unit you actually buy and mount.

Scaling up once more, multiple modules are connected into an array - modules wired in series into 'strings' to build voltage, and strings in parallel to build current - sized to the power the project needs and rated in kilowatts. A small home rooftop might be a few kilowatts of a handful of modules; a large commercial roof or facade, many tens or hundreds. The array, with its mounting and wiring, is what physically becomes the generating surface - and in BIPV, that surface *is* the roof, facade or glazing rather than a separate layer bolted on top.

This hierarchy matters to a designer for a very practical reason: it is modular and granular, which gives real design freedom but also real constraints. You can scale generation up or down by adding or removing modules, and you can shape an array to a surface. But because cells within a module and modules within a string are wired together, they are electrically interdependent - which is why, as the next lesson shows, shading one part can drag down far more than its share, and why the layout of cells and modules is not a purely visual choice. For now, hold the clean picture: a cell makes a whisper, a module is many cells wired to a useful voltage, an array is many modules sized to the job - and the honest performance and wiring design of that array is engineering, done with the manufacturers' data and the codes.

Efficiency is the number everyone quotes and many misunderstand. For a solar cell or module it means one specific thing: the share of the sunlight energy landing on it that comes out as electricity. If 1000 W/m2 of sunlight falls on a module and it delivers 200 W of electrical power per square metre, its efficiency is about 20 percent - the rest of that incoming energy is lost, mostly as heat and to the fundamental limits we met in lesson 1.2 (photons too weak to free an electron, surplus energy above what is needed wasted, reflection and internal losses). Efficiency is therefore always well under 100 percent, and honestly so; a perfect converter is physically impossible, and even the theoretical ceiling for a simple silicon cell is far below 100.

What are the honest ranges? For the mainstream crystalline-silicon modules that dominate the market, commercial module efficiencies broadly sit in the high-teens to low-twenties percent, with premium products a little higher and the best laboratory cells higher still (lab records are not what you can buy). Thin-film technologies are generally somewhat lower in efficiency but have other advantages this course explores later (Module 3). Every one of these figures is illustrative and moves as the technology improves - treat them as a sense of scale, not a specification, and read the actual product's verified data. The key mental model is simply that a good modern module turns roughly a fifth of the sunlight on it into electricity, and that this fraction, while modest-sounding, is enough to make solar one of the cheapest sources of new power on earth when there is area to work with.

It is also worth separating *cell* efficiency from *module* efficiency: a module is always a little less efficient than its bare cells, because the frame, gaps between cells, glass and wiring take up area and add small losses. And rated efficiency is measured under idealised laboratory conditions (next section) - real surfaces, hot, dusty, imperfectly angled and partly shaded, deliver less. None of this is a criticism of the technology; it is the discipline of reading efficiency honestly. Efficiency tells you how hard each square metre works - and the next idea is why that matters far more in some places than others.

Here is a point that reframes efficiency for BIPV specifically. If you have unlimited, cheap, unshaded space - a big open field or a vast warehouse roof - efficiency matters surprisingly little to whether the project works: you simply add more modules to reach the power you want, and lower-efficiency modules just take more area, which is fine if area is free. This is why large ground-mounted solar farms can happily use less-efficient, cheaper modules. In that world, cost per watt often beats efficiency.

But buildings, and BIPV above all, usually live in the opposite world: area is scarce and precious. A facade has only so many square metres; a small urban roof is quickly filled; a shading fin or a glazing panel offers a limited surface that must also do its architectural job. When the area is fixed and limited, the only way to get more power out of it is higher efficiency - more watts from every square metre. So for a compact roof, a generating facade, or any surface where you cannot simply spread out, efficiency becomes the decisive property, often worth paying a premium for. This is one honest reason BIPV projects frequently reach for higher-efficiency (and costlier) modules, and why the trade-off between efficiency, cost and appearance is so central to designing with BIPV (Module 5).

There is a further BIPV twist: integration and aesthetics often *reduce* the effective efficiency of a surface - coloured cells, semi-transparent glazing, or cells arranged with visual spacing all sacrifice some output for appearance or daylight. So on an area-limited surface you are frequently spending efficiency in two directions at once (for looks and for light) while needing it most. Holding this clearly is part of designing honestly: on a generous, cheap, sunny area, chase cost per watt; on a scarce, valuable, architectural surface, efficiency (watts per square metre) is king, and every aesthetic choice that lowers it must earn its place. The exact modules, their verified efficiencies and the resulting area-versus-power balance for a real project are worked out with the manufacturers' data and the engineers - your job as a designer is to know which world a given surface lives in and to weigh efficiency accordingly.

To compare modules at all, the industry needs a common yardstick, and that is rated power measured under standard test conditions (STC). A module's headline rating - its watt-peak (Wp), or for arrays kilowatt-peak (kWp) - is the electrical power it produces under a defined laboratory reference: an irradiance of 1000 W/m2 (recall that round number from lesson 1.1), a specified cell temperature of 25 degrees Celsius, and a standard spectrum of light. So a '400 Wp' module is one that produces 400 watts under exactly those conditions. The word 'peak' signals that this is a reference maximum, not an everyday figure - it is the label on the tin, defined so that a 400 Wp module from one maker can be fairly compared with a 400 Wp module from another.

The honesty this demands is important, because STC is deliberately idealised and real surfaces almost never meet it. Actual irradiance is usually below 1000 W/m2; cells in the sun run far hotter than 25 degrees (often 40-60 or more), and since PV loses efficiency as it heats, a hot module produces meaningfully less than its Wp rating - a caveat that bites hard in Indian heat and on poorly ventilated facades. Add dust, imperfect angle, shading, wiring and inverter losses, and the energy a system actually delivers over a year is always some fraction of what its rated kWp might naively suggest. This is precisely why rated power (kWp, a capacity) and actual yield (kWh generated, an energy) are different things and must never be conflated - the gap between them is real system engineering, captured by a performance ratio that engineers estimate with proper tools.

For a designer, the takeaways are practical. Wp and kWp are a fair way to compare and to size at a first pass (a rough sense of how much capacity a surface might hold), and STC is what makes that comparison meaningful - but they are a reference condition, not a promise of output. Rated power sits alongside efficiency (watts per square metre under STC) and area to give you an honest feel for a surface's potential. The binding numbers - the actual yield in kWh, the temperature and soiling losses, the system sizing and the payback - belong to qualified engineers using the manufacturers' verified data, the real site conditions and the governing standards (module standards such as the relevant IEC series, and in India the applicable IS/IEC and codes). Read the spec sheet fluently; defer the guarantee.