Types of solar panels: which one to choose

Both are silicon; what changes is how that silicon was formed. In monocrystalline, each cell is cut from an ingot that is a single continuous crystal, with its whole atomic structure aligned. In polycrystalline, the silicon is melted and left to solidify in a mould, forming many small crystals with boundaries between them.

Those boundaries are the origin of the difference. They act as obstacles for the electrons freed by light, and part of the energy is lost there. That is why, for the same surface, monocrystalline delivers more power. It also explains the appearance: a single crystal reflects light uniformly and looks solid black, while the multiple crystals of polycrystalline produce that characteristic bluish, streaked look.

The price difference that justified polycrystalline has narrowed considerably: making monocrystalline ingots stopped being the expensive process it once was. Finding it in a new proposal today is more a sign of old stock in a warehouse than of a design decision.

A module's efficiency is what percentage of the solar energy reaching it is converted into electricity. Current residential modules sit around 20-23%.

The usual mistake is to read it as a measure of quality. It is not: it is a measure of density. A more efficient panel delivers the same power in less surface, and that is all. If your roof is large, a slightly less efficient panel can give you exactly the same kWh a year for less money.

Where it does rule is on tight roofs. Since each kWp takes up around 5 to 7 usable m², gaining a couple of efficiency points can be the difference between covering a good share of your consumption or falling short. The right question is not 'which is more efficient?' but 'do I have roof to spare or not enough?'.

Within monocrystalline there are several cell architectures, and that is what usually appears on the datasheet as an acronym.

PERC was the standard for years: it adds a reflective layer on the rear face of the cell to make use of the light that passes through. TOPCon and HJT are the following generations, with better behaviour under temperature, lower annual degradation and a more usable rear face, which makes them the natural basis for bifacial modules.

You do not need to master the differences. What is useful is knowing that these acronyms describe different generations, that the more recent ones usually bring a better temperature coefficient and lower degradation, and that both parameters are written on the datasheet. Compare those numbers rather than the acronym.

A bifacial panel also generates from the light reaching it from behind, the light bouncing off the surface below it. It sounds like free gain, but it depends entirely on two things that have nothing to do with the panel.

The first is albedo: how much light whatever is underneath reflects. A white roof, light gravel or clear ground return a fair amount; dark tiles or brown metal sheeting, very little. The second is separation: if the panel is flush against the roof, barely any light reaches its rear face however light the surface is.

The gain usually quoted in the industry runs from 5% to 15%, but it is a range with widely separated ends for a reason: it covers everything from the bad case — low mounting over a dark surface — to the good one. On a conventional residential roof, with the structure a few centimetres up and dark tiles, the reasonable expectation is the low end, and there the premium is rarely recovered.

Where they do shine is on ground mounting, on elevated structures, over light-coloured membrane roofs and in agricultural installations with tall structures. If your installer proposes bifacial, ask them to explain what is underneath and at what height they will sit.

The datasheet capacity is measured in a laboratory with the cell at 25 °C. On a Colombian roof at midday the cell works well above that figure, and every extra degree takes off capacity.

The temperature coefficient quantifies that loss. On current silicon modules it ranges between −0.26% and −0.40% per degree. The difference between the two ends of that range, applied to a cell at 65 °C, comes to around five percentage points of instantaneous output.

Compare that with what efficiency contributes: between a 21% panel and a 22% one there is one point of difference in area needed. Between a good temperature coefficient and a mediocre one, in a hot climate, there are several points of output every day for 25 years. On the coast and in hot lowlands, that parameter deserves more attention than nominal efficiency.

'25-year warranty' means nothing on its own, because a panel carries two different warranties.

The product warranty covers manufacturing defects and usually runs from 10 to 15 years, although high-end manufacturers now offer 25. The performance warranty is something else: it guarantees a minimum percentage of output over time, expressed as a curve — a somewhat larger drop in the first year and a maximum annual degradation after that — down to a final value usually around 85-90% of the original capacity.

Two panels advertised with '25 years' can have different curves, and that difference is energy and money. Ask for the curve, not the headline. And check that the manufacturer has representation in Colombia: a warranty that requires exporting the module to enforce it is not a practical warranty.

The panel technology influences efficiency and the area you will occupy, but the decision also depends on capacity per panel, the manufacturer's warranty and the quality of the inverter it will work with.

A highly efficient panel with a mediocre inverter or a poor installation will not perform as expected. Assess the whole system, not just the panel: the difference between well-ventilated mounting and one flush to the roof, or between a well-designed string and one with shading, comfortably exceeds the difference between two panel models of similar grade.

These are the ones that most often lead to overpaying or underperforming.