One question decides which panel technology you should buy, and it is not a question about panels. It is: has my roof run out before my kilowatts have?
Go and measure the roof before you read any datasheet. If the answer is no — if you have more roof than you need — then the efficiency comparison below is almost entirely irrelevant to you, and a vendor leading with it is selling the wrong feature.
What efficiency actually buys
A panel's efficiency decides how many square feet a kilowatt occupies. That is all it decides. It does not change how many units a kilowatt generates. A 3 kW array of polycrystalline panels and a 3 kW array of TOPCon panels on the same roof, in the same sun, make broadly the same number of units. One takes more space doing it.
| Technology | Area per kW | Area for 3 kW | Roof needed at 80% usable |
|---|---|---|---|
| Polycrystalline | 115 sq ft | 345 sq ft | 431 sq ft |
| Mono PERC | 95 sq ft | 285 sq ft | 356 sq ft |
| TOPCon / N-type | 80 sq ft | 240 sq ft | 300 sq ft |
| Bifacial | 78 sq ft | 234 sq ft | 293 sq ft |
The last column is the one to use, because only about 80% of a roof holds panels once the water tank, the stair head, the parapet shadow and walking space are taken out. That rule and the reasoning behind it are in the roof space guide.
The test that settles it
Two numbers, one comparison. Take a 400 sq ft terrace.
The compact panels buy you one extra module — 0.545 kW, worth roughly 766 units a year in a region with 5 peak sun hours, or about Rs 6,100 at a Rs 8 tariff.
Now ask what you actually need. If your consumption points at 3 kW, that extra panel is surplus you may not be paid much for, and the premium bought you nothing. If your consumption points at 4 kW and the roof is 400 sq ft, the compact panel is the only way to get there.
Pricing the premium properly
Do not argue about efficiency percentages. Convert the premium into the price of a kilowatt and compare it with what a kilowatt costs anyway.
Worked on the roof above:
Rs 33,000 for a kilowatt that would otherwise cost Rs 55,000 or more. On a constrained roof that is a good buy, and it is the clearest case for paying the premium.
Run the same sum on a 900 sq ft roof where you only want 3 kW. The extra capacity unlocked is zero, the division is undefined, and the premium is pure cost. Same panels, same price, opposite verdict — because the constraint changed, not the technology.
What each technology actually is
Polycrystalline
The older construction — cells cast from multiple crystal grains, visibly blue and speckled. Cheapest per watt, largest per watt. It still appears in quotes, usually the cheap ones, and it is a reasonable choice on exactly one condition: you have roof to spare and you want the lowest capital cost. At 21% more area than mono PERC, that condition fails more often than people expect.
Mono PERC
Single-crystal cells with a passivated rear surface. This is the default in Indian residential solar now, it is what most quotes are built around, and it is the sensible baseline to compare everything else against. If a vendor cannot tell you which of these four he is supplying, that is a bigger problem than the choice itself — the quotation guide covers how to pin it down.
TOPCon / N-type
A newer cell structure, more efficient per square metre, and usually quoted with a lower temperature coefficient and lower claimed annual degradation than mono PERC. The temperature point is the one with real substance in India, where panels lose 8 to 12% of output to heat. Ask for the temperature coefficient off the datasheet rather than accepting "better in heat" as a claim.
Bifacial
Generates from the rear face as well as the front. The rear-side gain depends entirely on what is underneath and how high the panel sits — reflective surface, open mounting, clear air gap. Flat on a dark RCC terrace at low height, the rear contribution is small. Its front-side efficiency is good, so it earns its place in the table on area alone, but do not pay for the bifacial gain unless the mounting design actually delivers it.
What we do not vary by technology
Our 25-year projection applies 0.6% annual degradation to every panel type. N-type modules are often sold on a lower figure, and they may well deliver it, but 0.6% is the level the performance warranties assume and we would rather project the floor than the brochure.
Generation per kW is also held constant across technologies. The differences that exist are small, second-order, and swamped by shading, orientation and dust — all three of which you can influence far more than the cell structure.
When the premium is genuinely worth it
| Situation | Verdict |
|---|---|
| Roof too small for the capacity you need | Pay the premium. This is the case it exists for. |
| You want to keep part of the terrace usable | Pay it — fewer square feet of panels means more terrace left |
| Flat site, high ambient heat, long cable runs already optimised | Worth asking for the temperature coefficient comparison |
| Plenty of spare roof, capacity already met | Do not pay it. You are buying density you cannot use. |
| Heavy shading you cannot design out | Spend the money on layout and optimisers instead |
| Budget is the binding constraint, roof is not | Mono PERC, or polycrystalline if the area genuinely fits |
Three of those rows are about geometry and one is about heat. None of them is about the word "efficiency", which is how the choice is usually sold.
Panel count, and why it matters more than wattage
Higher-wattage modules mean fewer panels for the same kilowatts, fewer mounting points and fewer connections. They are also physically larger and heavier, which matters on a small or awkward roof where a 585 W module may simply not fit the available rectangle when a 450 W one does.
Whatever the wattage, do the multiplication yourself. Panel wattage times quantity is your real system size, and it is the number the DISCOM inspection compares against your approval. The roof area calculator works both directions — roof space needed for a target size, or the largest system a measured roof will take — and compares all four technologies side by side on the same area.
Questions people ask
Which panel type gives the most units?
Per kilowatt installed, broadly the same. Per square foot of roof, the order in the table above. If your roof is not full, the first answer is the one that applies to you.
Is polycrystalline a bad choice now?
Not bad, just area-hungry. It needs 21% more roof than mono PERC for the same output. On a large flat terrace with a tight budget it still works. On a typical urban roof it usually costs you capacity you wanted.
Should I mix panel types on one roof?
Not within a string. Mismatched panels pull a string down to the weakest member, which is the same mechanism that makes shading so damaging. If you must mix, each type needs its own string and the inverter has to support it.
Do bifacial panels really generate 10 to 20% more?
Only with the mounting and the surface to support it — height, air gap, and something reflective underneath. On a conventional low-mounted rooftop installation, expect the rear gain to be modest. Judge the quote on the front-side rating unless the design specifically engineers for the rear.
My vendor offers mono PERC and TOPCon at the same price. Which one?
TOPCon, with no further thought. The premium is the only argument against it, and at equal price there isn't one.
Does panel type change the cleaning or maintenance?
No. Dust settles the same on all of them, and the cleaning routine is identical. Bifacial on a raised frame is slightly more awkward to reach, which is a mounting consequence rather than a panel one.