How to choose panels for a small roof or a balcony

In short: On a small roof, choose the module format before the efficiency. Only whole modules count, so the way a module's length and width divide your roof sets how many fit, and efficiency can only work on that count. On our 4.5 by 3.5 m example roof, 2382 x 1134 mm modules fit 3 for 1.88 kWp, and the larger 2384 x 1303 mm ones fit 2 for 1.40 kWp.

Efficiency and watts per square metre are one question

With little space, the obvious move is to buy the most efficient module you can find. Efficiency matters. On a roof that takes a handful of modules, though, it isn't the first thing to settle.

There seem to be three ways to get more power onto a small area: a more efficient module, a module with more watts per square metre, or a module whose size happens to fit. The first two are the same thing. Rated power is measured at standard test conditions, with 1000 W/m² of light on the module, so watts per square metre is module efficiency times ten. A 22 % module puts 220 W on every square metre it covers.

So the real choice is between efficiency and format, meaning the module's outer length and width.

The example roof

Most of the catalogue's 14,005 modules share a handful of formats, because manufacturers build around a few cell sizes and cell counts. The article on module sizes shows the clusters. I took the five most common formats and, for each, the median rated power and the power of the most efficient tenth (the 90th percentile).

The roof is 4.5 m along the eaves and 3.5 m up the slope, with 0.3 m kept free on every side and 0.02 m between rows. These are the roof-fit selector's defaults, and they leave a usable area of 3.9 m by 2.9 m. Modules go in a plain grid, all portrait or all landscape, whichever fits more.

Counting the modules that fit

Doing the count once by hand shows where modules get lost. Per orientation you need two numbers: how many fit across the 3.9 m width, and how many rows fit up the 2.9 m slope. Every row but the last needs the 0.02 m gap after it, which is why 0.02 m is added on both sides of the row calculation.

1762 x 1134 mm: four in landscape

In landscape each module takes 1.762 m of the width: 3.9 / 1.762 = 2.2, so two per row. Each row takes 1.134 m plus the gap: (2.9 + 0.02) / (1.134 + 0.02) = 2.5, so two rows. Four modules.

Portrait fits three across (3.9 / 1.134 = 3.4) but only one row up the slope, because (2.9 + 0.02) / (1.762 + 0.02) = 1.6. Three modules, so landscape wins.

2384 x 1303 mm: two, whichever way you turn it

In portrait, 3.9 / 1.303 = 2.99. Three side by side would need 3.909 m, 9 mm more than the roof has, so it's two per row. Up the slope only one row fits: (2.9 + 0.02) / (2.384 + 0.02) = 1.2.

Landscape is no better: one per row (3.9 / 2.384 = 1.6) and two rows ((2.9 + 0.02) / (1.303 + 0.02) = 2.2). Two modules either way.

2382 x 1134 mm: three in portrait

Long but narrow. In portrait, 3.9 / 1.134 = 3.4, so three fit across, and one row fits up the slope. Three modules.

The figure below runs the same count on the same roof. Change the size or the margin, add a chimney and move it, and watch which modules survive.

Format against efficiency on the example roof

The first table describes the modules: how common each format is in the catalogue and the median rated power of a module in it.

The second puts each format on the roof. "Fit" is the count from the arithmetic above; the kWp columns are the roof's total with median modules and with modules from the most efficient tenth. Read across a row to see what efficiency adds within one format, and down the rows to see what the count does.

The count multiplies everything else. One module more or less moves the total by a whole module's power, while a better module of the same format moves it by the gap between the last two columns. Set those two against each other in the table. The largest module fits the fewest here, so its extra watts per module have to cover a missing module before it can lead.

What a step up in efficiency costs

With the format fixed, a more efficient module of the same size is the only lever left, so it helps to know whether it costs more per watt. The table shows the median price per watt of matched single modules in Polish shops, before VAT, in euro. Poland is the one market in our offer data with enough matched offers in every band.

Market medians mix cheap shops with dear ones, so the cleaner test is inside one shop. Of the 13 Polish shops that list matched modules in both upper bands, 11 have a higher median price per watt for the more efficient ones. The median same-shop difference is 6 %.

The lowest band isn't part of that test. Its median compares different modules in different shops, so don't read it as the price of lower efficiency.

What this means when you buy

Choose the format first

The count of whole modules depends only on how the module's length and width divide your usable area. Efficiency has nothing to do with it.

Measure the roof and take off the edge clearance your mounting system and local rules need. Then run the roof-fit selector: it tries every module size in the catalogue on your dimensions and ranks them by kWp. Compare efficiency only within the format that wins. How much power the roof should hold in the first place follows from your consumption and your site; the article on how many panels a household needs sets out that calculation.

Measure to the centimetre

In the example, the largest format missed a third module across the roof by 9 mm. Ask your installer whether their edge clearance is a fixed requirement or a default that can be tightened. On a small roof, a few centimetres of margin can decide a whole module.

Then compare efficiency on price

Whether the step up in efficiency is worth it for you is a price comparison between two specific modules of your chosen format. The panel comparison calculator runs it with your own prices.

Use the power tolerance as the tie-breaker

With three or four modules, every watt per module shows up in the total. A positive-only power tolerance is a small but real difference between two modules of the same rated power, and I'd use it to settle a tie.

Balcony solar: a small roof with stricter rules

A balcony adds legal limits to the geometry. In Germany, the Renewable Energy Sources Act (EEG 2023, section 8(5a)) gives plug-in solar devices a simplified connection when they total up to 2 kW of installed module power and up to 800 VA of inverter power, behind the household's meter. Other countries set different limits, or none, so check the rules where you live.

How many modules the 2 kW allows depends on each module's power. At the median power of the 1722 x 1134 mm format, 4 modules fit within it; at the median of the 2384 x 1303 mm format, 2. The 800 VA limit is a separate cap on what the inverter feeds into the house at any moment, whatever the modules could produce.

A railing fixes one dimension. Unless it's taller than the module is long, the module hangs with its long side along the rail, and the railing's length alone decides how many fit.

Flat on the rail or tilted out

A module hung flat against the railing stands vertical. At European latitudes a vertical module catches less sun over a year than one tilted back toward the sky, so brackets that angle it outward usually recover some of that loss.

The price is that a tilted module stands off the railing and the wind can get behind it. Either way, a balcony module hangs on a structure that wasn't designed for it, and wind acts on area. Check the mechanical load rating and the mounting before the watts.

Example modules and live prices

When this was written, the format with the most kWp on the example roof at median power was 2382 x 1134 mm. Below is the module in that format with the most shop offers at the time, with its five cheapest live offers. Each price is printed as the shop shows it, with or without VAT.

Cheapest current offers for Jinko Solar JKM620N-66HL4M-BDV, prices read 9 Oct 2026.

Each price is the figure the shop printed, on its own side of VAT.

For comparison, the catalogue's modules with the most power per square metre, whatever their format, picked by the rule in the caption:

Top 3 by power per square metre, one module per datasheet, among 12,655 modules of 300 W or more with a stated cell technology and a stated size, as of the last catalogue sync.

Limits of this example

It's one roof. A different roof can change the order of the formats completely; what carries over is the method, and the roof-fit selector does the same arithmetic for your dimensions.

The grid is plain: one orientation, no chimneys, vents or windows, no fire setbacks beyond the uniform margin. Real obstacles and setbacks split the free area into smaller patches, and each patch becomes its own small-roof problem. The count also ignores the clamp gap, a centimetre or two, that a mounting system leaves between neighbouring modules in a row, so a real installation would miss that third module by more than 9 mm.

An installer can sometimes fit one more module by mixing orientations, which the plain grid doesn't try.

The catalogue figures are what the datasheets state, at standard test conditions rather than on a roof; the comparison holds because every module in the tables is rated the same way. The prices cover one market and matched offers only, and they move daily. The exact counting rules for every figure are under "Figures and denominators" below.