By Vvon Engineering Team | Published | Updated
Calculate module area, then assess walkways, tilt, shading, access and the distinction between DC kWp and inverter AC capacity.
Solar EPC & street lighting RFQ checklist (TXT)
The number quoted in most conversations is the glass area, and the glass area is the easy part. It is a division and a multiplication, and it is the same anywhere in the world.
Read the rated power and dimensions of the proposed module. Divide the target DC capacity by module power, round to a feasible module/string count, then multiply by module area. For illustration only, 200 modules rated 500 W each provide 100 kWp; if each occupies 2.2 m², their combined face area is 440 m². Substitute the actual datasheet values before quoting a roof capacity.
Then the useful question begins, because that is the area of the modules, not the area of roof you need to put them on.
Roof geometry and the mounting arrangement determine the additional area required for spacing and access.
On a pitched sheet roof, modules are mounted coplanar with the sheet, lying parallel to it. No row shades the row behind, so the panels sit close to edge to edge and the only losses are walkways, ridge and eaves clearances and the edge setbacks. This is the efficient case, and it is why factory sheds carry so much more capacity than their footprint suggests.
On a flat RCC roof, south-facing tilted rows need spacing to limit inter-row shading; an east–west arrangement has a different layout and output profile. Select tilt, row pitch, access routes and setbacks through a site-specific shading and structural assessment. There is no single roof-to-module area multiplier that works for every arrangement.
| Roof arrangement | Area calculation | Additional allowances |
|---|---|---|
| Pitched sheet roof | Module count × actual module face area | Walkways, module gaps, safe edges, rooflights, obstructions and mounting constraints |
| Flat roof with tilted rows | Module face area plus row spacing from the shading model | Access, fire routes, parapet shading, ballast or anchoring and equipment platforms |
Those roof-area figures assume a clear rectangle with nothing on it, which no real roof is.
The most frequent mismatch between two quotations for the same roof is not the module make. It is that one contractor has quoted 100 kilowatts of inverter capacity and the other has quoted 100 kilowatts peak of modules, and on a plant designed with a DC oversize those are meaningfully different quantities of glass, of area and of money.
Ask the question directly, in writing: is the 100 KWp figure the DC array capacity or the AC output rating, and what is the DC to AC ratio. Then compare the two proposals on the same basis. Everything else in a rooftop comparison is easier once that is settled.
Ask for a layout showing module dimensions, mounting, walkways, setbacks, obstructions and the shading assumptions. A satellite-based capacity is preliminary until access, structural capacity and roof geometry have been checked.
Consult the relevant edition and the documents for the exact equipment and site: JinkoSolar: model datasheets, certificates, installation manuals and warranties; IEC 62548-1: PV array design requirements.
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