How much roof space does a solar system need?

Start with the capacity you need

Roof area is a result, not a starting point. Decide the capacity first, from the customer's needs and budget, and then see whether the roof can hold it. A customer who wants a larger system than the roof allows needs to hear that early.

Capacity is quoted in kilowatts, and an array and an inverter are rated differently, so say which one you mean. The glossary entry on kW and kWp explains the difference.

Measure the usable area, not the total area

The total roof area is never the area you can use. Take the measured area and remove everything that blocks or shades it.

• Water tanks, stair covers, vents and other structures on the roof.
• Parapet walls and nearby buildings, trees or poles that cast shade at any hour of the day.
• Strips kept clear for walking, maintenance and fire or safety access.
• Parts of the roof that cannot carry the load or that the customer wants kept free.

Allow for spacing, tilt and orientation

Modules on a flat roof are usually tilted, and tilted rows cast shade on the row behind them, so they need space between rows. A sloped roof lets modules lie close to the surface but fixes their direction.

Because layout, tilt and spacing change the area used, the same capacity can need a different amount of roof on two sites. This is why a rule of thumb for area per kilowatt is unreliable, and this guide gives none.

Turn the area into a module count

Take the area of one module from its datasheet, add the spacing your layout needs, and divide the usable area by that figure. Multiply the number of modules that fit by the module rating to see the capacity the roof can hold.

If the result is below the capacity the customer wants, the options are higher-rated modules, a smaller system, or another roof. Do not squeeze modules into shaded or blocked areas to reach a number.

What to state on the quotation

Write down the assumptions the capacity rests on: the usable area, the module make, model and rating, and the shading you found. If a survey has not been done, say that the capacity is subject to a site survey. If the project takes a scheme subsidy, name the modules' DCR status as well.

Step by step

1. 1

   Fix the capacity

   Decide the capacity first and say whether it means the array or the inverter.

2. 2

   Measure the usable area

   Remove tanks, structures, shaded parts and access strips from the measured roof.

3. 3

   Allow for the layout

   Add the spacing the tilt and the orientation need.

4. 4

   Count the modules

   Divide the usable area by the area of one module and its spacing, using the datasheet.

5. 5

   State the assumptions

   Write the usable area, the module details and any shading on the quotation.

Common mistakes

• Using a rule of thumb for area

  Layout, tilt and module size change the area, so a fixed figure per kilowatt can be wrong for a particular roof.

• Counting shaded or blocked areas

  Modules in shade or on blocked parts lose output, so leave them out of the usable area.

• Forgetting access space

  Strips kept clear for walking and maintenance are not usable for modules.

• Promising a capacity before a survey

  Say that the capacity is subject to a site survey until one has been done.

What to check before you send

• The capacity is stated and says whether it means the array or the inverter
• The usable area excludes shaded and blocked parts and access strips
• The module count comes from the datasheet area, not from a rule of thumb
• The quotation states the assumptions and any shading found
• A site survey has been done, or the quotation says it is still needed

Sources

• MNRE office memorandum on the DCR verification portal, read 10 Oct 2026