Choosing a rooftop solar size is a matching exercise. The system should fit the property's electricity demand, physical roof and connection conditions rather than simply filling every available square metre.
Begin with electricity consumption
Review recent bills and calculate the pattern of monthly electricity units. Separate normal months from seasonal peaks. If the property has a smart meter or interval data, identify how much electricity is used while the sun is available.
The bill amount alone is not enough because tariffs, fixed charges and taxes can change. Electricity units provide a clearer starting point for estimating how much solar energy the site may be able to use.
Decide what the system should achieve
Some customers want to offset a practical share of annual consumption. Others have a strict budget, limited roof area or a future expansion plan. Agreeing on the objective helps prevent both oversizing and undersizing.
A planning calculation usually considers:
- annual electricity consumption
- expected energy produced per unit of installed capacity
- system losses and operating conditions
- the share of solar production likely to be used or credited
- usable, structurally suitable and sufficiently unshaded roof area
These inputs should be visible in the proposal so they can be checked.
Check the roof before finalising capacity
Total roof area is not the same as usable solar area. Water tanks, access paths, parapets, nearby buildings, trees and equipment can reduce the space available. Row spacing, drainage access and safe maintenance routes also matter.
A site survey should record dimensions, orientation, shading, roof condition, mounting options and the proposed cable route. Structural suitability may require a specialist assessment, particularly for older or lightweight roofs.
Include future loads carefully
Planned air conditioning, machinery, pumps or vehicle charging can justify designing for future demand. However, expected loads should be documented instead of added as a vague allowance. The timing of those loads is as important as their total energy use.
Treat the result as a range
Early calculators are useful for comparing scenarios, but the final selection should follow an engineering assessment, applicable distribution-company requirements and an equipment-specific design. Ask the installer to explain why the recommended size is appropriate and what would happen if consumption changes.
