Depends on the type of roof. If you have a flat roof, very likely most of the space can be utilized for solar PV. However, if you have a pitched roof, utilizing the whole roof area might not be productive since solar PV generation in Malaysia is optimized by facing south at a certain tilt angle. There are also installations facing east/west in the country. In conclusion, installation sizing should depend more on your consumption to be in line with the NEM spirit of consumption first.