The Design process for an on-grid system is very different than that of an off-grid system. An on-grid system typically does not have batteries and certainly does not require batteries except in case of a grid-outage. An off-grid system requires an in-depth knowledge of the electrical loads and battery maintenance by both the installer and the customer. In general, off-grid systems require more planning, consultation, and industry knowledge, they are also more expensive (mainly due to the cost of the batteries) and often require more existing infrastructure such as non-electric heat sources or hot water sources.
The design process for a grid-tied system is relatively simple, as the installer does not need to know what the electricity is being used for. A grid-tied system also does not need to meet the entire electrical demand of the property. Most installers use satellite imagery and your historical annual kWh usage (commonly found at the bottom right hand side of your NSP bill) to determine how many panels can fit on the roof or ground and what the production will be for that size of system.