Unfortunately, the GHG Protocol standards do not permit offsetting of emissions via "avoided emissions", such as rooftop solar generation sent to the grid.
If a company generates more solar electricity than it consumes and exports the excess to the grid, it cannot subtract these emissions from its gross or net GHG inventory.
Electricity from the grid comes from a range of sources, both renewable and non-renewable. This is accounted for when each country calculates its electricity emissions factors. The overall grid mix means that the emissions from 1kWh from the grid cannot be directly offset by generating and returning 1kWh to the grid.
Under the standards, a company may choose report the sold electricity separately as “avoided emissions” under a project level accounting methodology, as the exported renewable energy helps reduce emissions from grid electricity generation, but this does not change the totals of the inventory. Emissions can only be subtracted if they are classified as permanent carbon removals i.e. if CO2 is being removed from the atmosphere, e.g. via carbon sequestration of trees.
There is no way for a company to have the solar power sold to the grid recognised as negative emissions under the standards. It might be possible to register the electricity as a Renewable Energy Certificate (REC), meaning a contractual arrangement to sell renewable energy. However, this still will not impact the generator's inventory as the net zero electricity would be recorded by the company who purchases the RECs.
For more information on the standards as pertains to Scope 2: Purchased Electricity, see the GHG Protocol Standard