As others mentioned in regards to offsetting embodied carbon with solar panels, it’s not too straightforward because of the regional and temporal differences in electricity generation sources. In some markets already saturated with solar energy, there is a possibility that someone’s PV system won’t even be displacing any more fossil fuels than is already happening for various reasons. That said, you could use the average grid CO2 emissions intensity per kWh (like from the EIA for the United States) and compare it with the intensity of a kWh generated with PV - the difference is your “offset”. LEED and Living Building also provide resources for calculating this.
An alternative is to purchase actual carbon offsets which then fund initiatives like afforestation, carbon capture, and research into decarbonaztion of the materials we use in construction in the first place. The value of installing PV versus pursuing other carbon offsets for decarbonization is whole another discussion though.