"Every watt of energy that is produced from solar is one watt less that needs to be produced by burning fossil fuel", true.
And every EV or ICE that is added, ultimately results in the same amount of fossil fuel being burnt. Where else is the energy required to propel an EV going to come on? Remember, the EV or ICE your purchased is an additional load on the system. And all additional load to the electricity system must come from fossil fuels since all solar/wind is sold first. Then when you add your EV you've increased the electricity needs, which can only come from coal, until all electricity is produced from solar/wind, which is a long long way off.
You can't even install rooftop solar to reduce the CO2 emissions. You've installed roof-top solar, then decide whether to purchase an EV or ICE: 1) EV charge from roof-top solar, therefore no CO2 emissions 2) ICE uses fossil fuels and the roof-top solar is fed into the electricity network, thereby reducing coal combustion, therefore no net CO2 emissions
