We get completely free power between 9PM and 9AM each day - and pay higher (than fixed price) rates between 9AM to 9PM.
If Stormwatch is triggered during the daytime, then we're paying for all of the electricity charging the 4 PowerWalls (assuming they were each at 35% charge to get started - that's about 32 KWh of energy - or about $5 at the rates we're paying).
If I notice this is happening and the event (such as a hurricane) won't happen any earlier than the next daytime, I will manually turn off Storm Watch and then turn it back on after 9PM.
However, since this only happens in very rare cases - such as a hurricane or recently during a possible Texas winter storm, having to pay to fully charge the PowerWalls doesn't have a huge impact on our electricity bill.
Though once the PowerWalls are full, during the daytime (if there's sun) we'll be sending all excess solar power back to the grid - and under our plan, we don't get any credit for that...
Again, that won't have a huge impact on the bill for a day or two - and probably better not to worry about that - and instead focus on preparing for the storm...