The short answer is... it depends. It is possible, but to do so your roof needs to be big enough that you can get enough solar energy to provide at least a day (and night) worth of power from the sun. It needs to be able to cover your usage during the day and still have enough power to charge the powerwalls to cover your usage during the night when the sun is down.
Additionally you need to have enough powerwalls to cover your usage during the night.
Likely the system should be at least a little bit oversized so that even if you have a rainy day (or even a few rainy days in a row) that there will be enough reserve in the powerwalls to cover you during those low production days.
So it is possible, but a lot of it depends on just how much electricity you use, how many powerwalls you have, and how big your solar system is.
If you are connected to the grid and just looking at coverage for an extended power outage, then you can get by with a smaller system and fewer powerwalls under the assumption that you would be able to only power critical loads during the power outage or otherwise cut back on your power usage.
My system is set to be installed in about a week. I’m getting a 15.12kW solar system (panels, though, not a solar roof) and 4 powerwalls. I am connected to the grid, but I picked this size of a system because it should be big enough to allow me to run my house indefinitely during an extended power failure, including use of my central AC, limited cooking, clothes washing and drying, etc. I will need to be careful to try to conserve power during an extended power failure, but assuming I do so then I should be able to run indefinitely.
