If it is underpowered it likely means that its affect on the employment rate will end, which in the long run could affect the income of the city. But since silos are very small, it would have a minimal effect.
Any facility that is unpowered, at the moment, can be entered and walked around and stuff left in there. Vehicles and ships too can be docked in facilities that have the right abilities with no power.
All that will happen if they don't have power is that their effect on morale/crime/ER will not take place, and you cannot use their abilities (production, mining etc).
The difference between a powered and unpowered silo/warehouse is that if they are owned by a mining faction with an active mine in the same city, and the mine's capacity is reached, then the automatic RM transferal happens only to the powered silos/warehouses. With the unpowered ones you would need to manually move those RMs from the mine.
