The amount of space needed depends on how much storage you want and can afford to cover your needs while the grid is out. The chemistry also matters. Sealed lead acid batteries can take up more space and require a custom or pre-made cabinet. Lithium ion batteries are pre-wired and often come in a wall or floor-mounted enclosure or cabinet. Another important factor is where your critical loads sub-panel will be located. This is the electric panel that will contain all the circuits in your home that will be powered by the battery bank when the power is out.