There are several ways. The simplest and least expensive is to connect an extension cord from the system to your refrigerator and a few essential lights and plug in other devices as needed. Probably not a good solution for long term, but can get you through a short term power outage.
Another solution is to install (usually need an electrician) an auxiliary electrical panel which connects to some of the circuits which you want to use in emergencies. Some of these panels have circuit breakers which can connect either to utility or emergency power.
A third process is to install (again, qualified technician) a transfer switch that transfers all of your electrical loads to the emergency system. This can be a manual switch or and automatic relay. You have to be careful in this situation not to turn on too many devices at once, as it may overload the emergency power system (depending on its capacity) and trip some breakers.
Grover--- Resident prognosticater. If you need specifics, contact me at firstname.lastname@example.org.