If you don't have, or expect to have a 200 amp generator, then a 200 amp DPDT switch is a waste of money.
An interlock on the panel, allowing say a 30 amp breaker in the upper right, fed by the generator, and the main breaker, to be interlocked so both cannot be on at the same time, is doable for most situations.
The GenerLink does the same thing, powers the panel with up to 30 amp from a generator, but does it at the meter. This may be a better choice if you have a Main Lug panel, or you would have to add a main breaker conversion to use the interlock kits. Difference is cost GenerLock is pricey, and that the Power Company will have to approve and install it, as its their meter it is plugging in underneath.
Square D QO interlock is about $50+ on Ebay plus another $50 for a generator inlet box plus wire and installation.