In the US:

Multiply the number of connectors you plug in to run all your equipment by 20, and then multiply by the line voltage at the socket - usually 120.

The result will be the maximum number of watts required.

(This assumes each of the outlets are at the common 20 amp rated capacity, on a 115 or 120 Volt line.)

Anywhere in the world:

For better accuracy, you can add the "maximum input watts" (for the power input, not the signal plugs) on the nameplates of all the pieces you'll plug in. You can also multiply max amps x line voltage to get the watts for those that don't quote watts directly. Then (in either case) add all the watts.

Either of these will be a fairly conservative estimate, since all your plugins won't likely draw maximum power at the same time - and/or you may be sensible enough not to run everything at maximum; but "derating" may be risky if you only have a few plugs running.

If you can plug more than one device into a multi-socket outlet that has all the sockets on the same circuit breaker, you can use the number of "breakers" instead of the number of plugs, which may bring the estimate down some too.

John