Very good info in here. The S9 requires at least 1600W at 220V. If you are going to use two 120 V power supplies, these will need to be at least 800W each, but you may want to go with 900W or higher. A good rule of thumb is to use a higher wattage power supply rather than going lower or you will most likely burn up your power supply and probably the S9 with it.

You want a dedicated circuit for each antminer. This means you have one outlet connected to one breaker in your panel. You do not want any other appliances to be plugged into the same circuit as your antminer or you will most likely trip your breaker or worse.

You could potentially use a 1600 W power supply at 120V. You would need a dedicated outlet for it. As mentioned in another post, you would have to make sure each rail had enough power for a minimum of 2 cards each on the antminer. You may even need adapter cables to have enough connectors for each board.

If you use use two 120 V power supplies, you will want to wire (14 gauge or 20 amps) an outlet to a 15 to 20 amp breaker. You can use the same outlet to power both power supplies since you should only use about 13.5 amps with two power supplies.

I bought a double pole 20 amp breaker. This means it has two wire connections for your "hot" wire. You can run a dedicated wire from one terminal to one outlet and another from the other terminal to another outlet. This will allow you to run two S9 antminers that each have 2 power supplies.

If you are going to use a 220V power supply, you will need at least two 15amp breakers. You will need a special outlet that will accept a 220 V plug connection as it will contain 2 "hot wires" (or one 120V wire from each breaker). Alternatively, you could use a 30 amp breaker with 2 poles and wire each terminal to your 220V outlet.

Other than that you need to make sure your electrical panel can handle multiple antminers. Your panel should say how many amps it is rated for. Lets say you have a 100 amp panel. Figure out all that appliances that could potentially run at the same time and add up the rated wattage for each (all should have a label with wattage requirements i.e. 1.2 kW aka 1200 watts). Example Fridge 700W, Dryer 1200W, Oven 800W, Microwave 1200W, AC unit 1200W and 2 antminers at 1600W each. They all add up to 8300 watts. If we divide 8300 watts by 120V you are at 69 amps. This leaves you some room for 2 more antminers as each will use about 13.5 amps. You do not want to use more current than what your panel is rated for. If needed, you can just make sure you aren't using two appliances at the same time that would put you over 100 amps.

As mentioned, you should always turn the main breaker off if you are going to be installing any new breakers or outlets. If you don't normally work with electricity, you have no business being inside it. It takes less than an amp to kill and you are playing with enough to kill you 20 times over or more.