To begin with I would like to say that this is for the United States Grid system running at 60 cycles.I have seen a lot of posts on the site that are wrong when it comes to electricity.

REMEMBER THE FIRST STEP IS TO REMOVE THE POWER FROM THE CIRCUIT

The power company charges you based on the Kilowatt Hour just like stated on the monthly bill. 1 kilowatt hour is equal to 1000 watts. So if your charge is $.19 per KWH and you run a 1000 Watt bulb for 20 hrs a day your charge will be $3.80. This will be the same with 240V because it is still running the 1000W bulb.

The higher voltages can save you money on the initial cost though. See as the voltage goes up the amperage goes down. This allows you to install a smaller size wire to run the same piece of equipment. 120V with 1000W is 8.33A while the same equipment at 240V is 4.167A So basically if you’re on a 15A breaker you can see the benefit of being able to run additional equipment on the same size wire.

Breaker sizing should always be at 80% of rated capacity.
Single pole breakers (one handle) 110V
15A is normally used with 14-2 gauge wire.
20A is normally used with 12-2 gauge wire.
30A is normally used with 10-2 gauge wire.

Double pole breakers (2 handles) are the same wire size but with an extra wire normally red added to the others. 14-3, 12-3, 10-3 and so on. This is what you should be using for 240V.

GFCI breakers would be a good idea for a grow room in case of water contact. Normally they are cheaper then the wall type. Of course this would depend on the comfort level with electrical.

I will add more to this as time becomes available. Feel free to post any questions.

good basic electrical info. The only thing is you stated that a GFCI (ground fault circuit interrupt) breaker is cheaper than a GFCI outlet? If I understood this right, then I must disagree. A GFCI outlet is considerably less expensive than a GFCI breaker, and serves the same purpose. With a GFCI outlet, you can install it in place of an outlet in the circuit and it will protect all outlets that are downstream from it.

You are correct. I get GFCI outlets for about $10 each and the breakers run around $25.