As long as the room the tank is in stays above 55F, you should be fine following the 3 watts per gallon rule.
Keep in mind that you should try not to run more than 75% of the max load on any circuit. What that means is that if you add up every electric device that is on the same circuit, you shouldn't exceed 1350 watts on a 15 amp circuit and 1800 watts on a 20 amp circuit.
So if you have a 15 amp circuit, the only thing you should plug into it is the heaters (you'd need to run pumps, lights etc on a different circuit).
This is pretty serious stuff...an overloaded circuit can lead to fire. Ideally, it would just trip your breaker, but there is a real risk of causing a fire.
I researched heaters for quite a while and I decided to use basically the same setup as Johnny. I'm using six 300W Eheim Jagers on a Ranco Temperature controller on my 770G.
A few months ago I noticed burn marks on the receptacle I had my Ranco controller plugged into. It was a 20 Amp circuit, so I should have been fine, but the receptable was only rater for 15amps (hence the burn marks). I upgraded it to 20 amps and all is good now.
I ordered my controller from Jehmco (they sell rebranded Ranco temp controllers). I would suggest going with four
300W Eheim Jagers and
ETCI-1R. Mouse over and click on the links to go to Jehmco's website.
Good luck!
