I use a single 300 wat titanium heating element controlled by my Oceanic chiller for my 210 gallon reef.
Some of these heating estimates posted seem way overkill to me. The equipment in my reef boosts temps as it is, and my heater is not on very much at all.
I have a 265 gallon planted tank with three 250 watt halides, and a single Stealth 250 watter keeps tank temp at 77 degrees no problem.
Dave