Accually we use up more energy then you'd think. Think of all the energy required to heat our bodies to 99 degrees and all the energy we use in all our muscles every day to keep us breathing, our heart beating, ect...
Converting dirt into food is a good idea, but only so much mass can be used, once all the stored mass is used up, waste won't provide enough energy to sustain the replicator and then they'll have to use parts of the ship to make more food.
But the dilithium crystals cannot have more energy than their mass, so at 100% efficency 1 pound of dilithium could only produce 1 pound of replicated stuff
Here's why: You can't get more mass out of energy then the mass you used to get the energy. What do I mean? Well, imagine someone created a power plant that could convert mass directly into energy at 100% efficency; and also created a replicator that used that energy to create matter at 100% efficency; you would only be able to synthesize as much mass as went into the power plant to create the energy for the replicator, thus rendering the replicator pointless. What do you guys think of this?
Oh, snap! Could Nova Six be one big organism in all it's carbon-based glory? What will happen to the Kerbals? Will Wehner ever believe in ponies? NASAFanboy strikes again!