The Energy Balance

Like anything in nature, the geodynamo needs to be driven by a constant source of energy. Whenever there is motion there is dissipation and unless something replaces the lost energy, the Earth's magnetic field would eventually die out. Below is a list of the main forms of dissipation and the main sources of driving energy. The amount of energy required can only be computed with assumptions and educated guesses and is probably not known to better than one or two orders of magnitude. A value that is often used is 3*10¹¹ watts. (Merrill 301)

Energy Dissipation

Fluid Viscosity (viscous heating)
Fluid viscosity causes dissipation of the fluid motion in the form of heat. (Merrill 301)
Magnetic Viscosity (joule heating)
Magnetic viscosity causes energy dissipation in the form of Joule heating. Magnetic viscosity is much greater than fluid viscosity in the Earth's core so this represents the dominant energy loss. It is difficult to interpret the actual loss involved though because the energy lost via Joule heating can end up being fed back into the system in the form of thermal convection. (Merrill 301)

Energy Sources

Tidal Friction
Tidal friction can contribute about 10¹²W of energy, which is enough to drive a dynamo. It is possible that the Milankovitch cycles have an effect on the magnetic field varation, but there is not enough evidence to say for sure. (Merrill 301)
Thermal Convection Below Mantle
Convection due to heat transfer at the core-mantle boundary gives some energy but not enough to be the sole driver of the dynamo. (Merrill 302)
Radioactivity / Thermal Convection at Core
It is possible that the radioactivity of the core generates enough heat to greatly enhance thermal convection. This was one of the first proposals, but now it is thought that there is not enough radioactivity to drive a dynamo in this way. (Jacobs 187)
Gravitational Collapse
As the molten outer core freezes onto the inner core its volume decreases. This change of volume causes everything on the outside to fall a bit which releases gravitational energy. (Merrill 302)
Latent Heat of Crystallization
As the inner core freezes lighter elements are released. These buoyant elements drive convection. This is probably the dominant source of convection. (Merrill 302)