Between the fuel pump and coolant pump, they tend to use about twice the amps of a forced air central system. Compare the power consumption specs on an Espar airtronic vs. same BTU Espar hydronic, for example.

The miserly solutions are all built around a fireplace like the Sig or Dickinson. The Espar and Webasto style systems have the advantage of being able to distribute heat evenly and controlled by thermostat, and having the system components hidden in engine spaces or other non-living areas, whereas the fireplace type units take up cabin living space and are "point" heating solutions.