Depends on the delivery mechanism. Our boiler (hot-water heat with radiators) has a pump that uses ~30 watts, and before we modernized our system it was entirel

Depends on the delivery mechanism. Our boiler (hot-water heat with radiators) has a pump that uses ~30 watts, and before we modernized our system it was entirely gravity-fed—hot water goes up to the living quarters, cold water comes back down to the basement. I haven't checked exactly, but I think our system uses around 100 watts total, and extracts 95% of the possible BTUs from the natural gas it consumes. The old system consumed basically nothing (just a bit to operate the thermostat controls), but was 80% efficient at best. Not surprisingly, forced air blowers consume more electricity—100-500 watts, according to my quick Googling.