The concept of MVDC collection network for renewable applications requires a high efficiency DC-DC converter to connect low voltage sources to the medium voltage network. The accurate measurement of low power losses is not possible with traditional electrical input-output power measurements. This paper adopts and extends a transient calorimetric method suited for the measurement on such converters. It is shown experimentally that this method can be applied to the measurement of the losses of an oil-immersed DC-DC converter. The maximum efficiency of the 83 kW, 1.2 to 6.6 kV prototype is measured at 99.16 ±0.04% (99.12…99.20%).