We present the Earth Climate Observatory space mission concept - currently studied in Phase 0 as a European Space Agency Earth Explorer 12 candidate - for the measurement of the Earth Energy Imbalance and the Earth Radiation Budget. Key innovations are 1) the differential Sun-Earth observation with identically constructed wide field of view radiometers, 2) an adequate sampling of the seasonal diurnal cycle with a constellation of polar precessing orbits, 3) complementary full angular coverage at high spatial resolution using wide field of view multispectral cameras.