Two main techniques have dominated the field of ecological energetics, the heart-rate and doubly labelled water methods. Although well established, they are not without their weaknesses, namely expense, intrusiveness and lack of temporal resolution. A new technique has been developed using accelerometers; it uses the Overall Dynamic Body Acceleration (ODBA) of an animal as a calibrated proxy for energy expenditure. This method provides high resolution data without the need for surgery. Significant relationships exist between rate of oxygen consumption (VO2) and ODBA in controlled conditions across a number of taxa; however, it is not known whether ODBA represents a robust proxy for energy expenditure consistently in all natural behaviours and there have been specific questions over its validity during diving, in diving endotherms. Here we simultaneously deployed accelerometers and heart rate loggers in a wild population of European shags (Phalacrocorax aristotelis) to compare behaviour specific estimates of energy expenditure from these two techniques. Compared with heart rate derived estimates the ODBA method predicts energy expenditure well during flight and diving behaviour, but underestimates the cost of resting behaviour We generate useable calibration relationships for cormorant species between ODBA and VO2 for resting, diving and flying in order to more accurately estimate energy expenditure. We find a good relationship between ODBA and VO2 for flight and resting, and a poor relationship during diving. The error associated with these calibration relationships mostly originates from the heart rate calibration rather than the error associated with the ODBA method. Compared to the heart rate method the ODBA method predicts energy expenditure close to the line of equality during flight and diving behaviour, but underestimates the cost of resting behaviour compared to heart rate estimates. The equations provide tools for understanding how energy constrains ecology across the complex behaviour of free-living diving birds.