I am trying to measure the efficiency of a DC/DC boost-converter in several operating points as precisely as possible. Right now I am measuring my input-voltage and output-voltage with a multimeter, and the input-current and output-current over a shunt-resistor with a multimeter as well. I think the four multimeters are pretty high quality (Flukes). I then calculate the input power and output power. Unfortunately it seems that my measurements are not precise enough, especially through the multiplication factors of the shunt and the multiplication of voltage and current, the resulting power seem to be off by a couple of percent (determined by repeated measurements of the same operating point).
Is there a better and more precise way to calculate the efficiency of my converter. I would obviously prefer a cheap solution, but if there is no way around it, I could also convince my boss to invest some money.