What is the measurement error of the power measurement?

The measurement error of the power measurement is considered low. This submits to the approximately 150 data points recorded during a test. According to usual error calculations1), at a power performance of 250 W a measurement error of 2% (5 watt) results for one data point. If all 150 data points are evaluated, the error in the measurement series is reduced to 0.7 W in case of the double statistical confidence interval and is thus to be regarded as low.

1) cf. Demtröder (2006): Experimentalphysik 1 – Mechanik und Wärme, Berlin: Springer Verlag. Page 29 ff.