  Quick U-building (QUB) is a method for short time measurement of energy
performance of buildings, typically one night. It uses the indoor air
temperature response to power delivered to the indoor air by electric heaters.
This paper introduces a method for estimating the expected measurement error as
a function of the amplitude and the time duration of the input signal based on
the decomposition of the time response of a state-space model into a sum of
exponentials by using the eigenvalues of the state matrix. It is shown that the
buildings have a group of dominant time constants, which gives an exponential
response, and many very short and very large time constants, which have a small
influence on the response. The analysis of the eigenvalues demonstrates that
the QUB experiment may be done in a rather short time as compared with the
largest time constant of the building.
