We present the results of a 25-year program to monitor the radio flux evolution of the planetary nebula NGC 7027 .
We find significant evolution of the spectral flux densities .
The flux density at 1465 MHz , where the nebula is optically thick , is increasing at a rate of 0.251 \pm 0.015 \% yr ^ { -1 } , caused by the expansion of the ionized nebula .
At frequencies where the emission is optically thin , the spectral flux density is changing at a rate of -0.145 \pm 0.005 \% per year , caused by a decrease in the number of ionizing photons coming from the central star .
A distance of 980 \pm 100 pc is derived .
By fitting interpolated models of post-AGB evolution to the observed changes , we find that over the 25-yr monitoring period , the stellar temperature has increased by 3900 \pm 900 K and the stellar bolometric luminosity has decreased by 1.75 \pm 0.38 % .
We derive a distance-independent stellar mass of 0.655 \pm 0.01 M _ { \odot } adopting the Blöcker stellar evolution models , or about 0.04 M _ { \odot } higher when using models of Vassiliadis & Wood which may provide a better fit .
A Cloudy photoionization model is used to fit all epochs at all frequencies simultaneously .
The differences between the radio flux density predictions and the observed values show some time-independent residuals of typically 1 % .
A possible explanation is inaccuracies in the radio flux scale of Baars et al . We propose an adjustment to the flux density scale of the primary radio flux calibrator 3C 286 , based on the Cloudy model of NGC 7027 .
We also calculate precise flux densities for NGC 7027 for all standard continuum bands used at the VLA , as well as for some new 30 GHz experiments .