This paper proposes a new closed-form expression for computing the power efficiency of delta-sigma (DS) radio frequency (RF) transmitters. Due to the large bandwidth of the pulsewidth modulated (PWM) signal at the output of the DS modulator, the peak efficiency of the power amplifier (PA) when driven by a PWM signal is considerably different than its peak efficiency when the input signal is a continuous sine wave. A weight averaging of the efficiency over the frequency bandwidth of the PWM signal is considered for accurate estimation and prediction of the power efficiency of DS RF transmitters. It is shown that, for a low-pass DS transmitter, the overall average power efficiency of the DS transmitter can be accurately estimated by the product of the modulated peak efficiency of the PA and the coding efficiency of the modulator. For bandpass DS transmitters, an additional parameter that accounts for the duty cycle effect on the efficiency of switching-mode PAs has to be considered. To validate the new proposed expression, a DS transmitter for WiMAX, CDMA and EDGE standards is designed, prototyped and tested. The comparison of the simulated and measured efficiencies of the DS transmitter proves the validity and accuracy of the proposed expression.