Steve beat me to it. There is a $4 x 8 SS alternative to 813s for RF and it's 20% more efficient.
The RF efficiency when transmitting is definitely more efficient. But, it's the OVERALL efficiency that really makes a solid state transmitter the winner.
Let's say you're on the radio for 5 hours some night. This is not atypical - at least not around here
Let's say you're running a KW output with a pair of 4-400s modulated by 833As. The RF driver is a Valiant and the audio driver is a pair of 2A3s with tube rectifiers, etc.
The RF amplifier tubes use 5 volts @ 14.5 amps each - so that's 5 volts @ 29 A for the RF amp. The modulators are 10V @20A (for both tubes). Then there's the Valiant - let's say it consumes about 100 watts in standby. That is probably being very optimistic, but what the heck! We'll say the audio driver uses about the same. So what's the stand by power budget?
145 watts for the RF finals. 200 Watts for the modulators. 200 watts for the RF and audio drivers. So we're burning up about 545 watts *IN STANDBY*.
You've consumed about 2275 watt-hours for doing nothing. Transmitting power input is not included at this time.
Your solid state rig might consume 20 watts in standby, so you'll consume 100 watt-hours in the 5 hours you're on the air - when in stand-by. Based on this along, the H.B. solid-state rig is 20 times more efficient, and I didn't add in the plate losses, etc... which will push the efficiency higher.
The difference is really quite amazing.