Well, the best of modern PC power supplies are about 85% efficient. That means that 85% of the wall AC power gets converted into DC power for use by the electronics. Older power supplies tended to be about 60-70% efficient, so you had more waste in the power supply. However, for completely closed systems where the manufacturer knows the absolute maximum wattage that will ever be drawn (like the Mac mini, or the older compact Macs,) you can achieve higher efficiency by designing the power supply for EXACTLY that wattage.
The modern "85% efficient 1000W" power supplies are only 85% efficient at close to their top draw. Which means that if you put one of these 1000W power supplies in a standard system, that will only draw 100-200W, you are going to be very inefficient.
If you're talking about efficiency in terms of 'performance per watt', then old systems are impossibly inefficient. While a 40 MHz 68040 may only use 5W of power, compared to an ultra-power-hog 3.6 GHz Pentium D at 150 W, the 3.6 GHz Pentium D is many orders of magnitude faster in performance that more than makes up for the higher power usage. Not to mention that all the other components in the older system are likely drawing almost as much power as the equivalents in the new system. So a choice between a 40 MHz 68040 drawing 95W at the wall and a 3.6 GHz Pentium D drawing 250W at the wall is really no comparison. And if we use a new Core 2 Duo instead of a Pentium D, the delta is even greater.
Just think: A PowerBook 190 and a MacBook Air have the same wattage power brick.
The modern "85% efficient 1000W" power supplies are only 85% efficient at close to their top draw. Which means that if you put one of these 1000W power supplies in a standard system, that will only draw 100-200W, you are going to be very inefficient.
If you're talking about efficiency in terms of 'performance per watt', then old systems are impossibly inefficient. While a 40 MHz 68040 may only use 5W of power, compared to an ultra-power-hog 3.6 GHz Pentium D at 150 W, the 3.6 GHz Pentium D is many orders of magnitude faster in performance that more than makes up for the higher power usage. Not to mention that all the other components in the older system are likely drawing almost as much power as the equivalents in the new system. So a choice between a 40 MHz 68040 drawing 95W at the wall and a 3.6 GHz Pentium D drawing 250W at the wall is really no comparison. And if we use a new Core 2 Duo instead of a Pentium D, the delta is even greater.
Just think: A PowerBook 190 and a MacBook Air have the same wattage power brick.