Yes, quite possibly.1
For tasks like web browsing and "office", your CPU and GPU load, and consequently net power consumption will be very low, most likely around 50W or even less. My Yorkfield-generation desktop with dedicated GPU (GTX650Ti) and 2 solid state disks uses 75-76W for such "mostly idle" tasks, but modern CPUs are a lot more more energy-efficient.
A 800W PSU would thus run at less than 10% of its maximal output rating. Probably closer to 5% than to 10%.
Yes, efficiency on modern PSUs has improved, and their load-efficiency graphs gain a lot steeper and have a much greater plateau than they used to have, but even certified PSUs are no panacea for loads in the single-digit percent range, let alone a cheaper PSU.
Even a very expensive 80+ "gold" or "platinum" certified PSU will not perform optimally at such low loads (the certification gives a guarantee for 20% load, not for 5%).
A computer with a weaker CPU/GPU and a weaker overall PSU (which maybe runs at 25-35% net power) may thus very well be more energy efficient and factually pull less electricity out of the wall plug.
EDIT:
My more recent Skylake system which has a 450W 80+ gold rated PSU idles at about 35W with integrated graphics only, and 45W with a nVidia 1060-6G, respectively (2 SSDs, 64GB RAM).
With the CPU under full load, it draws about 120W, with GPU also under maximum load, highest ever measurement was 280W. All measurements on the wall plug, thus these are overall (gross) values. 280W < 450W, no worries there. For a 800W PSU, that would be 35% of the PSU's capacity at 100% load.
1 One has to assume PSUs of similar quality with similar/identical efficiency certification, of course.
1@DMA57361 - Do you have any thoughts on the other part of his question, though, regarding a more powerful system drawing more power? – JNK – 2010-09-09T12:43:47.833
@JNK I'm just editing at the moment... – DMA57361 – 2010-09-09T12:45:43.200
@DMA57361 - Excellent! Not pushing, I just value your opinion from other posts you have made. – JNK – 2010-09-09T12:48:37.260
...and a +1 from me for the edit! – JNK – 2010-09-09T12:49:19.333
@JNK - No, that's fine, and please push (constructively!) whenever necessary. – DMA57361 – 2010-09-09T12:50:17.560
When the machines are under load, the power consumption becomes another ball game entirely. Does a chip at 50% load use 50% max power? probably not, so a 2Ghz chip at 50% probably drains more than a 1Ghz at 100% - but I'm having difficulty finding any facts on this at the moment. – DMA57361 – 2010-09-09T12:57:28.140
1+1 for mentioning the efficiency rate bell curve. Using over-rated PSUs on systems that do not need such capacity will waste electricity. It is best to get a PSU that will allow the machine to idle at around 60% of its rating, for optimum efficiency. – paradroid – 2010-09-09T13:18:08.407
@paranoid Usually the best efficiency of a PSU is reached anywhere between 20% and 80% usage and if you calculate for a machine to idle at 60% of the PSU capacity you will most probably reach higher than 100% in full use. For instance, in my latest computer my calculation using the max values for each components was 360w, so I bought a 430w PSU which is around 80% usage at 360w. This same system when idle consumes about 95w which is about 22% of PSU. This is still ok because it runs in the 20% 80% efficiency range. – jmbouffard – 2011-06-28T12:12:31.530