It should be safe enough but the PSU may be very inefficient with such a low load. Do you really want to draw 800 Watts to run a small PC, especially if it's going to be on all the time?It won't draw 800 watts all the time, it'll draw what it needs but do so with slightly more overhead.
You're right. However, the efficiency of some PSUs can drop to 40% at low loads, even if they have a high efficiency rating at full load. For example, such a PSU could draw as much as 500W to run a 200W system. An "80 PLUS" certified PSU is guaranteed to be at least 80% efficient even at 20% load, and should draw no more than 250W to power a 200W system. The OP should check if his 800W PSU is 80 PLUS compliant, although if he is not paying for his electricity, he may not care.
...and yes, I got all this off t'internet.
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