I have 850W PSU running on full capacity for about 3 month non-stop and it works fine 
If your kill-a-watt is registering 850W, then you PSU is only supplying 680 Watts if we assume 85% efficient at max power. If you bought a larger PSU, say 1000W and ran the same rig (680W draw) it would register 740W on the Kill-a-watt. So you are throwing away 100W per hour. Which is about $130 a year at .15c/kwh. Which is the cost of a 1000w psu about
If your PSU is less efficient than 85% at max, then you are wasting even more money.