The higher wattage PSU may be less efficient at your idle/low load levels than the lower wattage one.
An extreme example LZP-550 vs LZP-1000:
Power lost as heat vs load @ low load to 90W for the 550W is 7-8W, for the 1000W it's 14W
At 200W load, the LZP-550 has 13W of heat loss, the 1000W uses 15W
At 300W load, they are both around 21W.
Above 300W, the LZP-1050 is more efficient than the 550W.
Figure the LZP-850 is somewhere between those two.
1080p Gaming build: i5-4670K, Mugen 4, MSI Z87-G45, MSI GTX 760 2GB Gaming, 8GB 1866 RAM, Samsung Evo 250GB, WD Red 2TB, Samsung DVD burner, Fractal Define R4, Antec True Quiet 140 (2 front + rear) case fans, Seasonic X-560. 35-40W idle, 45-55W video streaming, 170-200W WoW, 200-230W Rift, 318W stress test (Prime95 + Furmark)