your question has been asked so many time on this forum. use the search box to view the numerous previous posts/replies or use google search keyword: "HP z820 bootblock date" either of the two will provide a answer (hint you have good news 2013 is what you want to see)
there is no dismantling required to read the power supply label, you simply remove the side cover (with power supply cord removed) and pull on the power supply handle ro remove unit, then read label which shows wattage, then slide supply back into slot..........no tools needed
or you can count the number of 6 pin GPU power cables the stock 850 supply has two the 1150 watt supply has 3
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