Wow. Literally every suggestion in here has been wrong. 900watts for 3x 5850s? lolz. Only run your PSU at 50%?
Here's a real rule of thumb: Get a quality PSU and run it up to 100% of its rated continuous power output.
Are you on drugs???
No, are you?
Long long post. I don't really want to address every point. I already stated that one can go with a 750 if one feels the need to play it safe. What I said was that suggesting a 900 watt PSU was retarded. There is absolutely no way to draw anywhere near that level of power with 3x5850s, unless maybe you have a full time DICE cooling a super high level OC.
While I agree fully that there's no way for 3x5850 to draw up to 900W, saying it's retarded is not true.
I guess the problem here is that we have very different definition of safe. Your definition of safe includes running a PSU at 100% load 24/7 and "paranoid safe" is just running at 80% load. For some of us, "safe" includes margins for thermal derate, degradation from deterioration over time and safety margins for load spikes. While 3% different in efficiency might seem insignificant, but when you're running high power 24/7, 3% will add up to a lot. In some places, that would pay for the difference between a 750W and a 900W in less than 12 months. That is the difference between running a PSU at 50% and at 80~100%.
Suggesting that someone throw money away for no reason is not really good advice. If you've deployed literally thousands of systems, one would expect you to know that. Are you going to take responsibility for someone throwing money into the gutter? Stupid question.
You see the difference is that you are telling them to do something risky without any caveats, in a way that makes it sound like your recommendation is the only way.
I take full responsibility for my comment because I lay out the lower and higher bound, stating why (general safety and optimal efficiency) and leave the decision of which way to go to the person. I didn't tell him "The rest of them are all idiots, just buy a 1200W, or a 1000W if you're a cheapskate!"
Alright, fair enough. although the riskiness of what I am suggesting is still overblown a bit. I have both calculated and empirical data on the general usage of a normal mining rig utilizing what he requested. I gave specific examples of exact make and model PSUs that are well known to be overdesigned to ensure clean and stable power at and above their rated power levels. Even after a few years (the general time-frame where the largest amount of degradation occurs) of relatively intensive usage, the bare specs, and "safe" suggestion will be fine. Now admittedly I didn't layout the case where the OP runs a full water loop with chiller in reservoir and extreme overvolts to murder gpus as fast as humanly possible, but I expect that in that case they might mention something like that. If not, well, caveat emptor, I don't live to warn everyone of every contingency life might possibly envision. Two standard deviations is enough for me.
although that is probably the best PSU calculator around, it is still flawed, and I wouldn't really use it to gauge my needs. I also disagree with your statement that gold rating will not pay off. Here is a simple example:
Say you have 3 PSUs, one is Gold, one is Silver, one is Bronze. Since we're building a beefy system lets say that they're running at 50% load for a 850Watt PSU, or drawing 425Watts.
The draw from the wall for each is --
Silver: 482.95Watts (+10.73Watts from gold)
Bronze: 500Watts (+27.78Watts from gold)
Over the course of 1 year, running a 24/7 mining load, assuming 11¢/kWh that turns out to be $26.77 difference between gold and bronze. Higher loads yield even higher savings (double the load is a little more than double the savings). Unless you don't plan on using the PSU very long, or your power is very very cheap, or you find a very cheap bargain PSU, gold rated PSUs will pay for themselves in savings over a bronze rated one most often in mining.