How many watts should a power supply be above the that of the components in the device?
20 watts, 50 watts, 100 watts, 200 watts, 500 watts, 700 watts, whatever makes you feel better. if you're talking more along the lines of a realistic "need" than a cushion of 50 watts, 100 for high end hardware (like a pair of gpus or 6+ core cpu overclocked) will do.
I presume there's no inherent downside of having a larger cushion.
The main factors would be (a) not having a short circuit and (b) having an optimal price.
no downside. any quality psu you buy has proper protections and buffer caps. price is the biggest factor in the end.
So if power consumption is 500W and your PSU (from a reputable company) is (for sake of argument) 510W, there's no risk in power problems?
for a quality psu there is no risk. and if the load goes over what the unit is rated for it will shut down safely.
Most modern systems containing 1 GPU will use under 400w at load. Brand means basically nothing when deciding a PSU's quality, same with the OEMs. It's all about the model and platform it's using.
I like 100-200w usually depending on specific components and desire to overclock or not.
A good rule of thumb is to get 30% more than your estimated draw. That leaves overhead room as well as room for upgrades (higher power graphics cards, more drives, etc.).
But quality is also very important.
The upgrading is a good point.
To what extent can a higher watt PSU cover an upgrade?
How many more years would a higher PSU last a person?
The extent depends on how much headroom the 30% gave you and the wattage of the new devices. For example, let's say you make a build that is estimated to use 400 W. 30% of that is 120, so you get a 550 watt PSU (round up from 520 to 550). That's approximately enough for another graphics card that uses around 150 watts. Drives use about 10 W max, if you wanted to add more. You can always get a wattmeter from Amazon or wherever to measure how much power your computer is actually drawing.
As for the number of years, that would really depend on a model-to-model comparison, not to mention what kind of load you are putting on the PSU (how much power, which rails, etc.). Higher quality PSUs have better engineering and components that provide better power to your computer, but also last longer. As far as I know (someone correct me if I'm wrong), a component as relatively simple as a PSU should not ever fail or perform any less than when it is new, assuming it was engineered correctly and well and has good components that last.
I disagree with the 30% extra. Mainly because SLI/Crossfire likely won't be consumed in the future as the better way to go is to sell the old GPU and buy a single more powerful one. By the time the GPU needs an upgrade, the current GPU in the machine would most likely be of bad value at the time.