Help with Power Supply Input Current?

[Tony Duell]

> 
> I have seen the specs for input current for various power supplies.
> Obviously, the higher the maximum output current, the higher the
> rated input current.
> 
> I would like to buy a power supply that is about double the usual
> capacity that I will need, but I am wondering how this choice
> might affect the UPS up time when the street power fails?
> 
> If the load for two power supplies is the same, will the input current
> be similar if the maximum output rating of one power supply is
> double the other power supply, assuming that the load is within
> the rating of the smaller power supply?

This depends on the efficiency of the power supplies, which is basically 
defined as the ration of output power to input power. This efficiency 
obviously depends on the design of the power supply (linears are a lot 
worse than switchets) and to some extend on the maximum power rating of 
the PSU. But for the supplies you're thinking of, it's likely to be 
pretty much the same.

In whcih case for a given output power (that is, for the same load 
connected to the PSU), the input power is going to be much the same no 
matter which PSU you use. And thus the amount of time your UPS will run 
it for is going to be much the same

-tony