does 600VA ups is enough for 650W psu???

Solution
Theoretically, 650W PSU will draw max around 1000VA using 0.6 power factor.
Practically, your PSU rarely or never does that and draw only around 400W or 660VA (again using 0.6 power factor). Yes, 600-750VA UPS should do.

Important:
I would recommend you to get an UPS with pure or true sinewave output, some PSUs have issues if your UPS does not have that. Ask the vendor if the PSU you pick has this.
UPS will shut itself down, if you try to draw more watts from it than it can manage.

And you should realize that 650 w PSU may draw alot more than 650 watts, depending on its certificate.
A bronze certified 80 PSU may draw 20% more power than it needs, and waste 20% as heat.
 



then,.. please suggest me a good ups for my psu...
thank you again
 
Theoretically, 650W PSU will draw max around 1000VA using 0.6 power factor.
Practically, your PSU rarely or never does that and draw only around 400W or 660VA (again using 0.6 power factor). Yes, 600-750VA UPS should do.

Important:
I would recommend you to get an UPS with pure or true sinewave output, some PSUs have issues if your UPS does not have that. Ask the vendor if the PSU you pick has this.
 
Solution

The majority of PC PSUs and other office equipment with SMPS are fine with modified sine wave UPS. A true sinewave UPS is only necessary if you want to run large transformers, AC motors, have picky SMPS or equipment that use AC transformers for power and have trouble with the extra harmonics.
 


A UPS should provide (or at least be capable of providing) the max potential draw of the PSU.
The entire point of the UPS is to protect your data so it should not be the weak link in the chain of components.

By slightly oversizing the UPS compared the PSU, you´d have the safety you want, which is why you bought a UPS in the first place.

As a rule of thumb, you´d go with the minimum VA rating you need is approximately 1.6 x load wattage.
In this case, it´d be 650 x 1.6 = 1040 VA * 100 * PSU efficiency rating (in percents)