Will a 500W SMPS consume more electricity than a 250W SMPS if they are powering computer with same specifications ? Or is the SMPS designed in such a way that it only draws the required power form the grid ?
[So far there is one "No" answer and one "Yes" answer. I'll add this "Depends" answer.]
If these were ideal switch-mode power supplies, then the answer would be "no, both power supplies would consume the same amount of power". But there is no such thing as an ideal SMPS, certainly not at the prices most consumers want to pay. So the realistic answer is "depends, but most likely yes".
The generalized answer is "depends" simply because you have not specified the efficiencies of the power supplies nor the characteristics of the PC load. Whichever PSU has better efficiency (at a given power draw) in converting AC electricity to DC electricity will consume less power from the wall outlet. This assumes a specific level of power draw or load from the PSU. A 500W 80Plus PSU could consume the same or less power than a non-certified 250W PSU for a computer always running at or near full load (i.e. rarely operating at idle), e.g. consuming 100 to 200 Watts. But if the computer was mostly idle and consuming only 50 Watts or so, then there's insufficient info to make an answer.
If the PSUs actually have identical efficiencies, then they would consume the same amount of power. For example a 200 Watt load and a 500W PSU operating at 80% efficiency will consume the exact same amount of energy as a 250W PSU operating at 80% efficiency. Since efficiency is defined as
output_DC_power / input_AC_power * 100, then input_AC_power is fixed in the expression
80 = 200 / input_AC_power * 100. Note that the actual efficiency of a PSU will typically vary according to the percentage of its maximum rating, i.e. the actual load versus its max rating, and tend toward zero efficiency at low or min wattage.
In general a higher-wattage PSU will consume more power than a lower-wattage PSU of similar efficiency for a specific low-wattage load. This is because the 80Plus specifications only tests PSU operation at 20% and more of the rated power output. An electrical load of less than 100 Watts (20% of 500W) is a small load that is outside the 80% efficiency operational requirement. I have verified this with a Kill-a-Watt meter, 500W & 380W 80Plus PSUs and a PC that drew only 40W at idle.
In the real world, the PC does not draw a constant amount of power. The PSU has to be sized to be more than the PC's max power draw, but the PSU should not have too much capacity so that the PC's draw at idle is still within 20% of the PSU's rated max (the guaranteed range of 80Plus certification) .
Depends on the specific power supplies. Efficiency usually varies with power output and is less at the high and low ends of rated power output. Some models have pretty flat curves and others do not. Right sizing an 80Plus supply is usually best.
The 500 needs more amps to run no matter if it's powering 250 or 500 watts, but the difference between the two running the same system specs will not be noticed on a electric bill. Anytime you have a higher watt device, whether it be a power supply or a fan or a tv, the higher watt device WILL NEED to use more amps then a similar lower wattage device. Even things like high tech digital lighting systems or energy saver appliances that are higher in wattage then the similar device it's being compared to will need more amps to run, therefore YES, you are using more electricity when you make the decision to use a higher watt device.
Yes. The supply needs a certain amount of power for its own operation, and the more powerful it is, the more power is needed.
If you want to reduce those, look for more efficient power supplies (e.g. 80Plus) and don't run the supply at its limit (in fact, this is always a good idea).