If there are no capacitors being drained, where does the power come from when the power turns on and the system is unplugged
You lost me there. Are you saying if you unplug the computer from the wall and press the power button, the computer starts to boot? If that is happening, there is a fault somewhere in the power supply.
You are absolutely correct that it would be capacitors storing that power, but the only power running through that front panel power switch is the +5Vsb voltage. That +5Vsb is removed (almost instantly) when you unplug the power supply from the wall (or flip the master power switch on the back of your PSU - if it has a master power switch). And that +5Vsb has a small current capability that is isolated (or is supposed to be as per the ATX Form Factor Standard) from the 4 main high current rails; +12VDC, +5VDC, +3.3VDC and -12VDC (though for most computers, the -12VDC is not used). When the power button is pressed to signal the PSU to turn off, those 4 main rails are supposed to turn off and have zero potential with respect to ground - meaning there should be no voltages being delivered to any other components as there are bleeder resistors in the PSU just for that purpose - assuming the PSU is working properly. And as mentioned earlier, that circuit the front panel button is connected to is a "momentary" circuit. Once the short is detected when you first push the button, the circuit ignores further input until the next short arrives (you press the button again).
We are talking about a desktop PC, right? And not a notebook?
Have you ever had a surge get through the UPS unit and damage equipment?
The only time I have heard of a surge getting through a UPS is when the transformer on the pole outside a client's house took a direct hit from lightning. That strike, besides starting the pole on fire, took out electronics in all 3 houses on that transformer. But short of a direct lightning strike from an angry Mother Nature, I have never seen or heard of an UPS failing to protect the connected equipment. Even if the batteries are failing and no longer capable of supporting the connected equipment during a power outage, they can still absorb massive surges - much better than a surge and spike protector.
I have seen UPSs of several brands fail - but in the process, they simply cut power to connected devices.
I have used APC UPSs for over 25 years and have never had to make a claim - and I live in
Tornado Alley. In fact, I still have my very first APC UPS, a
Smart UPS 900 that I got way back in 1994 and it still works great. I have it on my garage door opener! Other than having to replace the batteries a few times over the years (normal maintenance), it keeps humming along.
I currently have on this computer an
APC BackUPS XS 1500. It is supporting this computer with a 550W PSU plus my cable modem, wireless router, 4 port switch, home phone plus two 24" LCD monitors. I've had this UPS for about 4 years and never a problem. If I immediately power down completely (using the master power switch) my computer and monitors when the power goes out, this UPS will keep my network alive for at least 6 hours so I still have access via my wireless devices.
I actually have a "
Geek Squad" brand UPS on my home entertainment equipment supporting my 55" OLED TV, DVR, Blu-Ray player and AV receiver. It is made by Tripp Lite and it has worked flawlessly since I bought it too.
You may see "marketing hype" about pure sine wave outputs. That's all it is, hype. Any 1/2 way decent AC/DC power supply can handle the "stepped sine wave" or "stepped approximation" waveform just fine. They have for the last 25 years with no problems so there is no reason to believe the much more reliable and robust power supplies of today can't either. They are much more capable at handling power line anomalies than PSUs of yesteryear.
The ATX Form Factor standard requires all ATX power supplies to "hold" voltages for a mere 17ms (milliseconds) during abnormal power events. We (as humans) are not able to detect a flicker in the lights of just 17ms. But a "good" UPS can react easily within that time frame. Most have a cut-over or transfer time of 10ms or less.
And that's not just computer PSUs, but the power supplies for our modems, routers, switches, monitors and more. The only time you really need a "pure" sine wave output UPS is when used to support highly "sensitive" health monitoring and life support equipment as found in hospital intensive care units.
All computer and networking devices use DC voltages. It is actually easier to make a clean DC voltage converted from a stepped approximation sine wave than it is from a pure (round tops) sine wave because less filtering is required.
Do not forget that 99% of the time, our UPS is not "on-line". That is, it is not acting as a battery backup, except during a total power outage. It is just "in-line" acting as a sophisticated "automatic voltage regulator" (AVR). It is really for the AVR that we need a "good" UPS. Backup power during a full power outage is just the icing on the cake. But even then, power supplies do just fine with stepped approximation outputs.
The only real complaint I have with APC is they tend to use their reputation to charge more for their UPS. But they get away with it because they do make reliable products. I know many techs who won't by any brand but APC.
There is one downside to using a UPS. Most use SLA (sealed lead-acid) batteries and they have to be replaced about every 3 - 5 years. The UPS makers would like you to buy their expensive branded replacements, but there are sites that sell suitable replacements that are just as good, and often the exact same as the original, just a different brand label stuck to the side, but cheaper. I buy my UPS batteries from
Apex,
BatteryWholesale,
Battery Mart or
Batteryplex, depending on who has the best price
that day, as they change almost daily. Be sure to factor in shipping. Depending on the deal of the day, shipping can be significant, or free!