Well I wouldn't deny what you said but I'm using a 600W PSU I bought 10 years ago + a new luminous eco volt neo 1550 inverter on UPS mode and both together are able to keep my system running during power cuts if total system power consumption is below 300W. I'm saying 300W because I have tested my GPU running at 100% on furmark consuming 270W which is fine but as soon as I put more load like running CPU stress test it doesn't hold. Right now I have set power limit on my 3070 at 70% to combat this issue. BTW this is with 2-3 fans and 4-5 lights running in other rooms in the house. Now If I'm to guess I will blame the old PSU for having a degraded hold up time since it's so old now and pretty much at the end of its life. I'm not sure how well this holds for PCs with 600W+ power consumption which is why i'm interested in finding out what specs
@deepakvrao 's PC has and that it's not his UPS that's shitting the bed here due to having degraded performance like my PC's old PSU.
Also a bigger than needed PSU will help with hold up time because it will have higher amount of capacitors to stay within it's ATX's standards hold up time.
May I know what you had when you faced those frequent reboots ?