>Well. Suppose I tell my client: "You MUST change your 20 years OLD UPS cause Tore told me about" he tells me "Buy it" and then he still has the problem...

There are some circumstances where a UPS can make things worse:

1. UPS battery weak or dead

Most UPSs are set to kick in if voltage drops below some threshold, say 110V for a nominal 120V feed. If you have a voltage dip down to 100V, the UPS will kick in. If the UPS battery is dead, that will kill power to the connected loads (PC, network switch etc.). Ironically, many PC power supplies will accept input voltage as low as 90V and continue to function. So, in that case, the PC without a UPS would have kept going.

1a. UPS overloaded

This is a slight variation. The UPS battery can be OK, but if the unit is overloaded, when it kicks in due to low voltage or power interruption, it drops one or more loads to try to shed the overload condition. Those loads lose power.

The worst case I ever saw was 2 servers plugged into a 5 year old 350VA UPS.

2. Persistent low voltage

Some UPSs can use stored battery power to boost output voltage close to nominal levels. If you have persistently low voltage, eventually the UPS battery will be drained, whether the battery is brand new or not. Then, if the UPS is called on to try to bridge a brownout or blackout, there is no battery power available and all loads go down. Again, without a UPS in place, the attached loads may actually run OK on somewhat reduced voltage.

So - take some accurate measurements, see what your power situation is like. Depending on your findings, you may need to buy a more sophisticated unit that lets you adjust thresholds for voltage boosting, power failure etc.