If you connect your UPS directly to a generator you may be surprised to find that your UPS may not accept the power from the generator at all. Remember, a UPS monitors the input power’s quality and if it’s not within acceptable ranges, it switches to the UPS’s batteries. You could find yourself checking cables and circuit breakers wondering why the UPS is not getting power from the generator. The reality is that the UPS is getting power from the generator, but the UPS does not consider the power quality to be sufficient. So what do you do now? There is no single solution. Start by examining your entire configuration. First, the generator must be powerful enough to supply consistent power quality. This means that the generator should be able to provide the same quality of power as the the power demands change. You may find that the generator may need to be much larger than your actual power needs. One of the most common problems with generator output is the line frequency. Generator output frequency should be 60Hz. My experience has shown that many generators are off by a bit. Purchasing a line frequency meter is a good investment. They cost little and are well worth the money. Adjusting the governor on the generator is a simple matter. The frequency should be set to 60Hz. If the frequency varies significantly as the load on the generator changes, consider a larger generator.
Make sure that your UPS is generator compatible. You may check with the manufacturer or visit their web site for information on compatibility. If your UPS has a sensitivity setting, reduce the UPS sensitivity to power fluctuations. If your UPS switches between battery and line power constantly or remains on battery power, the power quality may be inadequate. Reducing the UPS sensitivity may resolve the issue.
It is worth noting that appliances, lighting, etc. are not as sensitive to power quality issues.