Using a UPS is a short term option. In some cases services need to be maintained longer -
Are there best practices for deploying a UPS with a generator?
When designing with a backup solution, compatibility between a UPS and Generator is usually one of the biggest issues. It can be complex and confusing.
If the generator isn't sized large enough, it may not be able to hold voltage and frequency within input tolerances when the UPS comes online. The UPS will view the generator as an unstable power source and never come off of battery. The UPS will have a set input voltage and frequency window; if you go outside the window the UPS will go on battery. You can get into a vicious cycle of going on and off of battery until your UPS crashes and your IT load are dropped.
Several reasons why the generator would be an issue:
• The generator is like a car engine. As it maintains RPMs, it maintains frequency. When the UPS transfers from battery to generator power, the engine may slow down a bit. You can compare this to driving a stick shift while stopped at a red light headed uphill. When you dump the clutch, if you aren't giving it enough gas (or if you are driving a pinto) the RPMs are going to drop down.
• Just like in the automotive world, the bigger the engine (generator here) the more it can handle the dumping of the clutch as the UPS comes off of battery. Bigger (aside from your budget number) is better
• Natural Gas is usually the least combustible fuel out there, and generators that run off of NG are slow to respond and may need to be sized larger than propane counterparts. Diesel is best, but usually predominant in the world of 50kW and larger, but it may present a lot more budget and maintenance issues if you don't have a maintenance program or run them weekly, etc.
• When looking at smaller generators (30kW and below) look at the governor. Electronic governors are quick to respond, mechanical governors are not and may add headaches.
The features of the UPS can also impact you configuration. Most single phase UPS products (20kVA and below) will come back online all at once. Larger three phase UPS products will have "current walk-in" and will slowly transition from battery anywhere from 5-60 seconds. As for sizing solutions with generators under 30kVA for single phase UPS applications, I usually lean on the err of caution being a manufacturer. We recommend the same if you are responsible as the decision maker.
• Generator should be sized 2 times the total load (UPS, A/C, an everything else) for NG and mechanical governor
• 1.5 times the load will work if you have a new generator, Propane or Diesel and an electronic governor.
• Any mix from the above puts you somewhere in the middle of 1.5 to 2 times.