UPS for generators: Six considerations to achieving generator-UPS harmony

Proper generator sizing is essential to ensuring the safety of both the UPS load and an organization’s personnel during a lengthy outage. Many companies expect the generator to accommodate air conditioning, emergency lighting, communications and other vital services. Yet if the generator isn't sized large enough, it won’t be able to hold voltage and frequency within input tolerances when the UPS comes online.

The bigger the generator engine, the more stable the frequency, and therefore the more it can handle as the UPS comes off of battery. With that in mind, whenever budgets permit, it is wise to size up and also allow for some potential growth with the generator friendly UPS. The general recommendations for UPS models up to 25 kW are as follows:

• 2 times the total load (including the UPS, A/C, and all other equipment that must remain online) for natural gas-powered and mechanical governor generators.
• 1.5 times the total load for propane- or diesel-powered generators and those with an electronic governor.

The most common generator fuel options include propane, natural gas and diesel, each of which comes with its own set of own advantages and disadvantages. Natural gas-operating generators can be slower to respond and may need to be sized larger than their propane counterparts. Diesel is widely considered the best fuel and is predominant for generator solutions 50 kW and larger; however, it has a short storage life and its cost can strain budgets.

Portable generators are equipped with a governor, which limits the speed of fuel being delivered to the engine to a safe level amid load changes. Electronic governors are quick to respond, while mechanical governors are slower and can also cause calibration challenges.

The type of UPS will also impact UPS generator compatibility and configuration, as not all can compensate for frequency variations without relying on the battery. Both standby and line-interactive UPSs use battery power to prevent frequency variations from affecting the protected load. A double-conversion, online UPS, on the other hand, recreates the sine wave and filters frequency variations as part of its normal operation, thereby preserving battery life. Because it constantly rectifies AC to DC and then inverts the DC back to AC, the online UPS produces an output that corrects for voltage and frequency deviations. For this reason, double-conversion technology is the most common for critical load applications, and the most advantageous type of system for generator integration.

Often you can avoid a range of potential problems and the UPS not working on generator power by first ensuring that the generator manufacturer has tested and approved the product’s intended use with UPSs. To ensure that optimal UPS vs generator sizing is achieved, it is wise to consult with both the UPS and the generator manufacturer prior to finalizing a purchase. In addition, capacities of prospective loads to be protected can generally be found in the manufacturer’s specifications.