As a rule of thumb, a genset continuous power rating should not be less than 1.5 times the UPS nominal power. The oversizing factor accounts for UPS efficiency, typically 96%, UPS power factor, typically 0.99, input harmonic distortion, typically 3% and adds up a power safety margin to prevent load steps from triggering a non-stable response in the genset’s output voltage. As mentioned above, though, other parameters should be considered to ensure proper operation during transient or no-load operation.

The Pros and Cons of soft-starting

Modern UPS do not exhibit any inrush power surge at start-ups. They gradually ramp-up to the required output power according to an adjustable “walk-in” time. This is the so called “soft-start” feature, which can also comprise a delayed start-up according to an adjustable “hold-off” time. Setting a hold-off time is useful when more UPS units are starting in parallel, so that the total UPS load is applied in gradual steps to the genset.

On the other hand, walk-in time relates to how steep the soft-start ramp will be. The shorter the setting, the steeper the ramp. It is important to note, that during the combined hold-off and walk-in time, the rectifier is not fully powered to supply the load, thus the storage batteries might still be partially discharging.

“Hold-off” and “walk-in” time. After an initial adjustable delay, or hold-off, UPS input current gradually increases, according to a preset ramp-up, or walk-in time.

On top of that, battery charging inhibition might be in place to assist the genset in gradual load ramping. Among other factors, problem-free changeovers therefore boil down to properly managing battery end of autonomy and UPS soft start settings. The latter is closely related to the specific genset output performance.