What size DG set do you need?

Because starting current — not running load — usually decides the rating. A direct-on-line motor draws roughly six times its full-load current for a few seconds; a star-delta starter cuts that to about a third; a VFD nearly eliminates it. Two sites with identical running loads can need very different gensets.

Two reasons: a genset should not run continuously at the top of its prime rating (sizing so running load sits near 70–80% leaves margin for surges and future loads), and the set must absorb the largest motor’s starting surge without a damaging voltage dip. The calculator sizes for both and picks whichever governs.

No — treat it as a well-informed starting point. UPS and VFD loads add harmonics that can call for an oversized alternator, altitude and ambient temperature derate output, and phase balance matters on mixed loads. Send the load list on WhatsApp and an engineer confirms the sizing free of charge, the same day.

Yes, and it is an expensive mistake twice over. Cummins application guidance is not to run a diesel set below about 30% of rated load for sustained periods: chronic light running glazes cylinder bores and passes unburnt fuel into the exhaust (wet stacking), which shows up later as slobber, fouled injectors and shortened life. If your genuine running load is small, size honestly and plan periodic loaded runs — or a load bank — rather than buying headroom you will never use.

Dramatically. Starting everything in one step forces the set to swallow every surge at once; stepped starting — largest motor first, then the rest — usually allows the smallest set for the same steady load. Two rules from Cummins application practice: life-safety loads may have to be up within about ten seconds of mains failure, and any motor that cycles on and off in service must be checked as if it starts last, with everything else already running.

Three families. Lightly loaded UPS systems with input filters can present a leading power factor, which a genset tolerates only in small doses before voltage control suffers — sequence lagging loads on first. Regenerative loads (lifts, cranes, hoists) push braking energy back at the set; unlike the grid, a genset can absorb very little, so other connected loads must soak it up. And rectifier-fed loads — UPS, battery chargers, VFDs — distort the waveform, calling for a larger alternator than the arithmetic suggests. Growth counts too: size for the load list you will have, not the one you have today.