Generator Size Calculator

Generator sizing is one of the line items in a business continuity plan that gets written once and then sits in a drawer for ten years, by which point the connected load has crept up, a second chiller has been installed, and the diesel set on the roof is no longer big enough to start the largest motor on the site. A hospital, a chilled distribution centre, or a tier-three data hall that loses primary supply during a storm finds out the hard way that the sizing margin was eaten by load growth nobody flagged.

The numbers move quickly with motor starting. A 75 kW chiller compressor with a direct-on-line starter draws six to eight times running current for a few seconds, and a generator sized only for steady-state kW will trip on the first restart attempt. Soft starters, variable-speed drives, and ride-through UPS systems all change the calculation, and a 500 kVA set that looked comfortable on paper can be marginal once the surge profile of the actual plant is layered in. The downstream cost of getting it wrong is a continuity event that the BCP was meant to prevent.

Purely Energy works with sites where standby generation interacts with the half-hourly demand profile, including Triad avoidance schemes, capacity market participation, and DSR contracts where the genset earns revenue while it waits. Generator size sets the upper limit on all of that, and a properly specified set is an asset, not just a depreciation entry. Pairing the engineering sizing with the right commercial wrapper is where the value gets unlocked.