How to Size a Standby Generator for Your Facility

Start with the total running load the generator must support, then add the starting (inrush) loads of motors, compressors, and other equipment that draw heavy current at startup. Also account for UPS rectifier and battery-charging loads. Nameplate sums alone are misleading. Comp-Utility models the running and transient loads to determine the true required capacity.

Motors and similar equipment can draw several times their running current at startup, and the generator must supply that surge while holding voltage and frequency within limits. If sizing ignores inrush, the generator may dip, stall, or fail to start critical loads. Proper sizing accounts for the largest motor starts and the sequence in which loads come on.

A UPS presents a nonlinear load and draws extra current to recharge batteries after an outage, which can stress a generator if not accounted for. Some designs oversize the generator or use UPS settings that limit charging current. Comp-Utility coordinates the UPS and generator sizing so they work together rather than fighting each other.

Step-loading (or block-loading) is the sequence and size of load increments applied to a generator as it comes online. Generators have limits on how much load they can accept in one step. Designing the load steps, sometimes with load-shed or sequencing, ensures the generator stabilizes. Comp-Utility designs the load-acceptance scheme as part of sizing.

Often yes, but with care. Oversizing for growth wastes capital and can cause wet stacking from light-load running, while undersizing forces early replacement. Modular or paralleled generator approaches can add capacity as you grow. Comp-Utility balances current load, realistic growth, and the risks of oversizing when recommending capacity.

An undersized generator may experience voltage and frequency dips, fail to start large loads, overload, or shut down under load, defeating its purpose during an outage. It can also wear faster under stress. Correct sizing that accounts for starting loads and UPS interaction prevents these problems, which is why engineer-led sizing matters.

An oversized generator costs more to buy and install and, if it routinely runs at very light load, can suffer wet stacking, where unburned fuel and carbon accumulate and harm the engine. Regular load bank testing mitigates this. Right-sizing, rather than simply buying bigger, avoids both the capital waste and the operational issues.

Comp-Utility's licensed Texas Professional Engineers size standby generators for facilities across Central Texas, modeling running and starting loads, UPS interaction, and step-loading, then designing the transfer and fuel system. Call (512) 346-0999 or email sales@comp-utility.com for an engineer-led generator sizing.