Standby generator systems are a critical part of resilient power infrastructure, especially where extended outages can affect business operations, safety systems, production environments or mission-critical equipment.

This guide provides practical planning guidance for understanding generator sizing, load behaviour, ATS integration, fuel autonomy, installation considerations and maintenance requirements.

A standby generator provides backup power during utility outages. Depending on the system design, it may start automatically from a controller and ATS signal or manually through an operator-controlled changeover arrangement.

Once the changeover system transfers load, the generator supports selected electrical circuits until utility power returns. Generators often work alongside UPS systems for critical loads because a generator does not provide zero-break transfer unless combined with UPS or other ride-through systems.

Generator capacity is normally stated in kVA, while the useful real power is expressed in kW. Many generator sets are rated at a power factor of 0.8, meaning a 100 kVA generator typically provides approximately 80 kW.

Generator loading matters because long-term light loading can cause wet stacking and poor engine performance, while excessive loading reduces spare capacity and can affect voltage and frequency stability.

Generator sizing must consider more than the normal running load. Motor starting, UPS rectifier behaviour, nonlinear loads, load steps, future expansion and environmental derating can all affect the selected generator size.

An ATS detects mains failure, sends a generator start signal, monitors generator voltage and frequency, transfers the load to generator supply and returns the load to mains when stable utility power is restored.

Open transition systems briefly disconnect the load before reconnecting to the alternate source. Closed transition systems overlap sources for a short controlled period and require more advanced design and approvals. Manual bypass or maintenance arrangements may be required to allow safe service or replacement of switching equipment.

UPS rectifiers can affect generator loading, especially when battery recharge current is added after an outage. Generator AVR response, frequency stability, harmonics, load steps and rectifier behaviour should all be reviewed during design.

Oversizing may be required where UPS loads are significant. Soft-start, walk-in and input current limiting features can help reduce generator stress where supported by the UPS.

Fuel planning should consider generator size, operating load, runtime requirement, tank volume, fuel condition and refuelling logistics. A site that needs eight hours of backup power must account for usable fuel volume rather than nominal tank size alone.

Generator controllers manage automatic start, stop, alarm, protection and monitoring functions. Deep Sea, ComAp-style and similar controllers can provide mains monitoring, generator status, alarm history and communication interfaces.

Supporting systems may include a battery charger, jacket water heater where applicable, remote monitoring, run-hour logging and service interval tracking.

Generator reliability depends on preventative maintenance, monthly inspections, test runs, periodic load bank testing, oil and filter services, coolant and belt inspections, battery checks, alternator checks and fuel condition management.

Long-term lifecycle planning should include service records, spares availability, fuel management, control system health, acoustic equipment condition and future load growth.

If a generator supports critical business operations, UPS systems, production equipment, server rooms, security systems, medical equipment or safety-related infrastructure, the design should be reviewed by an experienced Critical Power Specialist before procurement or installation.

This guide is provided for general technical guidance only. Final generator selection, protection, cable sizing, fuel system design, installation requirements and compliance must be confirmed against manufacturer data, site conditions, SANS requirements and applicable regulations.

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