Commercial Generator Sizing: What to Gather Before Selecting Equipment

Start with the equipment you need to support

A useful generator-sizing discussion starts with the intended loads and the building's operating objectives. It should not begin by treating the main service amperage as the required generator rating.

Genesis Prime Group's generator service offering includes sizing and load calculations. Owners in Miami-Dade, Broward and Monroe Counties can request a defined review as part of an installation, replacement or upgrade project.

This is a preparation guide, not a calculator or final engineering recommendation. It does not select a generator size from incomplete inputs.

Request a Generator Load-Analysis Scope.

Identify the actual loads

List the equipment and functions you want to support and explain which are essential to the proposed operation. Separate confirmed equipment from future additions and from items that are only being considered.

For a restaurant, identify the actual refrigeration and other equipment rather than writing only kitchen. For a property with elevators, pumps or other building systems, provide the available equipment documents and clarify the intended operating arrangement. Required emergency functions and optional operating preferences must be distinguished by the responsible design team.

Gather electrical and operating information

Existing one-line diagrams, panel schedules, nameplates and equipment specifications can help establish the required review. Voltage, phase and equipment characteristics matter. Conflicting or incomplete existing records may require verification in the field.

Manufacturer sizing resources account for loads and their operating sequence. Starting a motor or compressor can impose a different requirement from keeping it running, so adding only running loads is not a complete selection method.

Generac industrial sizing resources | Cummins sizing resources.

Do not confuse kW, kVA and an ampere rating

Generator real-power rating, apparent-power rating and equipment current describe different aspects of the system. The design review should use the actual load information and the selected equipment's ratings rather than silently treating all loads as unity power factor.

There is no universal conversion from a building main breaker to a final generator size. Even a correct electrical calculation from stated inputs does not settle motor starting, operating sequence, equipment compatibility or the applicable design requirements.

A transfer switch ampere rating also does not automatically specify the generator size. It is part of the electrical system design, not a substitute for the load evaluation.

Document the operating sequence and future needs

State which equipment needs to operate together, whether any loads may be managed and what changes are planned. If a staged operating sequence is proposed, it must be evaluated and implemented in the actual design; it should not be assumed merely to make a preliminary number smaller.

The review should also establish the proposed fuel, site conditions and equipment-selection constraints. Existing gas service or an available installation footprint does not complete the sizing exercise.

From preliminary review to a construction specification

The agreed professional scope should identify its inputs, assumptions, deliverables and unresolved items. A preliminary discussion should not be relabeled as a signed and sealed design or a permit approval.

Once the equipment and system interfaces are established, the installation proposal can address the physical work: support, electrical integration, fuel, access, testing and turnover.

Engineering and permitting coordination | Commercial installation.

Can a photograph of the main breaker determine the size?

No. It can provide background information, but the actual loads and system requirements need review.

Can we begin without complete electrical drawings?

Yes. Send what is available. The scope should identify the investigation needed to resolve missing information before final selection.

Will this page generate an equipment recommendation?

No. A numerical calculator is intentionally not provided here. Final sizing requires the defined equipment and professional review.

Request a Generator Load-Analysis Scope.

Planning information reviewed October 13, 2026. Project-specific requirements and scope must be confirmed before work begins.