Generator sizing begins with the loads expected to operate together, but it must also consider how those loads are applied and removed. A plant with a moderate running demand may still have a significant starting or restoration event. Describe the sequence and the equipment behaviour rather than relying on one total power figure.
Keep the assumptions visible
Separate measured demand, nameplate information and allowances for future loads. Record power factor, starting methods and site conditions where relevant. Uncertain inputs should remain identifiable so that a preliminary estimate is not mistaken for a final selection based on verified equipment data.
Step 3 — identify the hardest transient
- largest motor
- transformer energisation
- UPS recharge
- large block-load application
- multiple simultaneous loads
Step 6 — verify the candidate
- actual engine rating
- alternator capability
- transient-performance data
- duty definition
- manufacturer sizing data
It is appropriately sized for how the load actually behaves.
Engineering basis: standard single-phase and three-phase electrical relationships. Calculations are planning aids; final equipment selection requires application-specific verification.
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Read the rating conditions
Compare capacity under the intended duty, frequency and site conditions. Manufacturer rating categories can have different operating limits; use the actual product definition when moving from a load estimate to a candidate generator.
Review the proposed package
The selected engine, alternator and controls must be assessed together for the intended duty and environment. Installation interfaces and acceptance criteria also affect the decision. A complete sizing enquiry therefore includes the electrical requirement, operating profile and site information needed to examine the proposed package.
Start Engineering Review
Start Engineering Review. Share the application, location and information available so the next discussion can address your actual requirement.
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Engineering intent
Generator Sizing should be evaluated as part of the complete power system rather than as an isolated catalogue item. Operating duty, connected load, interfaces, protection, controls, installation conditions, testing requirements and lifecycle consequences determine whether the arrangement is appropriate.
Engineering considerations
If the generator is too large, it may operate inefficiently, consume unnecessary fuel, increase capital cost and create low-load operating problems. The right generator size is not found by asking only: The Reddy Generator Sizing Calculator gives a disciplined preliminary sizing direction — not a final engineering approval. The calculator should guide the customer from rough enquiry to structured engineering input. Display this near the top of the tool: Final generator sizing must be confirmed through load assessment, motor-starting review, duty selection, site conditions, cable/protection review, What type of facility are you sizing for?
| Data centre / IT | UPS, cooling, battery recharge, redundancy |
|---|
Selection and design considerations
Estimate operating autonomy using usable fuel, reserve and consumption assumptions. Compare ownership and rental considerations for different operating requirements. Every tool must clearly state its assumptions and limitations. Engineering calculators support preliminary decision-making; final equipment selection and system design require application-specific verification. A manufacturing facility, hospital, data centre, construction project and mine should not receive the same generic generator recommendation. Production continuity, motor loads, process stability, automation and maintenance planning. Generator, UPS, transfer, redundancy, fuel autonomy, monitoring and critical-load coordination. Critical-load continuity, starting reliability, automatic transfer, fuel planning and testing. Process continuity, electrical quality, redundancy and lifecycle reliability. Temporary generation, changing loads, distribution, mobility and project-stage power. Remote generation, large motor loads, fuel logistics and demanding operating environments.
Integration and protection considerations
IT/server load
| Usually UPS-linked; review UPS input data | Mixed building load |
|---|---|
| Use measured PF or conservative estimate | Final selection should use actual load schedule and measured data where available. It represents real working power — the useful power consumed by loads such as: For generator sizing, kW is essential, but it is not enough by itself. It represents apparent power — the total electrical capacity that the generator/alternator must support. 100 kVA / 80 kW at 0.8 PF So the generator can support 80 kW real power at 0.8 power factor, subject to rating, duty, derating and manufacturer conditions. Power factor describes how effectively apparent power becomes useful work. |
Testing, commissioning and verification
cable sizing, protection, site conditions and written quotation. A kW-to-kVA result is not final generator sizing. It is only one step in the sizing chain. Example 1 — 80 kW Load at 0.8 PF Result: 80 kW at 0.8 power factor requires 100 kVA apparent capacity before other sizing factors. Example 2 — 150 kW Load at 0.8 PF Result: preliminary apparent load is 187.5 kVA. This does not automatically mean a 187.5 kVA generator is final. The next step is to review motor starting, future expansion, duty and site conditions. Example 3 — 200 kW Load at 0.9 PF Result: preliminary apparent load is approximately 222 kVA.
Operating and lifecycle considerations
Most generator mistakes do not begin at installation. They begin earlier — when the wrong question is asked. What fuel, UPS, ATS, cable and maintenance system supports it? Reddy Power Intelligence turns generator selection from guesswork into structured engineering decision-making. A Generator Is Not Selected by kVA Alone. That is why calculators must guide the customer — not oversimplify engineering.
| Generator Sizing Calculator | Estimate required kVA based on load profile |
|---|---|
| kW to kVA Calculator | Convert real power to apparent power |
| Current Calculator | Estimate amperage for single/three-phase loads |
| Motor Starting Calculator | Review starting kVA and voltage-dip risk |
| Fuel Runtime Guide | Estimate backup duration from fuel and load |
| Fuel Cost per kWh Tool | Move from L/h thinking to energy economics |
Decision and application considerations
Preliminary generator range: ___ kVA to ___ kVA Preliminary running load suggests around 119 kVA apparent load. Generator selection may require higher capacity after motor starting, Motor starting and step-load acceptance must be verified before final generator selection. Battery recharge current, UPS input harmonics, generator voltage/frequency stability, cooling restart and ATS sequence must be reviewed.
| Engineering review required | Motors, UPS, critical loads or site issues exist |
|---|---|
| Ready for quotation review | Sufficient data for Reddy team to review |
| High-risk facility | Healthcare, data centre, public infrastructure, pharma, etc. |
The calculator should show red flags when user enters: Please request a Reddy sizing review before purchasing, renting or installing a generator. The calculator has identified motors, UPS loads, critical loads or site conditions
Direct answer
Generator sizing begins with the loads expected to operate together, but it must also consider how those loads are applied and removed. A plant with a moderate running demand may still have a significant starting or restoration event.
The practical decision for Generator Sizing should be based on verified load, equipment, interface and site information. Where equipment-specific data, regulatory applicability or a project study is required, this guidance does not replace that verification.
Related Reddy Generators pathways
- Power Engineering Centre
- Generator Finder
- Integrated Power Systems
- Industry Power Solutions
- Application Engineering Enquiry
Load-estimation control
Load estimation is not a simple sum of connected nameplates. Reliable source and distribution sizing requires the operating demand, diversity, starting duty, harmonics, criticality, future growth and emergency operating scenario to be defined before generator, transformer, UPS, cable and switchgear ratings are finalised.