Generator Sizing (Basic)

Sizing a generator for infrastructure loads

Generator sizing starts with real power (kW) and apparent power (kVA). Your equipment uses real power to do work, but the generator must supply apparent power that includes reactive components. The power factor (PF) converts between the two: kW = kVA × PF. Loads with motors or inductive components often have lower PF, which increases the kVA requirement. This calculator combines running load, PF, and headroom to estimate the minimum generator rating for stable operation.

Startup surge matters because many systems draw more current during startup. HVAC compressors, pumps, and motors can require 1.2x to 3x their running power for a short period. The surge factor captures this behavior so you do not under-size the generator. Headroom provides additional buffer for future growth, transient spikes, and to avoid continuous operation at 100 percent of nameplate rating, which can reduce reliability and efficiency.

This is a basic estimator intended for quick planning. Voltage and phase are included for documentation and help you align with typical site standards, but the output focuses on kVA and kW because those are the primary sizing figures. Use these results to select a generator class, then confirm against vendor specifications, fuel derating, altitude corrections, and harmonics from non-linear loads. All calculations run locally in your browser.

When sizing for critical loads, consider whether your generator will run in parallel with others or in a standby configuration. Parallel systems can split load and improve redundancy, but they also introduce synchronization and load-sharing complexity. For single-generator designs, aim for a rating that keeps typical operating load below 80 percent to reduce wear and improve fuel efficiency.

The model does not account for harmonics or non-linear load behavior, which can inflate kVA requirements beyond what a simple PF estimate suggests. UPS systems and switch-mode power supplies can introduce harmonic currents that require additional generator capacity or harmonic filters. Use this calculator as a baseline, then verify against manufacturer sizing guides for your specific equipment mix.

Formula

Adjusted load (kW): kW = runningW × surge × (1 + headroom/100) / 1000

Generator kVA: kVA = kW / PF

Example calculation

A 25,000 W running load with a 1.3 surge factor, 20 percent headroom, and 0.9 PF yields: kW = 25,000 × 1.3 × 1.2 / 1000 = 39.0 kW.

The kVA requirement is 39.0 / 0.9 = 43.3 kVA. A generator rated at 45 to 50 kVA would provide a safe buffer for startup and growth.

FAQs

What is kVA versus kW?

kW is real power used by the load, while kVA is apparent power supplied by the generator.

How is surge factor used?

The surge factor multiplies running load to cover startup inrush.

Why include headroom?

Headroom avoids continuous max loading and allows for growth.

Does phase selection change the result?

This basic model uses kW and PF; phase is included for context.

Is this private?

Yes. All calculations run locally.

How it works

This calculator scales your running load by surge and headroom, then converts to kVA using power factor.

Disclaimer

Generator sizing is a simplified estimate. Confirm with vendor data, site conditions, and local electrical codes.