Generator Size Calculator

By Reliant Electrical and Automation Inc.
This generator size calculator helps homeowners in Fresno, Clovis, and Madera estimate the correct Generac standby generator size for reliable backup power.

The calculator uses realistic, up-to-date appliance wattage and motor starting characteristics to estimate both continuous running load and start-up surge demand—the two factors that actually determine proper generator sizing. Natural gas (NG) and propane (LP) ratings are accounted for, along with typical residential usage patterns.

In just a few clicks, you’ll receive a recommended Generac Guardian generator size (10–26 kW) and an estimate of expected fuel consumption, helping you plan a system that starts critical loads, runs efficiently, and avoids costly oversizing.

Why Accurate Generator Sizing Matters
Undersized generators may fail to start A/C units, pumps, or other motor loads
Oversized generators waste fuel and increase installation cost
Proper sizing improves reliability, efficiency, and long-term performance

Generac Generator Size Calculator

Add the loads you want backed up. We’ll estimate running watts, starting surge, and recommend a Generac Guardian size (10–26kW) based on NG/LP output and typical surge behavior.

Home Size (sq ft)

Optional: used only for a light “baseline” estimate if you enable it below.

Fuel Type

Generac air-cooled units typically rate lower on NG vs LP.

Transfer Switch Package

We’ll show the matching Generac model # when available.

Sizing Headroom

Continuous headroom (keeps typical load under this % of generator rating)

80% target continuous loading

Appliances / Loads (Qty)

Other Load (Watts)

Use this for custom loads not listed (server rack, workshop tools, etc.).

Baseline (Optional)

Baseline adds ~0.6W/ft² running load to represent small always-on usage.

Notes: This is a planning tool. Final sizing should confirm HVAC LRA/starting, load-shed strategy, and site conditions.

Estimated Running Load

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Sum of selected loads (+ optional baseline).

Estimated Starting / Surge Requirement

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Approx. running + largest motor “extra” surge.

Recommended Generac Size

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Estimated Fuel Use (for recommended size)

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What this means

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Guardian 10–26kW (air-cooled) reference sizing + model mapping is based on Generac published specs. If you want liquid-cooled options added next, tell me your target range (e.g., 27–60kW).

Important Note

This tool is intended for planning and education. Final generator selection should always be confirmed with a professional load calculation and site evaluation by a licensed electrical contractor.