What size generator do I need for my home?

Generator size depends on which appliances you need to run simultaneously. A typical home needing A/C, refrigerator, lights, and a water pump requires 7.5-15 kW. Use our calculator with your specific loads for an accurate recommendation.

Why is starting wattage important for generator sizing?

Motors in appliances like A/C units, refrigerators, and pumps draw 2-3x their running wattage when starting. Your generator must handle the highest single starting surge on top of all running loads, or it will trip the overload protection.

How does altitude affect generator performance?

Generators lose approximately 3.5% of their rated power for every 1,000 feet above 1,500 feet elevation due to reduced air density. Our calculator automatically applies altitude derating to ensure proper sizing.

Generator Sizing Calculator - Backup Power Sizing Tool

New to this? Plain-English guide

Running vs Starting Watts — motors (fridges, pumps, ACs) need a brief surge of extra power to start — often 2-3x their running draw. Generators must be sized for that surge, not just the steady load.

kW vs kVA — kW is real usable power; kVA is what the generator nameplate often shows. They differ by the power factor (typically 0.8), so a 10 kVA set gives about 8 kW.

Power Factor — how efficiently your loads use the electrical current supplied. Homes are typically about 0.8. It's why kVA and kW aren't the same number.

Load Factor — how hard the generator works compared to its maximum. Running around 50-80% load is the sweet spot for fuel efficiency and engine life.

Days of Autonomy — how many days you want to run on stored fuel without refuelling — key for hurricane season when petrol stations may be closed.

Electrical Loads

Running: 0 W | Starting: 0 W

Generator Configuration

Fuel Type

Altitude (ft)

Ambient Temp (°C)

Power Factor

Safety Margin (%)

Fuel & Runtime

Runtime per Day (hrs)

Days of Autonomy

Currency & Fuel Cost

Currency

Fuel Cost per Gallon

Auto-populated typical price. Adjust to match your local fuel cost.

Project Management

Load Saved Project

Calculation Method: Generator sized for peak starting load (running total + largest single starting surge) with safety margin, altitude/temperature derating, and power factor correction. Fuel consumption based on 75% load at recommended generator size.

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Generator Sizing Guide

Running vs Starting Watts
Every motor-driven appliance (A/C, fridge, pump) draws significantly more power when starting than when running. This "starting surge" or "inrush current" typically lasts 1–3 seconds but can be 2–3x the running wattage. Your generator must handle the highest single starting surge on top of all running loads simultaneously.

Generator Types

Type

Best For

Runtime

Portable (3–10 kW)

Home backup, events

8–12 hrs/tank

Standby (10–50 kW)

Whole-house, auto-start

Continuous (piped fuel)

Industrial (50–200+ kW)

Commercial, critical

Continuous w/ tank

Fuel Comparison

Fuel

Efficiency

Notes

Diesel

Most efficient

Long shelf life, readily available in Caribbean

Gasoline

Moderate

Easy to find, shorter shelf life (3–6 months)

Propane

Less efficient

Indefinite shelf life, clean burning

Natural Gas

Less efficient

Continuous supply if piped, no storage needed

Altitude & Temperature Derating
Altitude: Generators lose ~3.5% of rated power per 1,000 ft above 1,500 ft elevation due to thinner air reducing combustion efficiency.
Temperature: Above 40°C (104°F), generators lose ~2% per degree C due to reduced cooling efficiency and lower air density. Caribbean ambient temps of 30–35°C are within normal operating range.

Caribbean Hurricane Preparedness
• Stock minimum 3 days of fuel (more for extended outages)
• Store fuel in approved containers away from the generator
• Never run a generator indoors or in enclosed spaces — CO poisoning kills
• Test your generator monthly; run under load for 30 minutes
• Use a transfer switch — never backfeed into the grid
• Keep spare oil, filters, and spark plugs on hand
• Priority loads: refrigeration, water pump, lighting, communication

Load Priority Strategy
Essential: Loads you must run (fridge, water pump, lights, medical equipment). Size your minimum generator for these.
Important: Loads that significantly improve comfort (washing machine, microwave, computer). Include if generator capacity allows.
Optional: Loads that are nice to have (TV, pool pump, water heater). Only run if generator is well within capacity.

Disclaimer: This calculator provides estimates for preliminary generator sizing. Actual performance depends on specific generator models, load characteristics, fuel quality, maintenance, and environmental conditions. Always consult a qualified electrician for transfer switch installation and load management.

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Results

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Total Running Load

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Peak Starting Load

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Recommended Generator Size

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Generator Rating (kVA)

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Load Factor

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Fuel Consumption

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Daily Fuel Cost

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Total Fuel Required

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Total Fuel Cost

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