Generator Size Calculator

Recommend a backup generator size from running and surge watts.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Continuous Running Watts (W)
Input the total continuous wattage required by all appliances and lights you intend to power simultaneously.
2
Enter Startup Surge Watts (W)
Input the additional peak wattage needed by motor-driven appliances (like refrigerators or well pumps) when they first start up.
3
Review your results
The calculator will recommend a generator size in watts and kilowatts, along with your total peak power need.

Example Calculation

A homeowner needs a backup generator for essential appliances that require 5000 continuous running watts and an additional 2200 watts for startup surge.

Continuous Running Watts (W)

5000

Startup Surge Watts (W)

2200

Results

7500 W

Tips

Prioritize Essential Appliances

During an outage, list your absolute essential appliances (refrigerator, well pump, furnace fan, essential lights). Calculate their combined running and surge watts first, as you may not need to power everything simultaneously.

Factor in Future Needs

Consider any future appliance purchases or increased power demands. It's often more cost-effective to buy a slightly larger generator now than to upgrade later.

Understand Surge vs. Running Watts

Motor-driven appliances require significantly more wattage for a brief moment at startup (surge watts) than they do to run continuously. Always account for the highest startup surge of any single appliance you plan to run.

The Generator Size Calculator helps homeowners and contractors determine the ideal wattage for a backup generator, ensuring it can handle both continuous running loads and sudden startup surges from motor-driven appliances. By inputting these critical wattages, the tool recommends a suitable generator size in watts and kilowatts. This precision prevents overloading and ensures reliable power during outages, aligning with National Electrical Code (NEC) guidelines for safe and effective installation in 2025.

Selecting the Right Backup Power for Your Home

Choosing the correct generator size is fundamental to reliable home backup power. An undersized generator risks frequent overloads, potentially damaging appliances or the generator itself, while an oversized unit consumes more fuel than necessary, leading to inefficiency. Homeowners must prioritize essential loads like refrigeration (600-800W running, 1200-2000W surge) and central AC (2000-4000W running, 5000-7000W surge) to ensure critical functions during an outage. The National Electrical Code (NEC) outlines specific requirements for generator installation, emphasizing proper sizing and transfer switch integration for safety and optimal performance.

Calculating Total Generator Power Needs

To determine the recommended generator size, the calculator sums the continuous running watts of all planned appliances and adds the single largest startup surge wattage from any motor-driven device that might start while other items are running. The result is then rounded up to a standard generator size to ensure adequate capacity.

Total_Peak_Need = Continuous_Running_Watts + Startup_Surge_Watts
Recommended_Generator_Size = Nearest_Standard_Size_GEQ(Total_Peak_Need)

Where GEQ means "Greater Than or Equal To". Standard generator sizes typically include 2000W, 3500W, 5000W, 7500W, 10000W, 12000W, 15000W, etc.

💡 Once you've determined the right size, you'll want to estimate operating costs. Our Generator Fuel Consumption Calculator can help you budget for fuel expenses.

Sizing a Generator for Essential Home Appliances

Let's size a generator for a homeowner whose essential appliances require the following:

Continuous Running Watts: 5000 W (e.g., refrigerator, lights, furnace fan)
Startup Surge Watts: 2200 W (e.g., from a well pump or central AC unit's compressor)

Step 1: Calculate Total Peak Need Total Peak Need = 5000 W (running) + 2200 W (surge) = 7200 W

Step 2: Determine Recommended Generator Size Looking at standard generator sizes (e.g., 2000W, 3500W, 5000W, 7500W, 10000W), the next size greater than or equal to 7200 W is 7500 W.

Therefore, a 7500 W generator is recommended. This provides sufficient power to run all continuous loads and handle the largest single startup surge without overloading the unit.

💡 For a more detailed home generator assessment, including fuel type considerations and a comparison table, our Generator Size for Home Calculator offers extended functionality.

Common Generator Sizes for Residential Use

Residential generators come in a range of sizes designed to meet varying household power needs. Portable generators typically range from 3,000 W to 7,500 W. A 3,000-watt portable unit might power a refrigerator, some lights, and a few small electronics. A 5,000-watt unit can handle more appliances, possibly a well pump or a small window AC unit. A 7,500-watt portable generator can often manage most essential circuits for a medium-sized home, excluding large central air conditioning. Standby (whole-home) generators, which are permanently installed, usually start at 8,000 W and can go up to 22,000 W or more. An 8,000-watt standby generator might cover essential circuits, while a 22,000-watt unit can typically power an entire average-sized home, including central air conditioning, ensuring seamless operation during an outage. These benchmarks help homeowners choose the right balance of capacity and cost.

Selecting the Right Backup Power for Your Home

Frequently Asked Questions

What is the difference between running watts and starting watts?

Running watts (or continuous watts) is the power an appliance needs to operate continuously once it's turned on. Starting watts (or surge watts) is the brief, extra burst of power required by motor-driven appliances (like refrigerators, air conditioners, or pumps) for a few seconds when they first start up. Starting watts are typically 1.5 to 3 times higher than running watts for these types of appliances, and a generator must be able to handle this peak surge.

Why is correct generator sizing important?

Correct generator sizing is crucial to ensure your appliances run efficiently and to prevent damage to both the generator and your equipment. An undersized generator will struggle to power all desired items, potentially tripping breakers or causing appliances to malfunction. An oversized generator, while providing ample power, will operate inefficiently at low loads, consuming more fuel per kilowatt-hour, leading to 'wet stacking' in diesel units, and increasing operational costs unnecessarily.

What are common wattage requirements for household appliances?

Common household appliances have varying wattage requirements. A refrigerator might need 600-800 running watts and 1200-2000 starting watts. A central air conditioner could require 2000-4000 running watts and 5000-7000 starting watts. A sump pump typically needs 800-1500 running watts and 1500-2500 starting watts. Smaller items like lights (60-100W) or TVs (100-300W) have much lower demands. Always check appliance labels for exact figures.

Should I prioritize running watts or starting watts for generator sizing?

You should prioritize both running watts and starting watts, but specifically, your generator must be able to handle the *total continuous running watts* of all simultaneously operating devices PLUS the *single highest starting surge watts* of any one motor-driven appliance that will cycle on during operation. This ensures the generator can sustain all loads and accommodate the brief power spike from the largest motor starting up, preventing overload.