A generator sizing calculator helps you determine the right generator wattage for your home by adding up the running and starting watts of every appliance you want to power during an outage. Choosing the correct size ensures your generator can handle motor startup surges without overloading, while avoiding the waste of buying more capacity than you need.
Select Appliances to Power
Check each appliance you want your generator to run. Wattage values are typical — adjust if your appliance differs.
Add Custom Appliance
Generator Recommendation
Load Breakdown
| Appliance | Running (W) | Starting (W) |
|---|
Estimated Fuel Consumption
Fuel estimates assume gasoline at ~$3.50/gal and typical portable generator consumption rates. Actual usage varies by generator model, load level, and altitude.
How to Use the Generator Sizing Calculator
Choosing the right generator size is critical for keeping your home comfortable and safe during a power outage. An undersized generator will overload and shut down — or worse, damage your appliances and the generator itself. An oversized one wastes money on fuel and purchase cost. This generator sizing calculator walks you through the process so you can find the perfect balance.
Step 1: Select the Appliances You Want to Power
Browse the appliance checklist and check the box next to every device you want running during an outage. The list includes common household items like refrigerators, freezers, sump pumps, air conditioners, heaters, and more. Each appliance shows its typical running watts and starting watts. Running watts are the steady power draw, while starting watts are the brief surge when a motor first kicks on — the generator must handle both.
Step 2: Adjust Wattage If Needed
The default wattage values are typical for each appliance, but your specific model may differ. Check the nameplate label on your appliance for its rated wattage. If your refrigerator draws 200W instead of 150W, simply edit the value in the checklist. You can also add custom appliances using the form at the bottom of the list — just enter a name, running watts, and starting watts.
Step 3: Calculate Your Generator Size
Click Calculate Generator Size to see your results. The calculator adds up all running watts and determines the peak startup surge — the highest starting wattage from any single appliance added on top of the total running load. It then adds a 20% safety margin and rounds up to the nearest standard generator size (3,000W through 20,000W) to give you a practical recommendation.
Step 4: Review the Recommendation
The results show your total running watts, peak startup surge, recommended generator size, and whether a portable or standby unit is the better fit. A detailed breakdown table lists every selected appliance with its wattage contribution. The fuel consumption estimate helps you budget for gasoline during extended outages — a key factor many homeowners overlook when shopping for a backup generator.
Tips for Choosing a Generator
For loads under 7,500W, a portable generator is usually sufficient and far more affordable. For whole-house coverage above 10,000W, consider a permanently installed standby generator that starts automatically when power goes out. Remember that you do not have to power everything at once — staggering heavy appliances like the AC and electric water heater can allow a smaller generator to handle your essential needs. Always place portable generators outdoors in a well-ventilated area to avoid carbon monoxide hazards.
Frequently Asked Questions
Is this generator sizing calculator free?
Yes, this generator sizing calculator is completely free with no account or signup required. All calculations run in your browser — your data is never sent to any server.
Is my data safe when using this tool?
Absolutely. Everything runs locally in your browser. No appliance data or results are stored on any server. You can close the page at any time and nothing is saved.
What is the difference between running watts and starting watts?
Running watts (also called rated watts) are the continuous power an appliance needs to operate. Starting watts (surge watts) are the extra burst of power needed for the first few seconds when a motor-driven appliance kicks on. Generators must handle the highest starting surge on top of the total running load.
Should I get a portable or standby generator?
Portable generators are best for loads under about 7,500 watts — they are affordable, mobile, and good for powering essentials during short outages. Standby generators are permanently installed, start automatically, and handle whole-house loads of 10,000 watts or more, making them ideal for frequent or extended outages.
How do I find the wattage of my appliances?
Check the manufacturer's label or nameplate on the appliance — it usually lists watts or amps and volts. If only amps are listed, multiply amps by volts (typically 120V in the U.S.) to get watts. You can also check the owner's manual or the manufacturer's website for power specifications.
Why does the calculator recommend a larger generator than my total watts?
The recommendation accounts for starting surge watts from motor-driven appliances like refrigerators, pumps, and air conditioners. It also adds a 20% safety margin so the generator is not constantly running at maximum capacity, which improves fuel efficiency and extends the generator's lifespan.
Can I run all my appliances at the same time on one generator?
It depends on the generator size. During an outage, it is common to stagger heavy loads — for example, turning off the AC before running the microwave. This calculator shows your total if everything runs simultaneously, but in practice you can manage a smaller generator by alternating high-draw appliances.
How much fuel does a generator use?
Fuel consumption depends on the generator size and load. A typical portable generator running at 50% load uses roughly 0.5 to 1 gallon of gasoline per hour. Larger standby generators running on natural gas or propane consume more. This calculator provides an estimate based on your total load and common consumption rates.