What Is the Generator Sizing Calculator?
The Generator Sizing Calculator is a free online tool designed for contractors and DIY builders who need quick, accurate calculations in the construction and building space. By entering your essential loads, large loads, additional custom load, you get instant results including running watts, starting watts, recommended size. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.
Why This Calculation Matters
Getting running watts right can make the difference between success and costly mistakes. In construction and building, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by contractors and DIY builders worldwide, giving you confidence that your numbers are correct. Use it to plan your projects with precision and avoid common pitfalls that trip up beginners.
When Should You Use This Calculator?
This tool is most useful when you know your essential loads and need to find the right running watts. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.
Generator Sizing Calculator
Select the appliances you need to power during an outage.
Common Generator Sizes and Capabilities
What each generator size can typically power
| Generator Size | Running Watts | Type | Can Power | Approx. Cost |
|---|---|---|---|---|
| 3,000W | 2,400W | Portable | Fridge + lights + chargers | $400-$800 |
| 5,000W | 4,000W | Portable | Essentials + furnace | $600-$1,200 |
| 7,500W | 6,000W | Portable | Essentials + well pump or AC | $1,000-$2,000 |
| 10,000W | 8,000W | Portable | Most circuits except AC + dryer | $1,500-$3,000 |
| 14,000W | 12,000W | Standby | Whole house minus central AC | $3,500-$6,000 |
| 22,000W | 18,000W | Standby | Entire house including AC | $5,000-$15,000 |
How to Use This Calculator
- Enter Your Essential Loads: Type or select your essential loads in the field provided. Use the most accurate value available for best results.
- Enter Your Large Loads (optional): Type or select your large loads in the field provided. Use the most accurate value available for best results.
- Enter Your Additional Custom Load (watts): Type or select your additional custom load in the field provided. Use the most accurate value available for best results.
- Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
- Review Your Results: Check your running watts, starting watts, recommended size. Use these figures to inform your next decision or compare against alternative scenarios.
How It Works
This calculator adds up the running watts of all selected appliances, finds the highest starting watt surge, then recommends a generator size with a 20% safety margin.
The basic rule:
- Running watts = sum of all selected appliance running wattages
- Starting watts = running watts + largest single starting surge (not all surge simultaneously)
- Recommended size = starting watts x 1.20 (20% safety margin)
- Motor-driven loads (pumps, compressors, refrigerators) draw 2-3x running watts at startup
- Resistive loads (heaters, lights, toasters) have equal running and starting watts
- Standard generator sizes: 3,000 / 5,000 / 7,500 / 10,000 / 12,000 / 17,500 / 22,000 watts
Never exceed 80% of continuous generator capacity for extended use. Motor loads should be started one at a time. Consider a whole-house standby generator (automatic transfer switch) for loads over 10,000 running watts.
Tips & Considerations
- Double-check your essential loads before calculating — even small input errors can significantly change your results.
- Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
- Pay attention to both running watts and starting watts — they work together to give you the full picture.
- Bookmark this page for quick access next time you need to plan your projects.
- If you're unsure about your additional custom load, start with a conservative estimate and adjust from there.
Frequently Asked Questions
What size generator do I need for my house?
For essential circuits only (fridge, freezer, furnace, lights, outlets): 3,000-5,000W portable. For essentials + well pump + sump pump: 7,500-10,000W. For whole-house including AC: 15,000-22,000W standby. Most homes need 5,000-7,500W for comfortable backup power.
What is the difference between running and starting watts?
Running watts is the continuous power needed to keep an appliance operating. Starting watts (surge watts) is the extra power needed for the first few seconds when a motor starts up — typically 2-3x running watts. Your generator must handle the starting surge of the largest motor.
Can I run my AC on a generator?
A window AC (10,000 BTU) needs about 1,500 running watts and 2,200 starting watts — a 3,500W+ generator handles this. A central AC (3-ton) needs 3,800 running watts and 5,700 starting — requiring a 7,500W+ generator minimum, often with nothing else running at startup.
How much fuel does a generator use?
At half load: a 5,000W portable uses about 0.5 gal/hr of gasoline, a 10,000W uses about 0.8 gal/hr. Natural gas standby generators use 150-300 cu ft/hr. Propane generators use 2-3 gal/hr. Running time on a full tank: 8-12 hours typical for portables.
Do I need a transfer switch?
Yes, for safety and code compliance. A transfer switch prevents backfeeding power into utility lines (which can electrocute lineworkers). Manual transfer switches cost $200-$500 installed. Automatic transfer switches (for standby generators) cost $500-$1,500 installed.
Portable vs standby generator?
Portable (3,000-12,000W): $500-$2,500, manual start, gasoline, 8-12 hr runtime, requires manual setup during outage. Standby (7,000-22,000W): $3,000-$15,000 installed, auto-start within seconds, natural gas or propane, unlimited runtime, permanent installation.