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 We Calculate This
This generator sizing calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
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.
When Would You Use This Calculator?
This generator sizing calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
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.