Generator Size Calculator Guide: Wattage, Sizing & Fuel Estimates
Quick Answer
- *For essentials only (fridge, lights, sump pump, phones): 5,000–7,500 watts.
- *For a full home including central AC: 10,000–22,000 watts.
- *Always account for starting watts — motor-driven appliances surge 2–3× their running wattage on startup.
- *Add a 20–25% safety margin to your total. Running at 50–75% load extends generator life.
How Generator Sizing Works
Generator sizing comes down to one question: how many watts do you need? Every electrical device has a wattage rating. Add up the ratings for everything you want to power simultaneously, account for motor startup surges, and you have your target generator size.
According to the U.S. Energy Information Administration (EIA), the average American household consumes about 10,500 kWh per year — roughly 1,200 watts of continuous draw. But peak demand (when the AC kicks on, the dryer runs, and the microwave fires up) can spike to 10,000+ watts. That peak is what your generator needs to handle.
Starting Watts vs. Running Watts
This is where most people get tripped up. Running watts (also called rated watts) are the continuous power a device needs. Starting watts (surge watts) are the brief spike of extra power required when an electric motor first starts.
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 150–400 | 400–1,200 |
| Sump Pump (1/2 HP) | 800 | 1,300–2,150 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Central AC (3 ton) | 3,500 | 7,000 |
| Well Pump (1/2 HP) | 1,000 | 2,100 |
| Microwave (1,000W) | 1,000 | 1,000 |
| LED Lights (10 bulbs) | 100 | 100 |
| Garage Door Opener | 550 | 1,100 |
Notice that resistive loads (lights, heaters, microwaves) have the same starting and running watts. Motor-driven loads (AC, pumps, compressors) can surge 2–3 times their running wattage. According to Generac's 2025 sizing guide, failing to account for starting watts is the number-one cause of generator overloads during power outages.
Step-by-Step Sizing Method
Step 1: List Your Essential Appliances
Write down everything you want to run during an outage. Be realistic — you probably do not need the clothes dryer running while the AC is on.
Step 2: Add Up Running Watts
Sum the running wattage of all listed appliances. This is your continuous load.
Step 3: Add the Largest Starting Surge
Identify the appliance with the highest starting wattage. Add its starting watts (minus its running watts, since running watts are already counted) to your total. Not all motors start simultaneously, so you only need to account for the largest single surge.
Step 4: Add a 20–25% Safety Margin
Multiply your total by 1.2 to 1.25. This accounts for power factor losses, aging appliances, and any loads you might have missed.
Example Calculation
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 200 | 1,200 |
| Sump Pump | 800 | 2,150 |
| 10 LED Lights | 100 | 100 |
| Furnace Blower | 800 | 2,350 |
| Phone/Laptop Chargers | 100 | 100 |
| Total Running | 2,000 | — |
Running total: 2,000W. Largest surge: furnace blower at 2,350W starting (1,550W above running). Total peak: 2,000 + 1,550 = 3,550W. With 25% margin: 3,550 × 1.25 = 4,438W. A 5,000W generator covers this with room to spare.
Generator Size Categories
| Size Range | Best For | Price Range (2026) |
|---|---|---|
| 2,000–3,500W | Camping, tailgating, a few essentials | $300–$800 |
| 5,000–7,500W | Essential home circuits during outage | $600–$1,500 |
| 10,000–12,000W | Larger homes, window AC units, well pumps | $1,200–$3,000 |
| 14,000–22,000W | Whole-house standby with central AC | $3,000–$6,000+ |
According to Consumer Reports (2025 generator buying guide), the most popular generator size sold in the U.S. is the 5,000–7,500W range, accounting for 42% of all portable generator sales.
Fuel Consumption Estimates
How long your generator runs depends on its fuel type, tank size, and load level. Running at 50% load instead of 100% can nearly double your runtime.
| Generator Size | Fuel Use at 50% Load | Fuel Use at 100% Load |
|---|---|---|
| 3,500W portable | ~0.4 gal/hr | ~0.75 gal/hr |
| 7,500W portable | ~0.7 gal/hr | ~1.3 gal/hr |
| 12,000W portable | ~1.0 gal/hr | ~1.8 gal/hr |
| 22,000W standby (LP) | ~1.5 gal/hr | ~2.8 gal/hr |
According to the Department of Energy, gasoline-powered generators cost approximately $0.12–$0.18 per kWh to operate, while propane units run $0.15–$0.22 per kWh. For reference, the national average grid electricity price is $0.16 per kWh (EIA, 2025).
Portable vs. Standby Generators
Portable Generators
Typically 2,000–12,000 watts. Run on gasoline. Require manual setup and connection via extension cords or a transfer switch. Average cost: $500–$2,000. Ideal for occasional outages and job sites.
Inverter Generators
A subtype of portable. Produce cleaner power (lower THD) safe for sensitive electronics. Quieter (50–60 dB vs. 70–80 dB for conventional). More fuel-efficient. Cost premium: 30–50% over conventional portables of the same wattage.
Standby Generators
Permanently installed. Run on natural gas or propane. Start automatically within 10–20 seconds of a power outage. Range from 7,500W to 150,000W. According to Remodeling Magazine's 2025 Cost vs. Value Report, a whole-home standby generator installation (average $15,000) recoups approximately 54% of its cost at resale.
Safety Considerations
The Consumer Product Safety Commission (CPSC) reports that portable generators cause an average of 70 carbon monoxide deaths per year in the U.S. Follow these non-negotiable rules:
- Never run a generator indoors — not in a garage, basement, or enclosed porch. Place it at least 20 feet from any door, window, or vent.
- Use a transfer switch to connect to your home's electrical panel. Backfeeding through an outlet without a transfer switch can electrocute utility workers.
- Let it cool before refueling. Gasoline on a hot engine or exhaust is a fire hazard.
- Store fuel safely in approved containers, away from the generator and living spaces.
Find the right generator size for your needs
Use our free Generator Size Calculator →Frequently Asked Questions
What size generator do I need for my house?
Most homes need 5,000–7,500 running watts for essentials (refrigerator, lights, sump pump, phone chargers). To power a full home including central air conditioning, you typically need 10,000–22,000 watts. Add up the running watts of everything you want to power, then add the highest single starting wattage surge.
What is the difference between starting watts and running watts?
Running watts (rated watts) are the continuous power an appliance needs during normal operation. Starting watts (surge watts) are the extra power needed for the first few seconds when a motor starts. Starting watts can be 2–3 times higher than running watts. Your generator must handle both.
How long will a generator run on a tank of gas?
A typical portable generator uses 0.5–1.5 gallons of gas per hour at 50% load. A 5,000-watt generator with a 5-gallon tank runs approximately 8–12 hours at half load. Runtime depends on load, engine efficiency, and tank size. Inverter generators are more fuel-efficient, often running 25–40% longer on the same fuel.
Can I run my air conditioner on a portable generator?
A window AC unit (5,000–10,000 BTU) needs 500–1,500 running watts and 1,000–3,000 starting watts. A central AC unit (2–5 ton) needs 2,000–5,000 running watts with starting surges up to 7,500 watts. For central AC, you typically need a 7,500+ watt generator minimum.
Is it better to oversize or undersize a generator?
Always oversize slightly. Running a generator at 100% capacity continuously causes faster wear, higher fuel consumption, and a shorter lifespan. The sweet spot is 50–75% of rated capacity. Add a 20–25% safety margin to your total wattage calculation. An undersized generator can damage both your appliances and the generator through voltage drops and overheating.