How Generator Sizing Works
Generator sizing comes down to one concept: watts. Every appliance draws a certain number of watts while it runs (called running watts or rated watts), and many appliances draw a much higher surge of power for the first few seconds when they start up (called starting watts or surge watts).
Anything with a motor -- refrigerators, air conditioners, sump pumps, well pumps -- will pull two to three times its running wattage at startup. Resistive loads like space heaters, toasters, and incandescent lights draw the same wattage whether they are starting or running.
The Key Formula
Watts = Volts x Amps. If an appliance label says 120V / 10A, it draws 1,200 watts. Need to convert between amps and watts? Use our amps-to-watts calculator.
Your generator must have enough running watts to handle everything operating at the same time, plus enough starting watts to cover the highest startup surge on top of that running load. Get this wrong and the generator overloads, trips its breaker, and shuts down -- or worse, damages sensitive electronics.
Common Appliance Wattage Chart
The table below lists typical running and starting wattages for common household appliances. Your specific appliances may vary -- always check the nameplate on the appliance itself for the most accurate numbers.
| Appliance | Running Watts | Starting Watts | Notes |
|---|---|---|---|
| Refrigerator | 150 W | 600 W | Compressor motor surge |
| Sump Pump (1/2 HP) | 800 W | 2,000 W | High surge -- motor load |
| Central A/C (3 ton) | 3,500 W | 6,000 W | Largest residential load |
| Window A/C (10,000 BTU) | 1,200 W | 3,600 W | 3x surge typical |
| Electric Water Heater | 4,500 W | 4,500 W | Resistive -- no surge |
| Well Pump (1/2 HP) | 1,000 W | 2,500 W | Deep well pumps draw more |
| Microwave | 1,000 W | 1,000 W | Check nameplate wattage |
| Toaster | 1,200 W | 1,200 W | Resistive -- no surge |
| Coffee Maker | 1,000 W | 1,000 W | Resistive -- no surge |
| LED Lights (10 bulbs) | 100 W | 100 W | ~10 W each |
| TV (LED, 55") | 200 W | 200 W | Varies by size and type |
| Desktop Computer | 300 W | 300 W | With monitor |
| Garage Door Opener | 600 W | 1,400 W | Motor surge on open/close |
| Electric Dryer | 5,400 W | 5,400 W | 240V circuit -- large load |
| Washing Machine | 500 W | 2,500 W | Motor surge during spin |
| Dishwasher | 1,800 W | 1,800 W | With heated dry cycle |
| Space Heater | 1,500 W | 1,500 W | Resistive -- no surge |
| Hair Dryer | 1,875 W | 1,875 W | Resistive -- no surge |
| EV Charger (Level 2) | 7,200 W | 7,200 W | 240V / 30A continuous |
| Furnace Blower | 800 W | 2,500 W | Critical for gas/oil heat |
Always Check the Nameplate
The wattages above are typical averages. Your refrigerator, well pump, or A/C unit may draw more or less than shown. Look for the manufacturer nameplate on the appliance -- it lists volts and amps. Multiply them together to get running watts.
Step-by-Step Generator Sizing
Follow these five steps to calculate the generator size you need. We will walk through a real-world example afterward.
Step 1: List Everything You Want to Power
Write down every appliance and device you need running during an outage or on the job site. Be realistic -- you probably do not need the electric dryer and the A/C and the EV charger all at the same time.
Step 2: Add Up All Running Watts
Total the running watts for every item on your list. This is your base running load.
Step 3: Find the Largest Starting Watt Item
Look through your list and find the single appliance with the highest starting wattage. This is the biggest surge your generator will need to handle.
Step 4: Add the Largest Starting Surge to Running Total
Take your total running watts from Step 2 and add the difference between the starting watts and running watts of your largest motor. This gives you the peak demand number: Total Running Watts + (Largest Starting Watts - Largest Running Watts) = Peak Demand.
Step 5: Add a 20% Safety Margin
Multiply your peak demand by 1.2 to add a 20% safety buffer. Running a generator at 100% capacity constantly reduces its lifespan and leaves no room for error. The result is your minimum generator size.
Worked Example: Home Power Outage
Suppose you want to keep these items running during a storm:
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 150 | 600 |
| Furnace Blower | 800 | 2,500 |
| Sump Pump (1/2 HP) | 800 | 2,000 |
| LED Lights (10) | 100 | 100 |
| TV | 200 | 200 |
| Microwave | 1,000 | 1,000 |
| Well Pump (1/2 HP) | 1,000 | 2,500 |
| Total Running Watts | 4,050 | -- |
Step 3: The largest starting-watt items are the furnace blower and the well pump, both at 2,500W starting. Pick either one since they likely will not start at the exact same instant.
Step 4: Peak demand = 4,050 running + (2,500 starting - 800 running for furnace blower) = 5,750W.
Step 5: With a 20% margin: 5,750 x 1.2 = 6,900W.
A 7,500-watt generator is the right choice for this scenario. It covers the peak demand with headroom to spare.
Generator Sizes: What Can They Power?
Here is a quick reference for common generator sizes and what they can realistically handle:
2,000 - 3,000 Watts
Best for: Bare essentials and camping
- Refrigerator, LED lights, phone chargers, TV
- Not enough for anything with a large motor or heating element
- Good backup for a small apartment or RV
5,000 - 7,500 Watts
Best for: Most home backup needs
- Refrigerator, furnace blower, sump pump, well pump, lights, TV, microwave
- Can handle one large motor starting at a time
- The most popular portable generator range for homeowners
10,000 Watts
Best for: Whole house minus central A/C
- Everything above plus washing machine, dishwasher, electric water heater
- Can run a window A/C unit but not central air
- Common for job sites running multiple power tools
15,000 - 20,000+ Watts
Best for: Whole house including central A/C
- Central A/C, electric dryer, EV charger, and everything else simultaneously
- Typically requires a standby generator with an automatic transfer switch
- Powers your home as if the grid were still on
Portable vs Standby Generators
The two main categories of generators serve very different purposes. Understanding the differences helps you make the right investment.
Portable Generators
- Output range: 1,000 - 12,000 watts
- Fuel: Gasoline (most common), dual-fuel with propane
- Startup: Manual pull-start or electric start
- Cost: $300 - $2,000
- Pros: Affordable, movable, no installation required
- Cons: Must be started manually, limited runtime, outdoor use only, requires extension cords or manual transfer switch
Standby Generators
- Output range: 7,500 - 150,000 watts
- Fuel: Natural gas or propane (piped supply)
- Startup: Automatic via transfer switch -- turns on within seconds of an outage
- Cost: $3,000 - $15,000+ installed
- Pros: Hands-free operation, unlimited fuel supply (natural gas), powers entire home
- Cons: Expensive, requires professional installation, permanent outdoor pad, needs an automatic transfer switch
If you are sizing a standby generator for a whole house, you will need a complete whole-house load calculation to determine your total demand and ensure the generator and transfer switch are properly matched to your electrical panel.
Inverter Generators vs Conventional Generators
You will see two types of portable generators at the store: conventional (open-frame) generators and inverter generators. The difference matters more than most people think, especially for sensitive electronics.
Conventional Generators
- Output: Raw AC power, sometimes with voltage fluctuation
- THD (Total Harmonic Distortion): 10-25%
- Noise level: 70-80+ dB (as loud as a lawnmower)
- Weight: 100-250 lbs for 5,000W+
- Cost: $400-$1,500 for 3,000-7,500W
- Best for: Power tools, work sites, motors, resistive loads
- Electronics safe: Not recommended for sensitive electronics without a surge protector
Inverter Generators
- Output: Clean sine wave (processed through inverter)
- THD: Under 3% (safe for all electronics)
- Noise level: 50-65 dB (quieter than a conversation)
- Weight: 30-120 lbs (much lighter)
- Cost: $500-$3,000+ for 1,000-7,000W
- Best for: Camping, tailgating, home backup with electronics, RVs
- Electronics safe: Yes — laptops, phones, TVs, medical devices
The key advantage of inverter generators is clean power. Computers, TVs, LED lighting, and medical equipment like CPAP machines can malfunction or be damaged by the 'dirty' power from conventional generators. If you are powering anything with a microprocessor, an inverter generator is the safer choice.
The trade-off is cost and maximum output. Inverter generators top out around 7,000 watts for portable models, while conventional generators can reach 12,000+ watts. For job site use where you are only running power tools and motors, a conventional generator is more cost-effective.
Parallel Operation
Some newer inverter generators support parallel operation — connecting two identical units together to double the output. This lets you start with one unit for light loads and add the second when you need more power, rather than running one large generator at partial load all the time.
Generator Fuel Consumption and Runtime
Knowing how long your generator will run on a tank of fuel is just as important as knowing the right wattage. Nothing is worse than running out of fuel at 2 AM during a storm.
| Generator Size | Fuel Type | Tank Size (typical) | Runtime at 50% Load | Runtime at Full Load |
|---|---|---|---|---|
| 2,000W portable | Gasoline | 1 gal | 6-8 hours | 3-4 hours |
| 3,500W portable | Gasoline | 4 gal | 12-14 hours | 8-9 hours |
| 5,000W portable | Gasoline | 6.6 gal | 12-14 hours | 8-10 hours |
| 7,500W portable | Gasoline | 8 gal | 11-13 hours | 7-8 hours |
| 10,000W portable | Gasoline | 8 gal | 8-10 hours | 5-6 hours |
| 22 kW standby | Natural gas | Utility supply | Unlimited | Unlimited |
| 22 kW standby | Propane (500 gal) | 500 gal | ~200 hours | ~100 hours |
Quick Fuel Cost Estimate
A typical 5,000W gasoline generator burns about 0.5-0.75 gallons per hour at 50% load. At $3.50/gallon, that is $1.75-$2.63 per hour, or roughly $42-$63 per day of continuous operation. Propane is usually 10-20% cheaper per hour, and natural gas cheaper still.
Generators are most fuel-efficient at 50-75% of rated load. Running a generator at very low load (under 25%) wastes fuel and can cause carbon buildup in the engine — a condition called 'wet stacking' in diesel generators. Running at 100% load continuously shortens engine life. This is another reason the 20% safety margin in Step 5 is important: it keeps your generator in the efficient operating range.
Want to see how generator operating costs compare to your normal electric bill? Use our electricity bill calculator to estimate your typical grid electricity cost per day.
Generator Sizing Tips
Getting the wattage right is only part of the equation. Keep these practical considerations in mind before you buy or install.
Do Not Forget Starting Watts
This is the number-one mistake. A generator rated at 5,000 running watts might have 6,250 starting watts. If your peak surge exceeds the starting-watt rating, the generator will overload and shut down. Always compare your calculated peak demand against the generator's starting watt specification, not just the running watts.
Consider Altitude Derating
Gasoline and diesel generators lose roughly 3.5% of their rated power for every 1,000 feet above sea level. If you live at 5,000 feet elevation, a generator rated at 7,500W at sea level will only produce about 6,500W. Factor this in when sizing -- or choose a fuel-injected or inverter model that compensates better at altitude.
Fuel Type Matters
Gasoline provides the most watts per dollar but has a limited shelf life and is hard to get during widespread outages. Propane stores indefinitely and burns cleaner but produces about 10% fewer BTUs. Diesel generators are more fuel-efficient and durable, but the upfront cost is higher. Natural gas (for standby units) gives unlimited supply from your utility connection.
Transfer Switch Requirements
Never backfeed a generator into your panel through a dryer outlet or any other workaround -- this is illegal and extremely dangerous to utility workers. A transfer switch (manual or automatic) isolates your home from the grid and lets you safely connect the generator to your panel. The transfer switch must be rated for the generator's output and installed per local codes.
NEC Article 445 Requirements
The National Electrical Code covers generator installations in Article 445. Key requirements include proper grounding, overcurrent protection, disconnecting means, and nameplate markings. Standby systems also fall under NEC Articles 700, 701, and 702 depending on the type of system (emergency, legally required, or optional standby). Check our NEC code reference for details.
Size Your Generator in Seconds
Skip the manual calculations. Enter your appliances into our generator sizing calculator and get an instant recommendation with starting-watt analysis built in.
Generator Sizing CalculatorGetting the right generator size does not have to be complicated. List your appliances, total the running watts, account for the largest startup surge, and add a 20% buffer. When in doubt, round up to the next available generator size -- a little extra capacity costs far less than an undersized unit that cannot handle your loads.
For more help with electrical calculations, explore our breaker size calculator, electricity bill calculator, and load calculation tool.