1. Add running watts
Running total = sum of running wattsAdd the running watts of everything that will be on at the same time. Fridge 700 + sump pump 1,050 + furnace fan 800 = 2,550 W.
Running watts, starting watts and safety margin
Generator Size Calculator
This generator size calculator tells you how many watts you need. Pick the appliances you want to power, set quantities, and get the minimum running and starting watts, the right generator type and the amps it will supply.
Generator wattage calculator
Method
How to Calculate What Size Generator You Need
Motors in fridges, pumps and air conditioners need a short burst of extra power to start. Size your generator for that burst as well as the steady load, using these three steps.
2. Add the largest surge
Peak = running total + largest extraMotors start one at a time, so only the biggest extra starting load counts. The furnace fan adds 1,550 W, giving a 4,100 W peak.
3. Add a safety margin
Rating = running total × 1.2Running a generator flat out shortens its life. A 20% margin keeps it below 85% load: 2,550 × 1.2 = 3,060 W running.
Check the label. Exact wattage is printed on each appliance or in its manual. If only amps are listed, multiply amps by volts to get watts, or use the watts to amps calculator.
Reference
Generator Wattage Chart for Common Appliances
Typical running and starting watts used by the calculator. Starting watts only matter for appliances with motors or compressors.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Chest freezer | 500 | 1,500 |
| Sump pump (1/2 HP) | 1,050 | 2,150 |
| Well pump (1/2 HP) | 1,000 | 2,100 |
| Furnace fan (1/2 HP) | 800 | 2,350 |
| LED lights (10 bulbs) | 100 | No surge |
| Wi-Fi router and modem | 20 | No surge |
| Phone and laptop charging | 100 | No surge |
| CPAP machine | 60 | No surge |
| Garage door opener (1/2 HP) | 875 | 2,350 |
| Microwave | 1,000 | No surge |
| Coffee maker | 1,000 | No surge |
| Electric kettle | 1,500 | No surge |
| Toaster | 850 | No surge |
| Dishwasher | 1,500 | No surge |
| Electric range burner | 1,500 | No surge |
| Slow cooker | 250 | No surge |
| Electric pressure cooker | 1,000 | No surge |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Space heater | 1,500 | No surge |
| Box fan | 200 | No surge |
| TV (55-inch LED) | 150 | No surge |
| Gaming console | 200 | No surge |
| Electric water heater | 4,000 | No surge |
| Washing machine | 1,150 | 2,250 |
| Electric clothes dryer | 5,400 | 6,750 |
| Clothes iron | 1,200 | No surge |
| Hair dryer | 1,500 | No surge |
| Circular saw (7 1/4 in) | 1,400 | 2,300 |
| Air compressor (1 HP) | 1,600 | 4,500 |
| Electric drill (1/2 in) | 600 | 900 |
Quick guide
What Size Generator Do I Need?
Use this as a starting point, then confirm with the calculator using your own appliances.
| Use | Typical size | Generator type |
|---|---|---|
| Camping, phones, lights, a small fridge | 1,000 to 2,200 W | Small inverter |
| RV or camper with air conditioning | 3,000 to 4,000 W | Large inverter |
| Home essentials: fridge, sump pump, lights, furnace fan | 5,000 to 7,500 W | Portable |
| Most of a home without central air | 7,500 to 12,000 W | Large portable |
| Whole home including central air | 15,000 to 26,000 W | Home standby |
Why ours
Why This Generator Size Calculator Is Better
Many generator sizing tools add every starting watt together and push you toward a bigger, pricier machine than you need.
Correct surge maths
Only the single largest starting surge is added, which is how electricians size generators. You buy the right size, not the biggest.
Tap to build your load
Over 30 appliances in clear groups with search and quantity steppers. No long forms to fill in.
Custom appliances
Add anything with its own running and starting watts, from aquarium heaters to welders.
Adjustable safety margin
Choose 10%, 20% or 25% headroom and see running and starting ratings update instantly.
Generator type and amps
See whether an inverter, portable or standby generator fits, plus the amps it delivers at 120 V and 240 V.
Shareable load list
Copy your list for a dealer or electrician, or share a link that reopens every selection.
Guide
How to Use the Generator Size Calculator
Think about what you would need during a power cut or on your next trip, then follow these steps.
- Pick your appliances
Tap each appliance you want to run at the same time. Use the search box to find items quickly.
- Set quantities
Use the plus and minus buttons for items you have more than one of, such as freezers or lights.
- Add anything missing
Enter a custom appliance with its running and starting watts from the label.
- Choose a safety margin
20% suits most homes. Use 25% if you plan to add appliances later.
- Shop with your numbers
Look for a generator that meets both the running and the starting watts shown, then copy or share your list.
Safety
Generator Safety Essentials
The right size is only half the job. Running a generator safely protects your family, your home and utility workers.
- Never run a generator indoors or in a garage, even with the door open. Keep it at least 20 feet from doors, windows and vents.
- Install carbon monoxide alarms on every floor and test them before storm season.
- Never plug a generator into a wall outlet. Backfeeding can electrocute utility workers. Use a transfer switch or interlock fitted by a licensed electrician.
- Use heavy-duty extension cords rated for the load, and keep connections dry. Check cable size with the voltage drop calculator.
- Let it cool before refuelling and store fuel in approved containers away from the house.
Benefits
Benefits of Sizing Your Generator Correctly
A correctly sized generator costs less to buy, uses less fuel and keeps your appliances safe.
Avoid overloads and trips
An undersized generator stalls or trips its breaker when a fridge or pump starts, and can damage sensitive electronics.
Do not overspend
Every extra 1,000 watts adds cost, weight and noise. Buying for your real load keeps the price and fuel bill down.
Better fuel economy
Generators are most efficient at 50% to 75% load. A good margin keeps you in that range for longer runtimes per tank.
Plan for the long term
Compare the running cost of backup power with solar using the solar rooftop calculator and electricity cost calculator.
FAQ
Generator Size FAQ
What size generator do I need to run a house?
To run home essentials such as a fridge, sump pump, furnace fan, lights and a microwave, most homes need a 5,000 to 7,500 watt generator. Running most of the house without central air takes about 7,500 to 12,000 watts, and a whole home with central air usually needs a 15,000 to 26,000 watt standby generator.
How do I calculate what size generator I need?
Add the running watts of everything you want to use at the same time. Then add the extra starting watts of the single appliance with the biggest surge. Finally add a 10% to 25% safety margin to the running total and choose a generator that meets both numbers.
What is the difference between running watts and starting watts?
Running watts are the steady power an appliance uses while it operates. Starting watts, also called surge watts, are the short burst of extra power motors and compressors need for a few seconds when they switch on. Resistive appliances like kettles and heaters have no starting surge.
Will a 5,000 watt generator run a refrigerator?
Yes. A typical refrigerator uses about 700 running watts and needs around 2,200 watts to start, so a 5,000 watt generator can run it along with lights, a sump pump and other essentials, as long as the total stays within the generator rating.
Can a portable generator run central air conditioning?
Usually only a large one. A typical 3 to 4 ton central air conditioner can need roughly 3,500 to 5,000 running watts and much more to start, so most homes need a generator of 12,000 watts or more. A soft start kit can greatly reduce the starting surge.
What size generator do I need for camping or an RV?
A 1,000 to 2,200 watt inverter generator handles phones, lights, a laptop and a small fridge. An RV air conditioner usually needs a 3,000 to 4,000 watt generator, or a smaller one if the air conditioner has a soft start kit.
How many amps does a generator produce?
Divide the generator watts by the voltage. A 7,500 watt generator delivers about 62.5 amps at 120 volts or 31 amps at 240 volts. Each outlet on the generator has its own amp limit, so check the control panel too.
Is an inverter generator better than a conventional generator?
Inverter generators produce very clean power that is safer for computers and other electronics, run more quietly and use less fuel at light loads. Conventional portable generators cost less per watt and are better for large loads such as well pumps and power tools.
Can I plug a generator into a wall outlet?
No. Plugging a generator into a wall outlet, known as backfeeding, is illegal in many areas and can electrocute utility workers or start a fire. Use heavy-duty extension cords or have a licensed electrician install a transfer switch or interlock kit.
Be Ready Before the Next Power Cut
Use the generator size calculator to buy the right generator once, then check amps, cable sizes and cooling loads with our related tools.
Size my generatorWattages are typical values. Check your appliance labels and the generator manual, and use a licensed electrician for home connections.