Introduction
Picking the right generator size is one of the most important steps in backup power planning.2 A generator that is too small will overload and shut down when you need it most. One that is too big wastes fuel and money. Our Generator Sizing Calculator adds up the running watts and starting watts of every appliance you want to power, then applies a safety factor for extra headroom.
Pick the appliances you want to power from the lists of residential, HVAC, and tool loads, set how many of each, and add custom items for anything not listed. The results show your total running watts, total starting watts, and the recommended generator size in watts, kilowatts, and kVA, with a suggested unit size, a breakdown table, and a chart.
A built-in Power Unit Converter is also included so you can quickly convert between watts, kilowatts, kVA, horsepower, BTU/hr, and amps at 120V or 240V. This calculator gives you the numbers you need to buy with confidence, whether you are preparing for storm season, setting up a job site, or sizing a standby generator for your whole house.
How to Use Our Generator Sizing Calculator
Choose the appliances you want to power to find out what size generator you need. The calculator gives you running watts, starting watts, and a recommended generator size in watts, kilowatts, and kVA.
Generator Sizing and Power Unit Converter tabs: The Generator Sizing tab adds up your loads and recommends a size. The Power Unit Converter tab converts between power units.
Safety / Surge Factor: Pick how much extra power headroom you want. Standard adds 25% and works for most homes. Moderate adds 50% for homes with several motors. High adds 75% and is best for commercial use. The factor is applied to your total starting watts.
Appliance Quantities: Set the quantity for each appliance you plan to run. Appliances are grouped into Residential, HVAC & Climate Control, and Tools & Equipment sections. Each row shows the running watts and starting watts so you can see the load each item adds.
Custom Items: Click "Add Custom Item" to enter any appliance not already listed. Type the item name, its running watts, its starting watts, and the quantity you need.
Power Factor: Enter the power factor for your loads. The default is 0.8, which is typical for most homes. This value is used to convert kilowatts to kilovolt-amps (kVA), which many generators are rated in.3
Power Unit Converter Tab: Use this tool to convert between watts, kilowatts, kVA, horsepower, BTU/hr, amps at 120V, and amps at 240V. Enter a value, select the unit you are converting from, set the power factor for kVA conversions, and click "Convert" to see all equivalent values at once.
Generator Sizing: How to Pick the Right Generator for Your Needs
A generator is a machine that makes electricity when your normal power goes out. Picking the right size generator is one of the most important decisions you can make for backup power. If your generator is too small, it won't be able to run everything you need. If it's too big, you waste money on a machine you'll never fully use. Generator sizing means figuring out exactly how many watts of power your home or building needs so you can choose a generator that matches.
Running Watts vs. Starting Watts
Every electrical device uses a certain amount of power to keep running. This is called running watts (also known as rated watts or continuous watts). But many appliances, especially ones with motors like refrigerators, air conditioners, sump pumps, and well pumps, need a big burst of extra power for the first few seconds when they turn on. This burst is called starting watts (or surge watts). Motors typically need about four times as much power to start as they do to run.1 When you size a generator, you must account for both numbers. Your generator needs to handle the highest possible demand, which happens when one or more motor-driven appliances kick on at the same time.2
How to Calculate Your Total Power Needs
To find the right generator size, follow these steps:
- List every appliance and device you want to power during an outage.
- Write down the running watts and starting watts for each item. You can usually find this on the nameplate label or in the owner's manual.1
- Add up all the running watts to get your total continuous load.
- Add up the starting watts of everything that could start at the same time. A generator that starts on its own must be able to start every connected circuit at once.2 This calculator adds the starting watts of every item you select.
- Apply a safety factor, typically 25% to 50% extra, to give the generator breathing room and prevent overloading.
Understanding the Safety Factor
A safety factor is extra wattage added on top of your calculated need. It protects the generator from being pushed to its maximum capacity all the time, which causes wear and shortens its life. A 25% safety margin works well for most homes. If you have many motor-driven appliances or run a small business, a 50% margin is safer. For commercial buildings or job sites with heavy equipment, 75% provides the most protection.
Key Power Units Explained
Generator specifications use several different units, and it helps to know what they mean:
- Watts (W): The basic unit of electrical power. Most household appliances are rated in watts.
- Kilowatts (kW): Equal to 1,000 watts.3 Generators are often sold by their kilowatt rating.
- Kilovolt-Amps (kVA): Apparent power. To convert kVA to kW, multiply by the power factor.3 For homes this is usually taken as 0.8.
- Amps (A): The amount of electrical current. You need to know the voltage (120V or 240V) to convert between amps and watts.3
- Horsepower (HP): Often used for motors and pumps. One horsepower equals about 746 watts.3
Power Factor
Power factor is a number between 0 and 1 that shows what part of the apparent power is real power.3 A power factor of 1.0 means all the current does useful work.3 A low power factor does not mean power is lost; it means the wires carry extra current.4 Most homes have a power factor around 0.8. When a generator is rated in kVA instead of kW, you need the power factor to figure out how many actual watts it can deliver.3 The formula is simple: kW = kVA × power factor.3
Common Generator Sizes and What They Power
- 3,000 to 5,000W: Good for basic needs like lights, refrigerator, phone chargers, and a few small appliances.
- 7,500 to 10,000W: Can handle most essentials in a small to average home, including a window AC unit or sump pump.
- 12,000 to 17,500W: Powers a medium home with central air conditioning, kitchen appliances, and a well pump.
- 20,000 to 25,000W: Suitable for large homes or small commercial spaces with heavy electrical loads.
- 30,000W and above: Whole-house standby generators or commercial applications.
Portable vs. Standby Generators
Portable generators typically range from 1,000 to 17,500 watts. They run on gasoline and must be placed outdoors to avoid carbon monoxide poisoning.1 They are best for temporary or emergency use. Standby generators are permanently installed outside your home and connect to your electrical panel through a transfer switch.2 They often run on natural gas or propane and can start automatically when the power goes out.2 Standby units range from about 7,500 watts to over 50,000 watts and are the best choice for whole-house backup power.
Tips for Choosing the Right Generator
- Never run a generator at more than 80% of its rated capacity for extended periods. This prevents overheating and prolongs the machine's life.
- Decide which appliances are truly essential during an outage. You may not need to power everything at once.
- If you have electric heating or a large central AC system, these will be your biggest power draws by far.
- Consider your local climate. Homes in hot regions need to plan for air conditioning, while cold-climate homes should plan for heating blowers or electric heaters. Proper insulation can reduce your heating and cooling loads, which in turn lowers the generator size you need.
- Always check the appliance nameplate for exact wattage rather than guessing.1 Actual power use can differ from general estimates.