Check off the appliances you plan to run, adjust quantities, or add custom equipment manually. Your recommended generator size updates the moment you press Calculate.

Generator Size Comparison Table

What Is Generator Wattage?

Generator wattage is the amount of electrical power a generator can produce and deliver to the devices connected to it. Every generator has two published numbers: a running (rated) wattage it can sustain continuously, and a higher starting (surge) wattage it can supply for a few seconds when motor-driven appliances first switch on. Sizing a generator correctly means matching both numbers to the combined demand of everything you intend to run at once, with enough headroom that the unit is never asked to operate at its absolute limit for long periods. Undersizing causes tripped breakers, stalled engines, and premature wear; oversizing wastes money and burns fuel inefficiently, since generators run most efficiently between roughly 50% and 80% of rated load.

This calculator adds up the running watts of everything you select, works out the single largest starting-watt surge your system will experience, applies an industry-standard 20% safety margin, and then matches the result against the generator categories used by manufacturers such as Honda, Generac, Champion, and DuroMax.

Running Watts vs Starting Watts

Running watts (also called rated watts) are the continuous power a device draws once it is up and operating — a refrigerator compressor humming along, a TV displaying a picture, a well pump moving water. Starting watts (also called surge or LRA-driven watts) are the short burst of extra power, sometimes 2–3 times higher, that a motor or compressor needs for a fraction of a second to overcome inertia and begin spinning.

Devices with a simple resistive load — lightbulbs, toasters, space heaters — have no meaningful surge; their starting and running watts are the same. Devices with a motor or compressor — refrigerators, air conditioners, well pumps, table saws, air compressors — can spike dramatically for a moment. A generator only needs to survive the single highest surge in your lineup at any one instant, not the sum of every appliance's surge, because appliances rarely all start at exactly the same millisecond. That is why this calculator computes peak surge as your total running load plus the highest individual starting spike, rather than adding every starting watt figure together.

How to Choose the Right Generator

Start by listing every appliance or tool you would realistically run during an outage or on a job site, including anything that cycles on automatically, like a refrigerator or sump pump. Add up the running watts, identify the single highest starting-watt appliance, and calculate the peak surge your generator must survive. Add a safety margin of at least 20% so the unit is not constantly working at its ceiling, then compare that number against the wattage classes above to land on a category. From there, consider fuel type availability in your area, how portable you need the unit to be, noise restrictions in your neighborhood, and whether you want automatic transfer switch capability for whole-house coverage.

Generator Safety Tips

• Always run a generator outdoors, at least 20 feet from windows, doors, and vents, to prevent carbon monoxide from entering the home.
• Never plug a generator directly into a wall outlet; use a properly installed transfer switch or heavy-duty extension cords rated for the load.
• Keep the generator dry and on a level, stable surface, and let the engine cool before refueling.
• Install carbon monoxide detectors on every level of the home when using a backup generator.
• Match extension cord gauge to the amperage draw; undersized cords overheat and are a fire hazard.
• Do not overload the unit beyond its rated running watts, even briefly, outside of expected starting surges.

Portable vs Inverter Generator

Conventional portable generators use a simple engine-driven alternator; they are generally less expensive per watt and well suited to job sites and higher-wattage backup needs, but they produce less stable power and are louder. Inverter generators condition their output electronically, producing clean, stable sine-wave power safe for sensitive electronics like laptops, CPAP machines, and smart TVs, and they typically run quieter and more fuel-efficiently at partial load, though at a higher price per watt. Many households pair a mid-size inverter generator for everyday sensitive electronics with a larger conventional or standby unit for whole-home coverage.

Generator Maintenance Tips

• Change the oil after the first 20–30 hours of use, then every 50–100 hours thereafter.
• Run the generator under load for 20–30 minutes every month to keep the engine and battery in working order.
• Use stabilized fuel or drain the tank if the unit will sit unused for more than 30 days.
• Replace air filters and spark plugs on the manufacturer's recommended schedule.
• Store the unit in a dry, ventilated space and keep the exterior free of dust and debris.

Fuel Consumption Explained

Fuel consumption depends on generator size, load percentage, and fuel type. As a general rule, a gasoline generator burns roughly 0.4–0.6 gallons per hour for every 5,000 watts of load at around 50% capacity; propane and diesel units tend to run more efficiently per hour at higher loads but require larger storage tanks. Running a generator at a lower percentage of its rated capacity generally improves fuel economy and extends engine life, which is another reason to size the unit close to your realistic peak demand rather than buying far more capacity than you need.

Whole House Generator Guide

A true whole-house generator, typically in the 9,000–20,000+ watt range and permanently installed with an automatic transfer switch, is sized to cover central air conditioning, an electric range, a clothes dryer, and every branch circuit in the home simultaneously. Sizing one correctly usually starts from the home's electrical panel and square footage, then layers in the running and starting watts of major appliances the same way this calculator does for a portable unit. Standby whole-house systems run on natural gas or propane and start automatically within seconds of a utility outage, offering the highest convenience at the highest installation cost.

Common Generator Mistakes

• Sizing only for running watts and ignoring the starting surge of motors and compressors.
• Skipping the safety margin and buying a generator that sits at 100% load.
• Using undersized extension cords that overheat under load.
• Running the generator indoors or in an attached garage, risking carbon monoxide poisoning.
• Forgetting fuel storage and runtime needs during multi-day outages.
• Neglecting routine maintenance, leading to failure exactly when the generator is needed most.

Emergency Backup Planning

Effective backup planning starts with identifying your must-run circuits — refrigeration, medical equipment, sump or well pumps, and basic lighting — and sizing a generator around those essentials rather than the whole house. Keep a documented fuel supply plan, test the generator seasonally, and store a printed copy of your wattage calculation and startup steps with the unit so any household member can operate it safely during a stressful outage.

Example Calculations

Frequently Asked Questions

Related Calculators