Generator Sizing Calculator

Pre-Computed Generator Sizes for Common Scenarios

The table below shows recommended generator sizes for five common use cases. These calculations assume all listed appliances running simultaneously with the standard 25% safety margin. Click any scenario name to see the full appliance breakdown and detailed recommendations for that use case.

How Generator Sizing Works

Determining what size generator you need is a straightforward process once you understand the relationship between running watts, starting watts, and the way electrical loads interact. Every appliance that uses electricity has a running wattage, which is the continuous power it draws during normal operation. Motor-driven appliances like refrigerators, well pumps, air conditioners, and power tools also have a starting wattage, which is a brief surge of extra power needed to get the motor spinning. This starting surge typically lasts only a fraction of a second to a few seconds, but your generator must be capable of supplying it or the appliance will not start properly.

The key insight in generator sizing is that you do not simply add up the starting watts of every appliance. In practice, only one appliance starts at a time while the others are already running at their normal wattage. So the peak demand on your generator equals the sum of all running watts plus the largest single starting surge gap. The starting surge gap is the difference between an appliance's starting watts and its running watts. For example, a well pump that runs at 1,000 watts but starts at 2,100 watts has a starting surge gap of 1,100 watts. If your total running load is 2,500 watts and the well pump has the largest starting gap, your peak demand is 2,500 + 1,100 = 3,600 watts.

After calculating the peak demand, we apply a 25% safety margin. This margin accounts for several real-world factors: manufacturer wattage specifications are approximate, generators lose capacity at high altitudes and in hot weather, running a generator at maximum capacity continuously shortens its lifespan and increases fuel consumption, and you may need to power additional appliances you did not initially plan for. The safety margin ensures your generator operates comfortably within its rated capacity under a range of conditions.

The final step is rounding up to the nearest kilowatt to match standard generator sizes available on the market. Generators are typically sold in sizes like 2 kW, 3 kW, 5 kW, 7.5 kW, 10 kW, 12 kW, and so on. If your calculation shows you need 6,200 watts, a 7,000-watt (7 kW) generator is the appropriate choice. Going one size larger than your calculation is always safer than trying to squeeze into the minimum size.

Running Watts vs Starting Watts Explained

Understanding the difference between running watts and starting watts is the most critical concept in generator sizing. Running watts, also called rated watts or continuous watts, represent the steady-state power an appliance needs to operate. A 1,000-watt microwave draws approximately 1,000 watts of continuous power while cooking. A 150-watt refrigerator draws 150 watts whenever its compressor is running.

Starting watts, also called surge watts or peak watts, represent the extra burst of power needed for a brief moment when a motor first turns on. Electric motors require significantly more current to overcome inertia and begin rotating than they need to maintain rotation once started. This starting surge typically lasts less than one second for most household appliances, but it can be two to three times the running wattage for heavily loaded motors like well pumps and air compressor motors. A well pump rated at 1,000 running watts may need 2,100 starting watts for just a fraction of a second as the motor overcomes the weight of the water column in the well pipe.

Resistive loads like heaters, toasters, incandescent lights, and coffee makers have no starting surge because they contain no motor. Their starting watts equal their running watts, or can be considered zero additional surge. In our calculator, resistive loads show a starting wattage of zero, meaning they add no surge above their running wattage. This distinction is important because a load dominated by resistive appliances needs a smaller generator than a load dominated by motor-driven appliances, even if the total running watts are the same.

When multiple motor-driven appliances are on the same generator, load management becomes important. Avoid starting two large motor loads simultaneously. For example, do not turn on the well pump and the air compressor at the same time. If you stagger the startups, the generator only needs to handle one starting surge at a time while the other appliances run at their steady-state wattage. This is why our calculator uses the largest single starting gap, not the sum of all starting gaps.

Types of Generators

Portable Conventional Generators are the most common and affordable option, typically ranging from 3,000 to 12,000 watts. They run on gasoline and produce AC power directly from the alternator. Portable generators are suitable for emergency home backup, job sites, and outdoor events. Their main advantages are lower cost and higher power output per dollar. Disadvantages include louder operation, heavier weight, and less clean power output compared to inverter models. They should be used with a surge protector for sensitive electronics like computers and televisions. Portable generators must always be operated outdoors, at least 20 feet from the home, with the exhaust directed away from windows and doors to prevent carbon monoxide poisoning.

Inverter Generators use advanced electronics to produce a clean sine wave power output that is safe for all sensitive electronics without additional surge protection. They range from 1,000 to 7,000 watts and are significantly quieter than conventional portable generators, often running at 50 to 60 decibels compared to 70 to 80 decibels for conventional models. Inverter generators automatically adjust engine speed to match the load, which improves fuel efficiency at partial loads. They are ideal for camping, tailgating, home office backup, and situations where noise is a concern. Many models can be paralleled (two units connected together) to double the available power. The main disadvantage is higher cost per watt compared to conventional portables.

Standby Generators are permanently installed outside your home and connected to your electrical panel through an automatic transfer switch. When grid power fails, the transfer switch detects the outage and starts the generator automatically within seconds. Standby generators run on natural gas or propane (LP), eliminating the need to store and manage gasoline. They range from 7,000 watts for essential circuit coverage to over 50,000 watts for entire-home coverage including central air conditioning, electric cooking, and EV charging. The automatic operation means your home never loses power, even if you are away. Standby generators require professional installation, a concrete pad, and a transfer switch, making the total installed cost significantly higher than portable options. However, they also increase home resale value and provide unmatched convenience and reliability.

Generator Safety Tips

Generator safety is critical and should never be compromised. Carbon monoxide (CO) poisoning from generator exhaust is the leading cause of death during power outages. Always operate generators outdoors, at least 20 feet from the home, with the exhaust directed away from windows, doors, and vents. Never run a generator in a garage, basement, crawl space, or enclosed area, even with the door open. Install battery-operated CO detectors on every level of your home and near sleeping areas.

Never connect a portable generator directly to your home's wiring (known as backfeeding). This creates a deadly hazard for utility workers repairing power lines and can damage your generator and your home's electrical system. Instead, use individual extension cords to connect appliances directly, or hire a licensed electrician to install a transfer switch that safely isolates your home's circuits from the utility grid when the generator is running.

Allow the generator to cool before refueling. Gasoline spilled on a hot engine or exhaust can ignite instantly. Store fuel in approved containers away from the generator and any ignition sources. Turn off all connected loads before starting or stopping the generator. Keep children and pets away from the operating generator at all times. Follow the manufacturer's maintenance schedule for oil changes, air filter replacement, and spark plug maintenance to ensure reliable operation when you need it most.

Frequently Asked Questions

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Explore detailed wattage information and generator sizing guidance for specific appliance categories and use-case scenarios. Each page includes accurate running and starting wattage data, generator size recommendations, and tips for efficient load management.