Generator Wattage Calculator

What This Calculator Does

This calculator estimates the minimum generator wattage required to power a set of electrical devices. It accounts for both running (continuous) wattage and starting (surge) wattage, which is the extra power motors and compressors need briefly when they turn on.

By entering the running and starting wattage for each device you want to run simultaneously, the calculator sums the totals and recommends a generator size that can handle the highest combined load.

How to Use the Calculator

1. Enter the running wattage for each appliance or tool you plan to power. This is the wattage the device consumes during normal operation.
2. Enter the starting wattage for each device. For items without motors (lights, heaters, electronics), the starting wattage is typically the same as the running wattage. For motor-driven devices (refrigerators, pumps, power tools), the starting wattage can be 2–3 times higher.
3. Add as many devices as needed using the add button.
4. The calculator automatically sums the total running wattage and the highest starting wattage, then displays the recommended generator size.

Understanding Running vs. Starting Wattage

Generators are rated by their continuous output and their surge capacity. A generator must be able to handle both:

• Running wattage — The total wattage of all devices operating at the same time. This must not exceed the generator's continuous rated output.
• Starting wattage — The extra power required when a motor starts. This surge lasts only a few seconds, but the generator must be capable of supplying it. The calculator compares the sum of all running wattage plus the single largest starting wattage surge against the generator's surge capacity.

The calculator uses the larger of these two values to determine the minimum generator size.

Common Mistakes When Sizing a Generator

• Ignoring starting wattage — A refrigerator may draw 700 watts running but need 2,100 watts to start. If you only account for running wattage, the generator may stall when the compressor kicks on.
• Adding starting wattage for every device — Not all devices start at the same instant. The calculator correctly adds the single largest starting surge to the total running wattage, which is the standard engineering approach.
• Forgetting devices that run intermittently — A well pump or sump pump may only run occasionally, but when it starts, it adds a significant surge. Include all devices that could run simultaneously.
• Using peak wattage from device labels — Some appliances list peak or maximum wattage, not continuous running wattage. Use the rated running wattage for the running value and the peak or starting value separately.

Practical Use Cases

• Home backup power — Determine the generator size needed to keep essential appliances running during an outage: refrigerator, lights, furnace fan, sump pump, and a few outlets for charging devices.
• Construction or job site power — Calculate wattage for running multiple power tools simultaneously: circular saw, compressor, lights, and a radio.
• RV or camping — Size a portable generator for an RV air conditioner, microwave, refrigerator, and battery charger.
• Event or outdoor power — Plan generator capacity for sound systems, lighting, food warmers, and fans at an outdoor event.

Limitations

• This calculator provides an estimate. Always select a generator with a continuous rating at least 10–20% higher than the calculated total to provide a safety margin and account for voltage drop under load.
• It assumes all devices run simultaneously. If you plan to stagger usage, you may need a smaller generator, but the calculator errs on the side of safety.
• It does not account for power factor or harmonic distortion, which may matter for sensitive electronics. For critical loads, consider an inverter generator or consult an electrician.
• Motor starting requirements vary by device age, condition, and manufacturer. The values you enter should be based on the device's nameplate or manufacturer specifications.