Grid Surge Resource

Interactive Sizing & Electrical Reference

The Difference Between Running and Starting Watts

When powering appliances, especially from a generator or a solar inverter, you need to be aware of two different power ratings: running watts and starting watts.

  • Running Watts (Continuous Watts): The steady amount of power an appliance requires to operate continuously.
  • Starting Watts (Surge Watts): The brief, high burst of power required to start electric motors or compressors. This surge lasts only a few seconds but can be 2 to 4 times higher than the running watts.

Why Do Motors Surge? (Inrush Current)

Devices with inductive loads, such as electric motors, compressors, and pumps, require extra energy to overcome inertia and establish magnetic fields when they first turn on. This phenomenon is known as inrush current.

For example, a refrigerator compressor might run at a steady 200 watts. However, to get the heavy internal pump moving against the pressure of the refrigerant gas, it might draw 1,000 watts for a split second upon startup.

Generator Sizing Warning: The most common mistake when buying a generator is sizing it based only on running watts. If the generator cannot handle the surge wattage of your largest motor, it will overload and trip the breaker.

Resistive vs Inductive Loads

Not all appliances have surge watts. It depends on how they use electricity:

Load Type Characteristics Examples Surge Multiplier
ResistiveUses heat or light elements. No moving parts.Incandescent bulbs, space heaters, toasters.1x (No surge)
Reactive/ElectronicUses switch-mode power supplies.Laptops, TVs, LED lights.1x - 1.2x
InductiveUses electric motors or compressors.Refrigerators, AC units, well pumps, saws.2x - 4x

How to Calculate Total Power Needs

When sizing a power source, calculate the total running watts of everything you want to run simultaneously, then add the single highest starting wattage among those appliances.

Example Scenario

  • Refrigerator: 700 Running W, 2,200 Starting W
  • Lights (LED): 100 Running W, 0 Starting W
  • TV: 200 Running W, 0 Starting W
  • Microwave: 1,000 Running W, 1,200 Starting W

Total Running Watts: 700 + 100 + 200 + 1000 = 2,000 Watts

Highest Starting Wattage: The refrigerator has the highest starting requirement (2,200W). However, since we already counted its 700W running power, the additional surge is 1,500W.

Required Generator Size: 2,000 Total Running + 1,500 Additional Surge = 3,500 Watts.

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