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Power & Generator Tool

Transfer Switch Load Calculator

Total running watts, peak surge watts, and generator headroom from 13 common circuits and your generator's rated wattage, with a load gauge against the 80 percent capacity safe limit.

Pair with the Generator Size Calculator for picking the right unit, the Appliance Wattage Reference for individual load lookups, and the Generator Fuel Runtime Calculator for runtime planning.

Calculate transfer switch load

Check the circuits you want on the transfer switch, then enter your generator's rated wattage.

Starter setup loaded Essentials backed up by a typical 7,500 W portable: fridge, well pump, lighting, device charging. Adjust to match your circuits and generator size.

Circuits to back up

Check every circuit you intend to run simultaneously during an outage.

W
Enter the "running watts" rating, not the surge/peak watts.
Live load updates as you select
Load Analysis

3,900W running load

Within generator capacity. Peak surge: 12,900W.

3,900W Running watts
12,900W Peak surge watts
48% Generator headroom
Selected circuits · running (solid) + surge (light)

Solid bars are running watts, what the generator must sustain continuously. Light extensions show surge/startup watts, the brief spike when motors kick on. The dashed red line is your generator's running capacity; keep the solid portion below it.

How to use this calculator

Check the circuits you want backed up by your generator and enter your generator's rated running wattage. The calculator totals both running watts (continuous load) and peak surge watts (motor startup spikes). The headroom percentage shows how much capacity buffer you have, aim for at least 20% to avoid generator strain during extended runs.

Tips for success

  • Size to running watts, not surge. The surge spike lasts only a fraction of a second but must not exceed the generator's peak rating.
  • Stagger the biggest surge loads. Don't run the electric water heater or central A/C simultaneously with a well pump, these are the three biggest surge loads in most homes.
  • 10-circuit MTS is the common spec. A 10-circuit manual transfer switch is the most common installation for portable generators. Whole-home automatic transfer switches cover every circuit.
  • Keep 20 to 25 percent headroom. Generators last longer and burn less fuel at moderate load; running at 90 to 100 percent for hours stresses the unit.
  • Label every breaker clearly before install. Post-install testing and emergency operation are much faster when every breaker has a name and circuit number.

Practical note

Surge wattage (also called starting watts) matters most for motor-driven loads: well pumps, refrigerators, sump pumps, and A/C compressors. These motors draw 3–5× their running wattage for the first few seconds on startup. If multiple motors start simultaneously, the combined surge can trip a generator's overload protection. Stagger startup timing where possible.

Good use cases

Essential when planning a transfer switch installation for a portable or standby generator. Use this to verify your generator is sized correctly before purchase, or to decide which circuits to prioritize when adding a 6- or 10-circuit manual transfer switch.

Manual vs. automatic transfer switch

  • Manual transfer switch (MTS): You flip breakers manually. Less expensive ($200–$600 installed) and simpler. Common for portable generators.
  • Automatic transfer switch (ATS): Senses outage and switches within seconds. Required for standby generators. More expensive ($500–$2,000+ installed).
  • Interlock kit: A lower-cost alternative to a full transfer switch for some panel configurations. Prevents back-feed to the grid. Not legal in all jurisdictions, check local code.

FAQ

What is back-feeding and why is it dangerous?

Back-feeding occurs when generator power flows backwards through your meter and onto the utility grid. This creates a life-threatening hazard for utility workers restoring power. A transfer switch physically disconnects your home from the grid before connecting the generator, it is a legal and safety requirement. Never plug a generator directly into a wall outlet ("suicide cord") without a proper transfer switch.

How many circuits can a typical transfer switch support?

Manual transfer switches typically come in 6, 8, or 10-circuit versions. An ATS for a portable generator may cover the whole panel. Whole-home standby ATS systems cover all circuits. For most households, 8–10 priority circuits (refrigerator, furnace, well pump, lights, charging) cover the essential needs.

Can I install a transfer switch myself?

In most jurisdictions, transfer switch work involves the main panel and requires a licensed electrician and permit. Some interlock kits are DIY-installable but still require a permit and inspection in most areas. Verify local requirements, improper installation is both dangerous and a potential insurance liability.

What's the difference between running watts and surge watts?

Running watts is the continuous power draw once a device is operating. Surge watts is the brief spike when a motor first starts, refrigerators, pumps, and A/C compressors all pull 3–5× their running wattage for a fraction of a second on startup. Your generator must handle the surge, but it only needs to sustain the running load continuously.