Generator Sizing Calculator

Powering Through Outages

Whether you are preparing for seasonal hurricanes, winter ice storms, or rolling blackouts, a backup generator is an essential piece of emergency equipment.

However, buying the wrong size generator is a costly mistake. If your generator is too small, the internal breaker will instantly trip the moment you try to turn on your refrigerator, leaving you in the dark. If you buy a massive, oversized generator, you will burn through your expensive gasoline reserves in a matter of days while barely utilizing the engine's capacity.

To buy the perfect generator, you must accurately calculate the total electrical load of the appliances you absolutely must run simultaneously during an emergency.

Running Watts vs. Starting (Surge) Watts

The secret to sizing a generator is understanding that motors lie about how much power they need. This is the difference between Running Watts and Starting Watts.

• Running Watts (Continuous): The amount of power an appliance needs to stay on continuously. A refrigerator might use 800 Running Watts. A lightbulb might use 60 Running Watts.
• Starting Watts (Surge): Any appliance with an electric motor or compressor (refrigerators, well pumps, air conditioners, furnace blowers) requires a massive, split-second surge of electricity to overcome inertia and start spinning. This surge can be 2 to 3 times higher than the Running Watts. That same 800-Watt refrigerator might require 2,400 Starting Watts just for the first 3 seconds it turns on.

Generators are always advertised by their Starting Watts (e.g., a "5000-Watt Generator" usually only provides 4000 Running Watts). You must ensure your generator can handle the running load of all your appliances plus the starting surge of your largest motor.

How to Calculate Generator Size

You don't need a generator large enough to start every motor in your house at the exact same millisecond. You only need a generator large enough to handle the total continuous running load, plus the surge of the single largest motor turning on.

The Formula

1. List the Running Watts of every appliance you plan to run simultaneously (e.g., fridge, lights, TV, furnace fan).
2. Add them all together to find your Total Running Watts.
3. Look at your list and identify the appliance with the Highest Starting Watts (usually a well pump or refrigerator).
4. Add that single highest Starting Watt number to your Total Running Watts.
5. Add a 10% to 20% safety margin so you are not running the generator engine at 100% maximum capacity for days on end (which destroys the engine).

Example Calculation

During an outage, you want to run:

• Refrigerator: 800 Running / 2400 Starting
• Furnace Fan: 600 Running / 1500 Starting
• Lights & TV: 400 Running / 0 Starting

1. Total Running Watts: 800 + 600 + 400 = 1,800 Watts
2. Highest Starting Surge: The refrigerator has the highest surge (2,400 W).
3. Add Surge to Running Total: 1,800 + 2,400 = 4,200 Watts
4. Add 20% Safety Margin: 4,200 × 1.20 = 5,040 Watts

You need to purchase a generator rated for at least 5,000 Starting Watts (which will comfortably handle your 1,800-watt continuous load while absorbing the heavy surge of the fridge cycling on).