How to Use the Generator Size Calculator (Running & Starting Watts Explained)

Choosing the right generator size is one of the most common—and costly—mistakes people make when buying a portable generator. Many decisions are based on guesswork, rough estimates, or generic wattage charts that do not reflect real-world usage.

The Generator Size Calculator is designed to remove that uncertainty.
It helps you estimate both running watts and starting watts, so you can choose a generator that starts reliably, runs efficiently, and stays quieter under normal operating conditions.

This guide explains how the calculator works, what the numbers mean, and how to use it for different scenarios such as camping, RV travel, and home backup power.

Why Generator Sizing Is More Than Just Total Watts

Most people focus only on total wattage. In practice, generators fail or struggle for very different reasons:

  • Startup surge exceeds generator capacity
  • Too many devices start at the same time
  • Generator runs constantly near maximum load
  • Noise increases due to high load percentage

Accurate sizing requires understanding how power is used, not just how much power is listed on a label.

Running Watts vs Starting Watts

Before using the calculator, it is important to understand the two types of power demand it models.

Running Watts

Running watts represent the continuous power required to keep an appliance operating once it is already running.

Examples include:

  • Lights
  • Electronics
  • Refrigerators after startup
  • Fans and small appliances

These loads add together over time and determine how hard the generator works continuously.

Starting Watts

Starting watts represent the temporary surge of power required when certain appliances start.
This surge usually lasts only a fraction of a second but can be several times higher than running watts.

Appliances that commonly have startup surges include:

  • Air conditioners
  • Refrigerators and freezers
  • Pumps
  • Power tools

Many generators can handle running loads but fail to start these appliances reliably without enough starting capacity.

How the Generator Size Calculator Works

The generator size calculator is designed to estimate power requirements based on real appliance loads, not generic wattage rules or guesswork.
It separates generator demand into two distinct components: running watts and starting watts, then combines them into a conservative, real-world recommendation.

1. Two Types of Power Demand

The calculator accounts for two different types of electrical load:

  • Running watts
    The continuous power required to keep appliances operating.

  • Starting watts
    The brief surge of power required when certain appliances start, typically those with motors or compressors.

Many generators fail not because they cannot handle running load, but because they cannot handle startup surge. This calculator explicitly models both.

2. Selecting Appliances From the List

The calculator includes a list of common appliances used for home backup, camping, and RV applications.
Each appliance in the list has predefined values for:

  • Running watts
  • Starting watts (when applicable)

When you select appliances from the list:

  • All running watts are added together to determine total continuous load
  • Only the single highest starting watt value is applied, representing the largest startup surge likely to occur

This approach reflects how generators behave in real use: while many devices may run at the same time, large startup surges usually occur one at a time.

3. Adding Custom Appliances

If your appliance is not listed, the calculator allows you to add a custom appliance with:

  • Appliance name
  • Running watts
  • Starting watts

This ensures the calculation matches your actual setup, not an average or assumed one.

Default Starting Wattage Rule

If no starting wattage is entered for a custom appliance, the calculator assumes:

Starting watts = running watts

This default is intentional and conservative:

  • Many electronic devices do not have a meaningful startup surge
  • Some appliances have unknown or undocumented starting wattage
  • Assuming equal starting and running watts avoids underestimating generator requirements

You can override this value at any time if your appliance has a known startup surge.

4. How the Final Generator Size Is Calculated

The calculator determines generator size using the following steps:

  1. Total running watts
    All selected and custom appliances are combined to calculate continuous load.

  2. Peak starting surge
    The highest single starting watt value is identified and added to the calculation.

  3. Safety margin for real-world conditions
    A buffer is applied to account for load variation, efficiency loss, and real-world operating conditions.

The result is a conservative generator size recommendation designed to:

  • Start appliances reliably
  • Avoid overload conditions
  • Operate more efficiently and quietly
  • Reduce long-term wear on the generator

Using the Calculator for Different Scenarios

Camping and Outdoor Use

Camping setups usually involve:

  • Light electronics
  • Portable refrigerators
  • Cooking appliances

Noise sensitivity is often higher than power demand.

For camping-specific guidance, see:
Portable Generator for Camping: What Size and Noise Level Do You Really Need?

RV and Travel Use

RV systems introduce:

  • Air conditioner startup surge
  • Battery charging loads
  • Longer runtimes

Accurate starting watt calculations are especially important.

For RV-specific details, see:
Portable Generator for RV Use: Power Requirements Explained

Home Backup Power

Home backup scenarios often include:

  • Multiple simultaneous loads
  • Long operating periods
  • Nighttime noise considerations

Proper sizing helps avoid overloads and excessive noise.

Common Generator Sizing Mistakes

  • Ignoring startup surge
  • Oversizing based on maximum wattage alone
  • Assuming propane delivers full rated power
  • Guessing instead of calculating real loads

The calculator is designed specifically to avoid these errors.

Start With Real Numbers, Not Estimates

Generator sizing does not need to be complicated—but it does need to be accurate.

By accounting for both running watts and starting watts, and by allowing full customization of appliance loads, the Generator Size Calculator provides a practical, real-world approach to choosing the right generator size.

→ Use the Generator Size Calculator