Running Watts vs Starting Watts: What’s the Difference?

When sizing a portable generator, one of the most common points of confusion is the difference between running watts and starting watts.

Many generators appear powerful enough based on running wattage alone, yet still fail to start appliances like air conditioners, refrigerators, or power tools. In most cases, the issue is not total power — it is startup surge.

This guide explains what running watts and starting watts mean, why both matter, and how they are used to calculate the correct generator size.

What Are Running Watts?

Running watts (also called continuous watts) represent the amount of power an appliance requires to operate normally after it has started.

Examples of running loads include:

  • Refrigerator maintaining temperature
  • RV air conditioner after startup
  • Microwave during cooking
  • Battery charger operating in steady state
  • Lights, electronics, and small appliances

Running watts are relatively stable and predictable. Once all appliances are running, this is the power level your generator must sustain continuously.

What Are Starting Watts?

Starting watts (also called surge watts or startup watts) are the additional power required for a short moment when certain appliances start.

Appliances that commonly require starting surge include:

  • Air conditioners
  • Refrigerators and freezers
  • Pumps and compressors
  • Power tools with motors
  • Furnace blowers

This surge typically lasts from a fraction of a second to a few seconds, but it can be two to three times higher than the running wattage.

Why Do Some Appliances Need Extra Power at Startup?

Appliances with motors or compressors face mechanical resistance when starting from rest.
Overcoming that resistance requires extra electrical current, which translates to higher wattage.

Once the motor reaches operating speed, power demand drops back to normal running levels.

This is why a device may be rated at 1,500 running watts but briefly require 3,000 watts or more during startup.

Why Running Watts Alone Are Not Enough

Adding only running watts is one of the most common generator sizing mistakes.

A generator that matches your total running load exactly may:

  • Fail to start large appliances
  • Trip overload protection
  • Experience voltage drops
  • Shut down unexpectedly

These problems often occur even when total running watts seem well within the generator’s rating.

How Generator Size Calculations Use Running and Starting Watts

A practical generator sizing approach involves two components:

  1. Total running watts of all appliances that may operate at the same time
  2. The single largest starting surge among those appliances

Instead of stacking all starting surges together — which would dramatically oversize the generator — realistic calculations assume that most appliances do not start simultaneously.

This method produces a more accurate and usable generator size for real-world conditions.

You can see this approach in action using the generator wattage calculator, which automatically combines running loads with the largest startup surge.
→ Use the generator size calculator

Example: Why Starting Watts Matter

Consider a simple setup:

  • Refrigerator: 700 running watts / 2,200 starting watts
  • Microwave: 1,200 running watts
  • Lights and electronics: 300 running watts

Total running watts: 2,200 W
Largest starting surge: +1,500 W

A generator sized only for 2,200 W may struggle.
A generator sized for ~3,700 W will handle the load reliably.

This difference explains why many “2,000W generators” fail in real-world use.

Running Watts vs Starting Watts in RVs and Camping

In RV and camping scenarios, startup surge becomes even more critical due to:

  • Rooftop air conditioners
  • Battery chargers starting bulk charge
  • Limited headroom on smaller generators

This is why RV generator sizing often requires more capacity than expected, even when daily usage seems modest.

For RV-specific guidance, see:
→ Portable Generator for RV Use

How the Calculator Accounts for Both Values

The generator size calculator used on this site separates power into two parts:

  • Running watts from all selected appliances
  • Starting watts from the most demanding appliance

You can also add custom appliances by specifying:

  • Appliance name
  • Running wattage
  • Starting wattage

If no starting wattage is entered, the calculator assumes it equals running watts, which is appropriate for non-motorized loads.

This approach mirrors how generators are used in practice and avoids unnecessary oversizing.
→ Try the generator wattage calculator

  • Ignoring startup surge entirely
  • Adding all starting watts together unrealistically
  • Choosing a generator with no surge headroom
  • Assuming inverter generators eliminate startup surge (they do not)

Understanding startup behavior prevents most overload-related issues.

Conclusion

Running watts determine whether a generator can sustain your load.
Starting watts determine whether your appliances will start at all.

Accurate generator sizing requires accounting for both, using realistic assumptions about appliance behavior.

If you want a practical, appliance-based estimate rather than guesswork, calculate your running and starting watts together.

→ Calculate your generator size here