# Generator Sizing Calculator

> Select your appliances and calculate the total starting and running watts to find the right generator size

**Category:** Utility
**Keywords:** generator, sizing, wattage, watts, appliance, power, starting watts, running watts, portable generator, backup power, emergency, electrical
**URL:** https://complete.tools/generator-sizing-calculator

## How it calculates

The calculator uses a standard electrical engineering approach to generator sizing:

**Running Watts** are the continuous power an appliance needs to operate. The calculator sums the running watts of every selected appliance multiplied by quantity.

**Starting Watts** (also called surge watts) account for the extra power that motor-driven appliances need during startup. A refrigerator compressor, for example, may draw 1,200 watts for a brief moment even though it only runs at 150 watts. The formula uses the single largest starting surge because appliances rarely all start at the exact same instant.

**Formula:**
```
Total Running Watts = sum of (each appliance's running watts x quantity)
Highest Surge = max(starting watts - running watts) across all selected appliances
Total Starting Watts = Total Running Watts + Highest Surge
Recommended Generator = Total Starting Watts x 1.25
```

The 25% safety margin ensures your generator is not running at absolute maximum capacity, which extends its lifespan, improves fuel efficiency, and provides headroom for any loads you may have overlooked.

## Understanding running watts vs starting watts

Every electrical device has a running wattage, which is the power it consumes during normal operation. Resistive loads like light bulbs, space heaters, and toaster ovens draw the same wattage whether starting or running.

Motor-driven appliances are different. Refrigerators, air conditioners, sump pumps, well pumps, washing machines, and garage door openers all contain electric motors that require a brief burst of extra power to start spinning. This startup surge typically lasts only a fraction of a second to a few seconds, but it can be 2 to 3 times the running wattage.

If your generator cannot supply enough watts to cover the largest startup surge on top of everything else that is already running, the generator's circuit breaker will trip or the appliance simply will not start. This is why starting watts are critical to generator sizing.

## Generator size categories

Generators are commonly available in several size tiers:

- **2,000W Portable** - Small inverter generators for camping, tailgating, or charging a few devices. Quiet and fuel-efficient but limited in capacity.
- **3,000W Portable** - Powers a refrigerator, some lights, and a few small electronics. A good choice for minimal emergency backup.
- **5,000W Portable** - Handles a refrigerator, sump pump, several lights, a microwave, and small electronics. Popular for moderate storm outages.
- **7,500W Portable** - Runs most home essentials simultaneously. Can handle a window AC unit along with kitchen appliances and lights.
- **10,000W Portable or Small Standby** - Enough for central air conditioning or an electric range alongside other essentials.
- **15,000W+ Standby** - Whole-house generators that can power nearly every circuit in a typical home.

## How to use

1. Browse the list of common appliances and set the quantity for each item you plan to power simultaneously
2. For appliances not on the list, use the Add Custom Appliance section to enter a name, running watts, and starting watts
3. Review the results showing total running watts, peak starting watts, and the recommended generator size
4. Check the appliance breakdown table to verify your selections and see which appliance contributes the largest starting surge
5. Use the recommended generator size as a minimum when shopping, and consider going one size larger if your budget allows

## Tips for generator sizing

**Stagger your startup.** When using a generator, turn on motor-driven appliances one at a time. Let each one finish its startup surge before turning on the next. This reduces the peak load your generator must handle.

**Prioritize essentials.** During an extended outage, you may not need to run everything at once. A refrigerator, a few lights, phone chargers, and a Wi-Fi router may be enough, requiring a much smaller generator than your full household load.

**Check your nameplate data.** The wattage values in this calculator are typical averages. For the most accurate sizing, check the nameplate label on each appliance for its exact rated wattage and locked-rotor amps (which indicate starting watts).

**Consider altitude.** Generators lose about 3.5% of their rated power for every 1,000 feet above sea level. If you live at high altitude, size up accordingly.

**Think about fuel availability.** Larger generators consume more fuel. A 7,500W generator might burn 1 to 1.5 gallons per hour at full load, while a 3,000W inverter generator might use only 0.25 gallons per hour at quarter load.

## FAQs

**Q:** Why does the calculator only add the single largest starting surge instead of all starting watts? 

**A:** Appliances do not all start at the exact same moment. In practice, you stagger startup by turning on one motor-driven appliance at a time. The generator only needs to handle one large surge on top of everything else that is already running.


**Q:** What happens if my generator is too small? 

**A:** If the total load exceeds the generator's capacity, the built-in circuit breaker will trip and shut off power. In the worst case, an overloaded generator can overheat or damage connected equipment.


**Q:** What is the 25% safety margin for? 

**A:** Running a generator at full rated capacity for extended periods reduces its lifespan and burns more fuel. The safety margin also accounts for appliances you may have forgotten and for normal wattage variations.


**Q:** Can I run a central air conditioner on a portable generator? 

**A:** A 3-ton central AC unit needs about 3,500 running watts and 4,500 starting watts. Combined with other essentials, you typically need at least a 7,500W to 10,000W generator.


**Q:** Are the wattage values in this calculator exact for my appliances? 

**A:** The values are typical averages for common household models. Your specific appliance may differ. Check the nameplate label on your appliance for exact wattage, or consult the owner's manual.


**Q:** What is the difference between watts and amps? 

**A:** Watts measure total power. For standard 120V household circuits, watts equal volts times amps (W = V x A). A 15-amp circuit at 120 volts provides 1,800 watts.

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