Generator Size Calculator
Use this calculator to estimate the running and starting wattage required for your generator. Accurately sizing your generator ensures it can power your essential appliances without overload, providing reliable electricity when you need it most.
Add a buffer (e.g., 10-30%) for future needs, unexpected loads, or to prevent your generator from running at maximum capacity constantly.
Appliance List
Enter the running and starting watts for each appliance you plan to power. If starting watts are unknown, you can leave it blank, and the calculator will assume it's similar to the running watts for resistive loads, or you can estimate it (often 2-3x running watts for motor-driven appliances).
Generator Size Recommendation
Enter your appliance details and click "Calculate Generator Size" to see your power requirements.
Generator Size Recommendation
Total Continuous Running Watts Required: ${totalRunningWatts.toFixed(0)} Watts Total Starting (Surge) Watts Required: ${totalSurgeWatts.toFixed(0)} WattsApplying a ${safetyFactor}% safety factor: Recommended Generator Running Watts: ${recommendedRunningWatts.toFixed(0)} Watts (${(recommendedRunningWatts / 1000).toFixed(1)} kW) Recommended Generator Starting (Surge) Watts: ${recommendedSurgeWatts.toFixed(0)} Watts (${(recommendedSurgeWatts / 1000).toFixed(1)} kW) Choose a generator with a running wattage capacity equal to or greater than the recommended running watts, and a surge wattage capacity equal to or greater than the recommended surge watts. `; }
Understanding Generator Sizing: Running Watts vs. Starting Watts
Choosing the right size generator is crucial for ensuring your essential appliances run smoothly during a power outage or at a remote location. An undersized generator will struggle to power your devices, potentially leading to damage to both the generator and your appliances. An oversized generator, while capable, might be an unnecessary expense and less fuel-efficient.
What are Running Watts (Continuous Power)?
Running watts, also known as continuous watts, refer to the power an appliance needs to operate continuously once it has started. This is the steady power draw required to keep the device functioning. For example, a light bulb, a television, or a computer typically have a consistent running wattage.
When calculating your total running wattage, you simply add up the running watts of all the appliances you intend to power simultaneously. This sum represents the minimum continuous power output your generator must provide.
What are Starting Watts (Surge Power)?
Starting watts, or surge watts, refer to the extra burst of power an appliance needs for a brief moment when it first turns on. This surge is particularly common with motor-driven appliances like refrigerators, freezers, well pumps, air conditioners, and power tools. These devices require significantly more power to overcome initial inertia and get their motors spinning than they do to keep running.
The starting wattage can be two to three times (or even more) higher than the running wattage for some appliances. Your generator must be able to handle this momentary surge, otherwise, it will trip a breaker or fail to start the appliance.
When calculating total starting watts, you sum the running watts of all appliances and then add the single largest starting wattage from any one appliance. This is because it's unlikely that all motor-driven appliances will start at the exact same moment. The generator needs to be able to handle the continuous load plus the largest individual surge.
How to Use the Generator Size Calculator
- List Your Essential Appliances: Identify all the appliances you absolutely need to power during an outage. This might include a refrigerator, freezer, well pump, furnace fan, lights, and communication devices.
- Find Appliance Wattages:
- Check Appliance Labels: Many appliances have a label or plate (often on the back or bottom) that lists their running and sometimes starting wattage. Look for "Watts," "W," or "Amps" (if only amps are listed, multiply amps by volts to get watts, e.g., 10 Amps * 120 Volts = 1200 Watts).
- Consult User Manuals: Appliance manuals often provide detailed power consumption figures.
- Use Typical Values: If you can't find specific numbers, use the pre-filled values in the calculator as a starting point, or research typical wattages for similar appliances online.
- Input Values into the Calculator:
- Enter the "Running Watts" for each appliance.
- Enter the "Starting Watts" if known. If left blank, the calculator will assume it's equal to the running watts (suitable for resistive loads like lights, but motor loads will need a higher value).
- Specify the "Quantity" of each appliance.
- Apply a Safety Factor: The "Safety Factor" adds a buffer to your total wattage requirements. A 10-30% safety factor is recommended to account for minor inaccuracies, future appliance additions, or to prevent your generator from constantly running at its absolute maximum capacity, which can extend its lifespan.
- Click "Calculate": The calculator will provide your total continuous running watts and total starting (surge) watts required.
Example Scenario:
Let's consider a typical home backup scenario using the default values in the calculator:
- Refrigerator: 700 Running Watts / 2200 Starting Watts (1 unit)
- Well Pump (1/2 HP): 1000 Running Watts / 2000 Starting Watts (1 unit)
- Microwave (1000W): 1500 Running Watts / 1500 Starting Watts (1 unit)
- Lights (10 LED/CFL bulbs): 600 Running Watts / 600 Starting Watts (1 unit, representing 10 bulbs)
- Television: 100 Running Watts / 100 Starting Watts (1 unit)
- Computer: 300 Running Watts / 300 Starting Watts (1 unit)
- Furnace Fan (1/2 HP): 800 Running Watts / 2350 Starting Watts (1 unit)
- Sump Pump (1/2 HP): 1000 Running Watts / 2000 Starting Watts (1 unit)
- Coffee Maker: 1000 Running Watts / 1000 Starting Watts (1 unit)
- Hair Dryer: 1500 Running Watts / 1500 Starting Watts (1 unit)
With a 20% safety factor, the calculator would determine the total running watts by summing all running wattages (700+1000+1500+600+100+300+800+1000+1000+1500 = 8600 Watts). The largest individual starting wattage is 2350 Watts (Furnace Fan). So, the total surge watts would be 8600 + 2350 = 10950 Watts.
Applying the 20% safety factor:
- Recommended Running Watts: 8600 * 1.20 = 10320 Watts (10.3 kW)
- Recommended Surge Watts: 10950 * 1.20 = 13140 Watts (13.1 kW)
In this scenario, you would look for a generator rated for at least 10,320 running watts and 13,140 starting watts.
Important Considerations When Choosing a Generator:
- Prioritize Loads: You might not need to power every single appliance simultaneously. Consider what is truly essential during an outage.
- Stagger Appliance Starts: To minimize surge demand, avoid starting all motor-driven appliances at once. Turn on the highest surge appliance first, let it stabilize, then turn on the next, and so on.
- Fuel Type: Generators run on gasoline, propane, natural gas, or diesel. Consider fuel availability, storage, and run time.
- Generator Type:
- Portable Generators: More affordable, versatile, but require manual setup and refueling.
- Standby Generators: Permanently installed, automatically turn on during outages, but are more expensive.
- Noise Levels: Generators can be noisy. Check the decibel rating, especially if you have close neighbors.
By carefully calculating your power needs with this generator size calculator, you can confidently select a generator that provides reliable power for your home or business.