Determine the optimal Generac generator wattage for your backup power needs.
Calculate Your Generator Size
Enter the total heated/cooled square footage of your home.
Count large appliances that will run simultaneously.
Count essential circuits you need powered.
Count items with high starting or running wattage.
Estimate how many of your critical/high-demand items will run at the same time.
Your Estimated Generator Size
— Watts
Base Load— Watts
Appliance Load— Watts
Critical Circuit Load— Watts
High Demand Load— Watts
Formula: (Base Load + Appliance Load + Critical Circuit Load + High Demand Load) * Simultaneous Usage Factor = Total Required Watts.
Base Load is estimated per sq ft. Appliance/Circuit/High Demand loads are estimated per item.
Power Load Distribution
Visual breakdown of your estimated power load components.
Appliance Wattage Estimates
Item Type
Typical Running Watts
Typical Starting Watts
Refrigerator
150-200
700-1000
Central Air Conditioner (3-ton)
3500-5000
7000-10000
Electric Oven
2000-5000
2000-5000
Microwave
800-1500
800-1500
Well Pump
750-1500
1500-3000
Electric Dryer
1800-2500
1800-2500
LED Lighting (per bulb)
5-10
5-10
Ceiling Fan (per fan)
50-100
50-100
Laptop Charger
50-100
50-100
Television
100-300
100-300
Note: Wattage can vary significantly by model and brand. Consult your appliance manuals for precise figures.
What is a Generac Generator Size Calculator?
A Generac generator size calculator is an essential online tool designed to help homeowners and business owners determine the appropriate wattage capacity for a backup generator. Power outages can be disruptive and costly, making a reliable backup power source crucial. This calculator simplifies the complex process of assessing your power needs by considering various factors like home size, the number and type of appliances, and critical circuits you wish to power during an outage. By inputting these details, the calculator estimates the total wattage required, guiding you towards selecting a Generac generator that can meet your demands without being undersized or excessively oversized.
Who should use it? Anyone living in an area prone to power outages, individuals who rely on electricity for medical equipment, businesses needing to maintain operations, or homeowners seeking peace of mind during storms or grid failures should utilize a Generac generator size calculator. It's particularly useful for those unfamiliar with electrical loads and generator specifications.
Common misconceptions: A frequent misconception is that a generator only needs to power a few essential items like lights and a refrigerator. In reality, modern homes have numerous appliances and electronics that draw significant power, especially during startup. Another myth is that bigger is always better; an oversized generator can be inefficient and more expensive to purchase and operate. This calculator helps strike the right balance.
Generac Generator Size Calculator Formula and Mathematical Explanation
The core principle behind a Generac generator size calculator is to sum up the estimated power consumption of all devices intended to run during an outage and then apply a factor for simultaneous usage and startup surges. While specific algorithms may vary slightly, a common approach involves calculating different load types:
1. Base Load Calculation: This represents the continuous power needed for basic functions and smaller devices. It's often estimated based on the square footage of the home.
2. Appliance Load Calculation: This accounts for the running wattage of major appliances like refrigerators, freezers, and HVAC systems.
3. Critical Circuit Load Calculation: This sums the wattage of essential circuits such as lighting, fans, and medical equipment.
4. High-Demand Item Load Calculation: This includes items with particularly high starting or running wattage, like well pumps, electric dryers, or EV chargers.
5. Simultaneous Usage Factor: Not all devices run at 100% capacity simultaneously. This factor (expressed as a percentage) adjusts the total load to reflect realistic usage patterns.
Percentage of items expected to run at the same time.
% (0-100)
50% – 90%
Base Load Watts
Estimated wattage for basic home functions per sq ft.
Watts
1000 – 5000+
Appliance Load Watts
Sum of running watts for major appliances.
Watts
1000 – 10000+
Critical Circuit Load Watts
Sum of running watts for essential circuits.
Watts
500 – 3000+
High Demand Load Watts
Sum of running watts for high-wattage devices.
Watts
500 – 5000+
Total Required Watts
The final estimated generator capacity needed.
Watts
5000 – 25000+
The calculator uses average wattage figures for common appliances and circuits, which are detailed in the table above. It's crucial to remember that starting wattage (surge) for motors can be 2-3 times higher than running wattage. While this calculator focuses on running watts adjusted by a usage factor, for critical high-demand items like air conditioners or well pumps, ensuring the generator can handle their starting surge is paramount. Generac generators often specify both running and starting watts.
Practical Examples (Real-World Use Cases)
Let's illustrate how the Generac generator size calculator works with practical scenarios:
Example 1: Suburban Family Home
Scenario: A 2,200 sq ft home in a region with frequent winter storms. The family wants to power essentials plus their furnace, refrigerator, microwave, lights, fans, and a well pump.
Interpretation: This family needs a generator capable of at least 4000 running watts. Considering starting surges, especially for the well pump and potentially the furnace blower, a generator rated around 5000-7000 starting watts would be a safer choice to ensure reliability.
Example 2: Small Business Office
Scenario: A 1,500 sq ft office space that needs to maintain essential operations like computers, lighting, and a small server during outages.
Interpretation: The office requires a generator around 5500 running watts. Given the critical nature of computers and servers, ensuring stable power delivery is key. A generator in the 7000-9000 watt range would provide adequate capacity and buffer.
How to Use This Generac Generator Size Calculator
Using the Generac generator size calculator is straightforward. Follow these steps to get an accurate estimate:
Enter Home Square Footage: Input the total heated and cooled square footage of your property. This helps estimate the baseline power needed for general functions.
Count Major Appliances: List and count appliances like refrigerators, freezers, air conditioners, ovens, etc., that you intend to run during an outage.
Count Critical Circuits: Identify and count essential circuits such as lighting, fans, home office equipment, and medical devices.
Count High-Demand Items: Note any specific high-wattage appliances like well pumps, electric dryers, or electric vehicle chargers.
Estimate Simultaneous Usage: Determine the percentage of these items you realistically expect to operate at the same time. For example, if you'll run the furnace, lights, and microwave concurrently, that's 3 items out of potentially more. A factor of 70-85% is common.
Click 'Calculate Size': The calculator will process your inputs and display the estimated required wattage.
How to read results: The calculator provides a primary highlighted result showing the total estimated wattage needed. It also breaks down the contribution of different load types (Base, Appliance, Critical, High Demand) and shows the calculated simultaneous usage factor. This detailed view helps understand where the power demand originates.
Decision-making guidance: The primary result is your target running wattage. Always choose a generator with a higher rated wattage than your calculated need to accommodate starting surges (especially for motors) and ensure longevity. For instance, if the calculator suggests 5000 watts, consider a generator rated for at least 6500-7500 running watts, and check its starting (surge) wattage capabilities against your highest-demand items.
Key Factors That Affect Generac Generator Size Results
Several factors influence the accuracy of your generator size calculation and the ultimate choice of a Generac generator:
Appliance Wattage Variations: The wattage figures used are averages. Actual consumption varies significantly by brand, model, age, and efficiency. Always check the nameplate or manual for specific appliances.
Starting (Surge) Watts: Motors in appliances like refrigerators, AC units, and well pumps require a much higher wattage to start than to run. A generator must meet both running and starting requirements for these items.
Simultaneous Usage Patterns: Real-world usage is dynamic. The percentage entered reflects an estimate; actual usage might fluctuate, requiring a buffer.
Future Needs: Consider potential future additions like electric vehicle charging, home additions, or new appliances. Sizing slightly larger now can save costs later.
Inverter vs. Conventional Generators: Inverter generators provide cleaner power, ideal for sensitive electronics, but may have different sizing considerations. Conventional generators are often more robust for heavy loads.
Fuel Type and Efficiency: While not directly impacting wattage calculation, the type of fuel (gasoline, propane, natural gas) affects runtime, availability, and refueling logistics.
Voltage Requirements: Ensure the generator's output voltage matches your home's electrical system (typically 120V/240V split-phase).
Transfer Switch Capacity: The generator must be paired with a transfer switch rated to handle the total load.
Frequently Asked Questions (FAQ)
Q1: How accurate is this calculator?
A: This calculator provides an estimate based on typical wattage values and common usage patterns. For precise sizing, especially for complex electrical systems or specific high-demand appliances, consulting a qualified electrician is recommended.
Q2: What's the difference between running watts and starting watts?
A: Running watts (or rated watts) is the continuous power the generator can supply. Starting watts (or surge watts) is the extra power needed for a brief moment to start motor-driven appliances. Generators must have enough starting watts to handle the highest-demand item's surge.
Q3: Do I need a generator for my whole house?
A: Not necessarily. You can choose to power only essential circuits (lights, fridge, furnace, medical equipment) with a smaller generator, or power the entire house with a larger standby unit. This calculator helps determine the scope.
Q4: How much wattage do lights and fans typically use?
A: LED lights use very little, typically 5-10 watts each. Ceiling fans use around 50-100 watts. These are generally considered critical circuits.
Q5: What if I have a large air conditioner?
A: Central air conditioners are high-demand items. A 3-ton unit can draw 3500-5000 running watts and require 7000-10000 starting watts. Ensure your generator can handle this surge.
Q6: Can I run a well pump and other appliances simultaneously?
A: Yes, but you must ensure the generator's starting wattage can handle the well pump's surge, and its running wattage can support the pump plus all other simultaneously running devices.
Q7: What is a good simultaneous usage percentage?
A: A common estimate is 70-85%. It's unlikely every single appliance and circuit will be on at maximum load simultaneously. However, it's wise to err slightly higher to ensure capacity.
Q8: Should I buy a generator larger than the calculator suggests?
A: It's often recommended to select a generator with a running wattage slightly higher (e.g., 10-20%) than the calculated estimate to provide a buffer and extend the generator's lifespan. Always check the starting wattage rating.