Whole House Generator Size Calculator
Select the appliances you intend to power simultaneously during an outage to determine the required generator size. The calculator considers both continuous (running) and startup (surge) wattage.
Recommended Generator Size
This includes a 25% buffer for safety and optimal performance.
What is a Whole House Generator Size Calculator?
A whole house generator size calculator is an essential tool for any homeowner considering a backup power solution. Its primary purpose is to estimate the total electrical load your home requires, helping you select a generator that can handle all your essential (and non-essential) appliances during a power outage. Choosing the right size is critical: a generator that’s too small won’t be able to power your devices and may shut down, while one that’s too large will be inefficient and more expensive to run. This whole house generator size calculator simplifies the process by accounting for two types of wattage: running watts and starting watts. For more information on what is best for you, see this standby generator calculator.
The Formula and Explanation for Generator Sizing
The core of generator sizing lies in understanding the difference between running and starting (or “surge”) watts. Running watts is the continuous power an appliance needs to operate. Starting watts is the extra burst of power required for a few seconds when an appliance with a motor or compressor (like a refrigerator or air conditioner) turns on. Our whole house generator size calculator uses the following industry-standard formula:
Total Required Watts = (Total Running Watts of All Appliances) + (Highest Starting Watts of a Single Appliance)
This formula ensures you have enough power for the continuous load of all your devices, plus the largest single surge from your most demanding appliance. We then add a 25% safety buffer to this total to arrive at the recommended generator size, ensuring longevity and the ability to handle unexpected loads.
Appliance Wattage Variables
The wattage of appliances can vary. The table below provides typical ranges. For precise numbers, always check the data plate on the appliance itself.
| Variable (Appliance) | Typical Running Watts | Typical Starting Watts |
|---|---|---|
| Refrigerator/Freezer | 700W | 2200W |
| Central AC (24,000 BTU) | 3800W | 11400W |
| Sump Pump (1/3 HP) | 800W | 1200W |
| Electric Water Heater | 4500W | 0W |
| Clothes Washer | 1150W | 2250W |
Practical Examples
Example 1: Essentials-Only Home
A homeowner wants to power only the critical items during an outage.
- Inputs: Refrigerator (700 running / 2200 starting), Sump Pump (800 running / 1200 starting), and Several Lights (500 running).
- Calculation:
- Total Running Watts: 700 + 800 + 500 = 2000W
- Highest Starting Watts: 2200W (from the Refrigerator)
- Total Required: 2000W + 2200W = 4200W
- Recommended Size (with 25% buffer): 4200W * 1.25 = 5250W or 5.25kW
- Result: A generator around 5,500 watts (5.5 kW) would be suitable.
Example 2: Whole-House Comfort
A family in a larger home wants to maintain a high level of comfort, including air conditioning.
- Inputs: Central AC (3800 running / 11400 starting), Refrigerator (700/2200), Well Pump (1000/2000), Lights (500/0), TV (500/0), Microwave (1000/0).
- Calculation:
- Total Running Watts: 3800 + 700 + 1000 + 500 + 500 + 1000 = 7500W
- Highest Starting Watts: 11400W (from the Central AC)
- Total Required: 7500W + 11400W = 18900W
- Recommended Size (with 25% buffer): 18900W * 1.25 = 23625W or 23.6kW
- Result: A generator around 24 kW is needed for this whole house setup. Reviewing your backup power solutions is an important step.
How to Use This Whole House Generator Size Calculator
Using this calculator is a straightforward process:
- Survey Your Home: Go through the list of appliances in the calculator.
- Select Appliances: Check the box next to every item you plan to run *at the same time* during a power outage. Be realistic; you probably won’t run your dishwasher and clothes dryer while the AC is on.
- Review Results: The calculator will instantly update. The “Recommended Generator Size” is the most important number—this is the minimum kilowatt (kW) rating you should look for.
- Interpret Intermediate Values: “Total Running Watts” shows your continuous power demand, while “Highest Starting Watts” shows the biggest power surge the generator must handle.
Key Factors That Affect Generator Sizing
- Starting vs. Running Wattage: As explained, the surge demand from motors is often the single biggest factor determining the required generator size.
- Essential vs. Non-Essential Loads: Deciding what you truly need to power versus what you can live without is the first step in managing your power needs and budget. For a better understanding of your needs see this generator wattage guide.
- Central Air Conditioning: This is typically the largest single power consumer in a home, with a very high starting wattage. If you need to run your AC, expect your generator size requirement to increase significantly.
- Well Pumps and Sump Pumps: These are critical for many homes and have significant starting wattages that must be accounted for.
- Fuel Type (Propane vs. Natural Gas): While this doesn’t change the wattage calculation, it impacts performance. Natural gas is convenient but may provide slightly less power than propane for the same generator model.
- Future Power Needs: The 25% buffer our whole house generator size calculator adds isn’t just for safety; it allows for adding new appliances or for slight degradation in generator performance over time.
Frequently Asked Questions (FAQ)
Running (or rated) watts are the continuous watts an appliance consumes to operate normally after it has started up.
Starting watts are the extra power an appliance’s motor or compressor needs for a few seconds to start. This “surge” can be 2-3 times higher than the running watts.
The compressor in an air conditioner requires a massive amount of energy to get moving from a standstill. This initial kickstart is the highest load your generator will likely face.
No. This would lead to an incorrectly oversized generator. You only need to account for the items running simultaneously and add the single highest starting wattage, not all of them. Our whole house generator size calculator handles this correctly.
Not necessarily. An oversized generator runs inefficiently, wastes fuel, and can have a higher initial cost. It’s best to get a size that matches your calculated needs with a reasonable buffer. Getting help with home generator installation is very important.
This calculator provides a very reliable estimate based on typical appliance wattages and standard formulas. However, for a final decision, it’s always best to have a qualified electrician perform a professional load calculation.
They are both units of power. One kilowatt is equal to 1,000 watts. Generators are typically rated in kilowatts (kW).
The buffer ensures the generator isn’t constantly running at 100% capacity, which improves longevity and fuel efficiency. It also provides flexibility if you add new appliances or if an appliance draws slightly more power than listed.
Related Tools and Internal Resources
Explore more of our resources to make an informed decision about your backup power needs:
- Standby Generator Calculator: A tool focused specifically on permanently installed standby units.
- Generator Wattage Guide: A detailed list of wattages for hundreds of household items.
- How to Size a Generator: A comprehensive guide on the principles behind generator sizing.
- Backup Power Solutions: An overview of different types of backup power, from portable generators to whole-home systems.
- Home Generator Installation: Learn about the process and requirements for installing a standby generator.
- Best Home Generators of 2024: Our reviews and top picks for the year.