Generator Sizing Calculator
Use this calculator to determine the appropriate generator size (in Watts and kVA) needed to power your essential appliances and equipment during an outage or for off-grid use. Accurately sizing your generator prevents overloading and ensures reliable power.
Appliance Load Details
Enter the quantity, running watts, and surge (starting) watts for each appliance you plan to power. If an appliance does not have a significant surge, enter 0 for surge watts.
| Appliance | Quantity | Running Watts (per unit) | Surge Watts (per unit) |
|---|---|---|---|
| Refrigerator/Freezer | |||
| Central AC (3-ton) / Large Window AC | |||
| Well Pump / Sump Pump (1/2 HP) | |||
| Lights & Small Electronics (Combined) | |||
| Electric Water Heater / Oven Element | |||
| Custom Load 1 | |||
| Custom Load 2 |
Safety Margin: It's recommended to add a safety margin to account for future needs or unexpected loads.
Generator Sizing Results:
" + "Total Running Watts Required: " + finalRunningWatts.toFixed(0) + " Watts" + "Total Surge (Starting) Watts Required: " + finalSurgeWatts.toFixed(0) + " Watts" + "This translates to approximately:" + "Required Running kVA: " + finalRunningkVA.toFixed(2) + " kVA" + "Required Surge kVA: " + finalSurgekVA.toFixed(2) + " kVA" + "Note: Generator ratings are often given in 'peak' or 'surge' watts and 'continuous' or 'running' watts. Ensure your chosen generator's continuous rating meets your 'Total Running Watts Required' and its peak rating meets your 'Total Surge Watts Required'."; }Understanding Generator Sizing
Choosing the right size generator is crucial for ensuring your home or business has reliable backup power. An undersized generator won't be able to power all your essential appliances, while an oversized one can be an unnecessary expense and less fuel-efficient. This calculator helps you determine the ideal generator capacity based on your specific electrical loads.
Running Watts vs. Surge Watts
When sizing a generator, you need to consider two types of wattage:
- Running Watts (Continuous Watts): This is the power an appliance needs to operate continuously once it's started. Most appliances list their running wattage on a label or in their manual.
- Surge Watts (Starting Watts): Many appliances, especially those with electric motors (like refrigerators, air conditioners, well pumps, and power tools), require a brief burst of extra power to start up. This initial surge can be significantly higher than their running wattage. A generator must be able to handle this surge, even if it's only for a few seconds.
The calculator accounts for both. It sums up the running watts of all appliances you plan to use simultaneously and then adds the single largest surge wattage from any one appliance that might start while others are already running. This ensures your generator can handle both the steady load and the momentary power spikes.
How to Use the Calculator
- List Your Essential Appliances: Identify all the appliances and devices you absolutely need to power during an outage.
- Find Wattage Information: For each appliance, look for its running watts and, if applicable, its surge watts. This information is usually found on the appliance's data plate, in the owner's manual, or on the manufacturer's website. If you can't find surge watts, you can often estimate it as 2-3 times the running watts for motor-driven appliances.
- Enter Quantities: Input how many of each appliance you plan to run at the same time.
- Add Custom Loads: Use the "Custom Load" fields for any specific items not listed in the pre-defined categories.
- Consider a Safety Margin: It's always a good idea to add a safety margin (e.g., 10-20%) to your total wattage. This provides a buffer for future needs, unexpected loads, or the natural degradation of generator performance over time.
- Calculate: Click the "Calculate Generator Size" button to get your recommended running and surge wattage, as well as the equivalent kVA.
Typical Appliance Wattage Examples
Here are some common wattage values to help you estimate if you don't have exact figures:
- Refrigerator/Freezer: Running 600-800W, Surge 1800-2500W
- Central Air Conditioner (3-ton): Running 3000-4000W, Surge 6000-8000W
- Window Air Conditioner (10,000 BTU): Running 1000-1500W, Surge 1500-2000W
- Well Pump (1/2 HP): Running 800-1200W, Surge 1500-2500W
- Sump Pump (1/2 HP): Running 700-1000W, Surge 1200-1800W
- Microwave Oven: Running 800-1500W, Surge 1200-2000W
- Electric Water Heater: Running 3500-5000W (no significant surge)
- Electric Oven (per element): Running 1000-2000W (no significant surge)
- Furnace Fan (Blower): Running 700-1000W, Surge 1400-2000W
- Television (LED): Running 100-300W
- Desktop Computer & Monitor: Running 200-500W
- Incandescent Light Bulb: Running 60-100W
- LED Light Bulb: Running 5-15W
- Hair Dryer: Running 1200-1800W
Watts vs. kVA
Generators are often rated in both Watts (W) and kilovolt-amperes (kVA). Watts represent the "real power" consumed by the load, while kVA represents the "apparent power." The relationship between them is determined by the power factor (PF), typically around 0.8 for most residential loads and generators.
kVA = Watts / (1000 * Power Factor)
Our calculator provides both Watts and kVA to give you a comprehensive understanding of your generator requirements.
Important Considerations
- Simultaneous Use: The calculator assumes you will run all listed appliances simultaneously. If you plan to cycle certain high-draw appliances (e.g., only run the AC when the well pump is off), you can adjust your inputs accordingly.
- Fuel Type: Consider the fuel type (gasoline, propane, natural gas, diesel) and its availability and storage requirements.
- Installation: For whole-home backup, professional installation with a transfer switch is highly recommended for safety and compliance with electrical codes.
- Noise Levels: Generators vary significantly in noise output. Consider this, especially if the generator will be near living areas or neighbors.
By carefully calculating your power needs, you can select a generator that provides reliable and efficient power for your specific situation.