Generator Size Calculator
Estimate the wattage needed for your home or business during a power outage by selecting the appliances you plan to run. This calculator will help you determine the appropriate generator size.
| Appliance | Quantity | Running Watts | Starting Watts |
|---|---|---|---|
| Refrigerator (Energy Star) | |||
| Freezer (Energy Star) | |||
| Sump Pump (1/2 HP) | |||
| Well Pump (1/2 HP) | |||
| Window AC (10,000 BTU) | |||
| Microwave (1000W) | |||
| Coffee Maker | |||
| Hair Dryer | |||
| Lights (LED String/Small) | |||
| Television (LED) | |||
| Computer/Laptop | |||
| Medical Device (CPAP) | |||
| Space Heater (1500W) | |||
| Safety Margin (%) | |||
Recommended Generator Size:
Required Continuous Running Watts: 0 Watts
Required Starting (Surge) Watts: 0 Watts
Choose a generator that meets or exceeds both the continuous running watts and the starting (surge) watts.
Understanding Generator Sizing for Your Needs
When a power outage strikes, a reliable generator can be a lifesaver, keeping essential appliances running and maintaining comfort. However, choosing the right size generator is crucial. An undersized generator won't power your critical items, while an oversized one can be an unnecessary expense and consume more fuel.
Running Watts vs. Starting Watts
The most important distinction in generator sizing is between "running watts" and "starting watts":
- Running Watts (Continuous Watts): This is the power an appliance needs to operate continuously once it's turned on. For most resistive loads (like lights, heaters, coffee makers), the running watts are the only consideration.
- Starting Watts (Surge Watts): Many motor-driven appliances (like refrigerators, freezers, air conditioners, well pumps, and power tools) require a brief burst of extra power to start their motors. This initial surge can be 2-3 times their running wattage. While this surge only lasts for a few seconds, your generator must be capable of providing it.
Our calculator sums the running watts of all selected appliances and then adds the single highest starting wattage requirement from any one appliance to determine the total surge capacity needed. This is because typically, not all motor-driven appliances will start simultaneously.
How to Use the Calculator
- Identify Essential Appliances: Think about what you absolutely need to run during an outage. This might include your refrigerator, some lights, a TV, a computer, and perhaps a medical device.
- Enter Quantities: For each appliance you wish to power, enter the quantity you'll be using.
- Verify Wattages: The calculator provides common wattage estimates. However, for precise sizing, check the labels or owner's manuals of your specific appliances. Running watts are usually listed, and starting watts can sometimes be found or estimated (often 2-3x running watts for motors).
- Consider a Safety Margin: We recommend a 10-20% safety margin. This buffer accounts for potential future needs, slight inaccuracies in wattage estimates, or the possibility of an appliance drawing slightly more power under certain conditions.
- Calculate: Click the "Calculate Generator Size" button to see your estimated running and starting wattage requirements.
Interpreting the Results
The calculator will provide two key figures:
- Required Continuous Running Watts: This is the minimum wattage your generator needs to supply consistently to keep all your selected appliances running simultaneously.
- Required Starting (Surge) Watts: This is the maximum wattage your generator must be able to produce for a brief moment to start the highest-demanding motor-driven appliance while other items are already running.
When purchasing a generator, ensure its "rated running watts" meet or exceed your "Required Continuous Running Watts" and its "peak/surge watts" meet or exceed your "Required Starting (Surge) Watts."
Example Scenario:
Let's say you want to power a refrigerator (600 running / 1800 starting), a few LED lights (60 running / 60 starting), and a TV (100 running / 100 starting). With a 20% safety margin:
- Total Running Watts: 600 (fridge) + 60 (lights) + 100 (TV) = 760 Watts
- Highest Single Starting Watt: 1800 Watts (from the refrigerator)
- Required Continuous Running Watts (with 20% margin): 760 * 1.20 = 912 Watts
- Required Starting (Surge) Watts (with 20% margin): (760 + 1800) * 1.20 = 2560 * 1.20 = 3072 Watts
In this example, you would need a generator capable of at least 912 continuous running watts and 3072 surge watts. A 3500-watt peak / 2800-watt running generator might be a good fit, but always check the specific model's ratings.