What Size Generator Do You Really Need?
What Size Generator Do You Really Need? Powering Up Smartly
Power outages can strike without warning, bringing daily life to a standstill. Whether it’s a summer storm, winter ice, or grid maintenance, losing electricity means losing comfort, convenience, and sometimes even safety. A generator offers peace of mind, providing essential backup power. But stepping into the world of generators can be confusing, especially when faced with terms like watts, amps, running power, and starting power. One of the most critical decisions you’ll make is determining the right size generator for your needs. Too small, and you won’t be able to run the appliances you rely on. Too large, and you’ll waste money on purchase, fuel, and maintenance. So, what size generator do you really need? It’s not a simple answer, but a calculation based on your specific requirements.
Why Size Matters: The Goldilocks Principle
Think of generator sizing like choosing the right-sized engine for your car. An engine that’s too small won’t be able to get you up hills or carry much weight efficiently. One that’s too large is overkill, burning excessive fuel for no practical benefit. Generators work similarly:
- Too Small: A generator that’s undersized for your needs will struggle to power your appliances. Attempting to draw more power than it can provide can overload the unit, tripping breakers or even damaging the generator itself. More critically, it simply won’t run everything you need simultaneously, leaving you in the dark or without crucial services like refrigeration or heating.
- Too Large: Buying a generator with far more capacity than you need is a waste of money upfront. Larger generators cost more to buy, consume more fuel (even at partial load, though modern ones are more efficient), and often require more expensive maintenance. While having extra capacity seems like a good idea, excessive oversizing offers diminishing returns.
The goal is to find the "just right" size – enough power to comfortably run your essential (or desired) items without excessive waste.
Understanding the Basics: Watts, Running vs. Starting Power
To size a generator, you need to understand the language of electricity, primarily watts (W). Watts measure the rate at which electrical energy is used or produced.
- Running Watts (Rated Watts): This is the continuous power an appliance consumes once it’s up and running. It’s the steady demand it places on the generator.
- Starting Watts (Surge Watts): Many electrical devices, particularly those with motors (like refrigerators, air conditioners, pumps, power tools), require a brief surge of extra power when they first start up. This starting wattage can be significantly higher than the running wattage – sometimes 2 to 3 times more! This initial surge lasts only a few seconds but is critical for the generator to handle.
When sizing a generator, you need to consider both the total running watts of all appliances you want to power simultaneously and the highest starting watts of the single largest motor-driven appliance you might start while other things are running.
The Sizing Process: A Step-by-Step Guide
Determining the correct generator size involves a practical process of identifying your needs and calculating the power required.
Step 1: Identify Your Power Needs
The first step is to decide what you absolutely need or want to power during an outage. Will it be just the bare essentials for survival and safety, or do you want to maintain a degree of normal comfort? Make a list. Common essentials include:
- Refrigerator/Freezer (to preserve food)
- Lights (a few key rooms)
- Furnace Fan (for heat in winter) or AC (for cooling in summer – can be power-hungry!)
- Sump Pump (critical in flood-prone areas)
- Well Pump (if you rely on well water)
- Medical Equipment (oxygen concentrators, CPAP machines, etc. – often critical!)
- Basic Electronics (phone chargers, perhaps a TV or computer)
- Garage Door Opener
Think realistically about what you must have versus what would be nice to have. Powering a whole house with central air, electric stove, and multiple TVs requires a much larger, typically standby, generator than just keeping the fridge cold and a few lights on with a portable unit.
Step 2: List Your Appliances and Their Wattage
Once you have your list of essential items, find the power requirements for each. This is the most crucial (and sometimes trickiest) part.
- Check Appliance Labels/Manuals: Most appliances have a label (often on the back, bottom, or near the power cord) or a section in the manual that lists power consumption in watts (W) or amps (A). If it lists amps, you can roughly estimate watts using the formula: Watts = Volts x Amps. For standard household items, Volts are typically 120V. For larger appliances or 240V circuits (like some well pumps or AC units), use 240V.
- Look for Both Running and Starting Watts: For motor-driven appliances, look specifically for both ratings. Sometimes, the starting watts aren’t explicitly listed, but a range or multiplier (like "LRA" – Locked Rotor Amps, related to starting surge) might be.
- Use Online Resources: If you can’t find the information on the appliance, a quick online search for "[Appliance Name/Model] wattage" or "typical [Appliance Type] wattage" can give you good estimates. Be aware that wattages vary by model and efficiency.
Create a simple table or list for clarity:
| Appliance | Running Watts (W) | Starting Watts (W) (if applicable) | Notes |
|---|---|---|---|
| Refrigerator | 150 | 600 | Startup surge |
| Furnace Fan (med) | 300 | 900 | Often a motor |
| 60W Light Bulb | 60 | – | No surge |
| Microwave (700W) | 700 | – | Resistive load, no surge (mostly) |
| Sump Pump (1/2 HP) | 800 | 1600 | Significant motor surge |
| Laptop Charger | 50 | – | Low wattage, no surge |
| Medical Device (CPAP) | 50 | – | Critical, clean power might be needed |
Note: These are typical wattages. Always check your specific appliances.
Step 3: Calculate Your Total Running Watts
Add up the running watts of all the appliances you listed in Step 1 that you anticipate running at the same time during an outage.
Example (using the table above):
Refrigerator (150W) + Furnace Fan (300W) + 5x 60W Lights (300W) + Microwave (700W – if used during outage) + Sump Pump (800W – if running) + Laptop (50W) + CPAP (50W) = 2350 Running Watts (if all on simultaneously)
Self-Correction: It’s unlikely the sump pump and microwave run constantly and at the exact same moment as everything else. Be realistic about simultaneous usage. Maybe calculate a "minimum essential" total (fridge, furnace, lights, CPAP) and a "peak usage" total (adding the microwave or pump if needed). Let’s use the "minimum essential" for our running watt base: Refrigerator (150W) + Furnace Fan (300W) + 5x Lights (300W) + CPAP (50W) = 800 Running Watts. Now, add the potential for intermittent use: If the Sump Pump (800W) or Microwave (700W) might be used while the essentials are running, your potential running load could temporarily increase. Let’s stick to the base running load for the generator’s continuous rating calculation, but remember this potential variation.
Step 4: Determine Your Peak Starting Watt Requirement
This is where starting watts come in. Look at your list of appliances from Step 2 that have a starting watt requirement (motor-driven items). Identify the single appliance with the highest starting wattage.
Example (from table):
Refrigerator: 600W starting
Furnace Fan: 900W starting
Sump Pump: 1600W starting
The highest starting wattage among these is the Sump Pump at 1600W.
Now, take your Total Running Watts from Step 3 (let’s use the essential 800W) and add the Highest Single Starting Wattage from Step 4.
Example:
Total Running Watts (Essentials): 800W
Highest Single Starting Watt: 1600W (Sump Pump)
Required Starting Watt Capacity: 800W + 1600W = 2400W
This means your generator needs to have a starting watt rating of at least 2400W to handle the surge when the sump pump kicks on while the fridge, furnace fan, lights, and CPAP are running. The generator’s running watt rating needs to be at least your total running watts (800W in this example) to handle the continuous load.
Step 5: Add a Buffer
It’s wise to add a buffer to your calculated requirements. Appliance wattages can vary, older generators might not perform at peak efficiency, and you might find you need to power something else later. Adding 10-20% to your calculated running and starting wattage provides a safety margin.
Example:
Calculated Running Watts: 800W
Calculated Starting Watts: 2400W
Add 15% Buffer:
Running Watt Requirement + Buffer: 800W 1.15 = 920W
Starting Watt Requirement + Buffer: 2400W 1.15 = 2760W
Based on this example calculation, you would look for a generator with a Running Watt rating of at least 920W and a Starting (Surge) Watt rating of at least 2760W. You’ll likely find generators rated in round numbers like 1000 Running / 3000 Starting, or 1500 Running / 3500 Starting. Always choose a generator that meets or exceeds both your calculated running and starting watt requirements (plus buffer).
Beyond the Watts: Other Factors to Consider
While wattage is the primary factor for sizing, don’t forget these other considerations:
- Fuel Type: Gasoline (common for portable, limited storage life), Propane (cleaner, stores well, lower power output than gas usually), Natural Gas (requires hookup, unlimited supply but tied to grid), Diesel (efficient, common for very large units).
- Run Time: How long will the generator run on a tank of fuel at your expected load? Important for extended outages.
- Noise Level: Generators can be loud. Check the decibel (dB) rating, especially if noise is a concern for you or neighbors.
- Outlets and Voltage: Ensure the generator has the types and quantities of outlets you need (120V, 240V) and the appropriate amperage rating for connecting to your system or appliances.
- Portable vs. Standby: Portable generators are wheeled units you can move and start manually (or electric start). Standby generators are permanently installed, connect to your home’s electrical system via a transfer switch, and often start automatically when power is lost. Standby units are typically much larger and more expensive but offer seamless whole-house power.
FAQs
- Q: What happens if my generator is too small?
- A: It won’t be able to power all your desired appliances simultaneously. Attempting to draw too much power can trip the generator’s breaker, shut it down, or potentially damage the generator or connected appliances.
- Q: What happens if my generator is too big?
- A: You will have paid more for the generator than necessary, and it will likely consume more fuel than a properly sized unit, even at lower loads (though less of an issue with modern inverter generators). It’s inefficient and costly overkill.
- Q: Can I run sensitive electronics like computers and TVs?
- A: Yes, but it’s recommended to use an inverter generator. Inverter generators produce "cleaner" sine wave power, which is less likely to harm sensitive electronics than the "dirty" power from some conventional generators. Check the generator’s specifications or look specifically for "inverter" models.
- Q: Is it better to use gasoline or propane?
- A: Both have pros and cons. Gasoline is readily available but degrades over time. Propane stores indefinitely but typically provides slightly less power output per gallon than gasoline and requires separate tanks. Natural gas offers an unlimited supply if you have a hookup, but ties you to the gas line.
- Q: How do I connect a generator to my house?
- A: For safety and to avoid backfeeding power onto the grid (which is extremely dangerous for utility workers!), you must connect a generator through a properly installed transfer switch. For larger portable generators and all standby generators, this should be done by a qualified electrician. Never plug a generator directly into a wall outlet ("backfeeding").
- Q: My appliance lists amps, not watts. How do I convert?
- A: Use the formula: Watts = Volts x Amps. Most household items are 120V. So, a 10 Amp appliance on a 120V circuit is roughly 10A 120V = 1200 Watts. For 240V circuits, use 240V (e.g., a 15A appliance on 240V is 15A 240V = 3600 Watts).
Conclusion
Choosing the right size generator is a critical step in preparing for power outages. It requires a realistic assessment of your needs, a bit of detective work to find appliance wattages, and some basic arithmetic to calculate your total running watts and peak starting watt requirements. By following the step-by-step process – listing essentials, finding wattages, calculating running and starting needs, and adding a buffer – you can arrive at a power requirement number that guides your purchase. Don’t forget to consider other factors like fuel type, noise, and whether a portable or standby unit best suits your situation. Investing time in proper sizing ensures you get a generator that provides the reliable power you need, without being an expensive or inadequate burden, giving you valuable peace of mind when the lights go out. When in doubt, especially for complex setups or whole-house power, consult with a qualified electrician who can help you accurately assess your needs and recommend appropriate solutions.
