The #1 Mistake When Buying a Generator (It’s About Size)

The #1 Mistake When Buying a Generator (It’s All About Size)

The lights flicker, then plunge you into darkness. The refrigerator groans silent, and the hum of daily life ceases. Power outages are disruptive, sometimes dangerous, and often unpredictable. For many homeowners and businesses, a generator isn’t just a convenience; it’s a vital piece of preparedness equipment, offering a lifeline of electricity when the grid fails.

But buying a generator isn’t as simple as picking the shiny one off the shelf or the cheapest one online. There are many factors to consider: fuel type, noise level, portability, and features like automatic transfer switches. However, amidst all these considerations, one stands out as the most common and most detrimental error buyers make: getting the size wrong.

Yes, the #1 mistake when buying a generator is a failure to accurately assess your power needs, leading you to purchase a unit that is either too small or, less commonly but still problematic, too large for your requirements.

While buying a generator that’s slightly oversized might cost you a bit more upfront and consume a little extra fuel, the consequences of buying one that is undersized are far more severe. Let’s delve into why size is paramount and how failing to calculate it correctly can turn your investment into a frustrating, potentially damaging, and ultimately useless piece of equipment during an emergency.

Why Size Matters: Watts, Running, and Starting

To understand why size is so critical, you need a basic grasp of how electricity is measured and consumed. Electrical power is measured in watts (W) or kilowatts (kW, which is 1,000 watts). Every appliance, light bulb, and electronic device has a wattage rating, indicating how much power it needs to operate.

There are two key types of wattage you need to consider for most appliances, especially those with motors:

1. Running Watts (or Rated Watts): This is the continuous power an appliance needs to operate after it has started up. Think of it as the power needed to keep the fan spinning, the light glowing, or the refrigerator compressor running steadily.
2. Starting Watts (or Surge Watts): This is the extra burst of power an appliance, particularly one with an electric motor (like refrigerators, air conditioners, pumps, power tools), requires for a brief moment (a few seconds) to get going. This starting surge can be significantly higher – often 2 to 3 times – than the running wattage.

A generator is rated by its continuous (or rated) wattage and its surge (or peak) wattage. The continuous wattage tells you how much power the generator can provide consistently over time. The surge wattage tells you the maximum power it can provide for a short duration to handle the startup demands of appliances.

The Consequences of Getting It Wrong (Especially Too Small)

This is where the #1 mistake leads to problems. When you buy an undersized generator, its continuous wattage capacity is lower than the total running watts of the appliances you need to power simultaneously, or its surge wattage capacity is lower than the starting watts of the largest appliance you need to kick on.

What happens then?

• Overloading and Tripping Breakers: The generator’s internal circuit breakers will trip repeatedly as you try to connect too many appliances or start a large motor. This is frustratingly inconvenient and defeats the purpose of having backup power.
• Brownouts and Voltage Fluctuations: An overloaded generator can struggle to maintain a stable voltage and frequency. This results in "brownouts," where power is delivered below the required voltage.
• Damage to Appliances: Operating sensitive electronics or motor-driven appliances on unstable or insufficient power can cause them to overheat, malfunction, or suffer permanent damage. Modern electronics are particularly vulnerable to power surges and sags.
• Damage to the Generator: Continuously running a generator at or near its maximum capacity, or repeatedly tripping its breakers, puts excessive strain on the engine and alternator, leading to premature wear and potential failure.
• Ineffectiveness: The primary consequence is that the generator simply won’t be able to power the essential items you bought it for when you need them most. You might have lights, but your refrigerator won’t run, or your furnace fan won’t kick on.

While less catastrophic, buying a generator that is significantly too big also has downsides. It costs more upfront, consumes more fuel (even at lower loads, though less per watt generated at optimal load), is usually heavier and louder, and if it’s rarely run at a decent load, it can potentially suffer from "wet stacking" (unburned fuel and soot buildup) over time, requiring more maintenance. However, these issues are generally less severe than the functional failure and potential damage caused by an undersized unit.

Avoiding the Mistake: How to Calculate Your Needs

Preventing the #1 mistake requires a simple, but crucial, exercise: a power audit of your anticipated needs. Don’s guess! Follow these steps:

1. Identify Your Essential Needs: What must run during a power outage? Lights (specify rooms/types), refrigerator/freezer, furnace fan (if applicable), well pump (if applicable), medical equipment, essential electronics (phone chargers, a computer for work), perhaps a sump pump. Be realistic – you likely won’t run everything in your house at once.
2. List Appliances and Find Their Wattage: Go through your list of essential items. For each, find its wattage requirements. This information is typically on a label or plate attached to the appliance itself (often on the back, bottom, or near the power cord), in the owner’s manual, or sometimes on the manufacturer’s website.
3. Note Both Running and Starting Watts: For appliances with motors (fridge, freezer, pump, furnace fan, window AC unit), you need both numbers. For resistive loads (lights, heaters, coffee makers, toasters – though you might not run a toaster during an outage), the running wattage is usually the only figure you need, as their startup surge is minimal or non-existent.
4. Calculate Total Running Watts: Add up the running watts of all the appliances you anticipate needing to run simultaneously at any given time. This gives you the minimum continuous power output your generator needs.

   • Example: Fridge (150W running), a few lights (300W total), furnace fan (300W running), TV (100W). Total Running Watts = 150 + 300 + 300 + 100 = 850W.

5. Calculate Peak Starting Watts: This is the tricky part. You need your total running watts (calculated in step 4) PLUS the single largest starting wattage of any appliance you might need to start while everything else is running. You don’t add up all the starting wattages, only the highest one you’ll need to start at any moment.

   • Example: Using the same list (850W running). Let’s say the fridge needs 800W to start, the furnace fan needs 1200W to start, and the TV needs 50W to start. The largest single starting wattage is 1200W (for the furnace fan).
   • Peak Starting Watts = Total Running Watts + Largest Single Starting Watts
   • Peak Starting Watts = 850W + 1200W = 2050W.

6. Add a Buffer: It’s wise to add a safety margin, typically 10% to 20%, to both your total running watts and peak starting watts calculations. This accounts for potential inefficiencies, future needs, and ensures the generator isn’t constantly running at its absolute limit.

   • Example: Running Watts needed: 850W + 15% buffer = ~978W.
   • Peak Starting Watts needed: 2050W + 15% buffer = ~2358W.

Based on this example calculation, you would look for a generator with a continuous or rated wattage of at least 1000W (or 1kW) and a surge or peak wattage of at least 2400W (or 2.4kW). Always prioritize matching the continuous wattage first, then ensure the surge wattage is sufficient for your largest starting load.

Generators and Their Ratings

When you look at generator specifications, you’ll see two main power ratings:

• Rated Watts (Continuous Watts): This is the power the generator can produce reliably for extended periods. Match this number (plus your buffer) to your Total Running Watts calculation.
• Surge Watts (Peak Watts): This is the maximum power the generator can produce for a very short duration (typically seconds) to start motor-driven appliances. Match this number (plus your buffer) to your Peak Starting Watts calculation.

Crucially, make sure the generator’s continuous rating meets your continuous needs and its surge rating meets your surge needs. Don’t get tricked into buying a generator based only on its higher surge rating, as it cannot sustain that power output.

Conclusion

Buying a generator is an investment in preparedness and peace of mind. But that investment is compromised if you fall prey to the #1 mistake: misjudging the size. An undersized generator will be unable to power your essential appliances when you need them most, leading to frustration, potential damage to your equipment, and a false sense of security.

Taking the time to perform a simple power audit – identifying your critical needs, listing appliances, finding their running and starting wattages, and performing the calculation with a buffer – is a small effort that pays huge dividends. It ensures you purchase a generator that is truly capable of meeting your requirements, keeping your lights on, your food cold, and your essential systems running until grid power is restored. Don’t let a simple calculation error leave you in the dark.

FAQs

Q1: What’s the difference again between running watts and starting watts?
A1: Running watts are the power an appliance needs to keep running continuously (like a light bulb staying on). Starting watts are the extra surge of power needed for a moment to get motor-driven appliances (like a refrigerator or air conditioner) started up. The starting watts are usually much higher than the running watts for motor-driven items.

Q2: Do I need to add up the starting watts of all my appliances?
A2: No, you only need to add the single largest starting wattage of any appliance you might start to your total running watts figure. This accounts for the highest single power spike the generator will need to handle at any given moment while other things are already running.

Q3: Where can I find the wattage of my appliances?
A3: Look for a label or plate on the appliance itself, often near the power cord, on the back, or underneath. Check your owner’s manual or the manufacturer’s website. If you can only find amperage (A) and voltage (V), you can estimate wattage using the formula: Watts = Amps x Volts (for resistive loads) or Watts = Amps x Volts x Power Factor (for motor loads, Power Factor is typically 0.8). Using the listed wattage is always more accurate if available.

Q4: Can I just buy a really big generator to be safe?
A4: While better than buying one too small, a significantly oversized generator costs more upfront, burns more fuel, is heavier, and can potentially suffer from issues like "wet stacking" if rarely run under a sufficient load. Calculating your needs provides the best balance of capability and efficiency.

Q5: What happens if I try to run too many things on my generator?
A5: The generator’s circuit breaker will likely trip to protect itself and the connected appliances. This will cut power to everything connected. Repeatedly tripping the breaker can strain the generator.

Q6: Does the type of generator (inverter vs. conventional) affect the sizing calculation?
A6: The method for calculating your needs (running and starting watts) remains the same regardless of generator type. However, inverter generators are generally more fuel-efficient and produce cleaner power, making them better suited for sensitive electronics, but you still need to match their wattage ratings (continuous and surge) to your calculated load.