Don’t Buy Until You Read This: Sizing Your Generator

3 days ago

Don’t Buy Until You Read This: Sizing Your Generator

Power outages are inconvenient at best and dangerous at worst. When the lights go out, a generator can be a lifeline, keeping essential appliances running, protecting frozen food, and providing comfort and safety. But simply buying a generator isn’t enough. One of the most common and costly mistakes generator buyers make is getting the wrong size. Too small, and it won’t power what you need, potentially damaging the generator or your appliances. Too large, and you’ve wasted money on a unit that’s less efficient, louder, and harder to store than necessary.

This isn’t a decision to rush. Before you hand over your hard-earned cash, take the time to understand the crucial process of sizing your generator correctly. It’s simpler than you might think, and it ensures you get a generator that meets your specific needs perfectly.

Why Generator Sizing Matters (and What Happens If You Get It Wrong)

Think of your generator like an electrical heart for your home or campsite during an outage. Just as a weak heart can’t pump enough blood to keep a body functioning, a generator that’s too small can’t supply enough electricity to power your chosen appliances.

Too Small: If the total power required by the devices you plug in exceeds the generator’s capacity, you risk overloading the unit. This can cause circuit breakers to trip, the generator engine to struggle or shut down, and potentially damage the generator itself. Even worse, insufficient power can harm sensitive electronics. You’ve bought peace of mind but ended up with frustration and potential repair bills.

Too Large: On the flip side, a generator significantly larger than your needs is inefficient. It will consume more fuel than a smaller unit for the same load, costing you more in the long run. Larger generators are typically heavier, bulkier, and louder. Unless you have future expansion plans, buying excess capacity is simply a waste of money.

The goal is to find the "Goldilocks" generator – not too big, not too small, but just right for your specific power requirements.

Understanding the Jargon: Watts, Running Watts, and Starting Watts

Before we dive into the sizing process, let’s clarify the key terms you’ll encounter:

Watts (W): This is the fundamental unit of electrical power. It represents the rate at which electrical energy is consumed or produced. Generator capacity is almost always rated in watts or kilowatts (kW), where 1 kW = 1000 W.

Running Watts (or Continuous Watts): This is the power an appliance needs to operate continuously once it’s already running. It’s the steady power draw.

Starting Watts (or Surge Watts/Peak Watts): This is the extra burst of power some electrical devices need for a few seconds when they first start up. Appliances with motors (like refrigerators, freezers, air conditioners, pumps, power tools) have a significantly higher starting wattage requirement than their running wattage. This surge is needed to get the motor going from a standstill. This is critical for sizing.

Understanding the difference between running and starting watts is perhaps the most important concept in generator sizing.

The Step-by-Step Process for Sizing Your Generator

Sizing isn’t about guessing; it’s about calculating. Follow these steps to determine the right generator size for your needs:

Step 1: Identify What You Need to Power

Make a list of all the appliances, tools, or systems you might want to run simultaneously during an outage or while using the generator. Be realistic. Are you powering your entire house? Just essential circuits? Appliances for camping or an RV? Specific tools on a job site?

Example List (Home Essentials): Refrigerator, Freezer, Sump Pump, Furnace fan (gas furnace), several Lights (LED/CFL), Television, Computer/Laptop, Charging phones, Microwave, Coffee Maker.

Step 2: List the Running Watts for Each Item

Go through your list and find the running wattage for each item. This information is usually found on:

The appliance’s data label (often on the back or bottom). Look for "Watts (W)" or "Amps (A)" – if only Amps are listed, you can estimate Watts by multiplying Amps by the voltage (Watts = Amps Volts). For household items, assume 120 Volts (W = A 120).

The owner’s manual.

The manufacturer’s website.

Online appliance wattage charts (be aware these are estimates and can vary by model).

Example (Finding Running Watts):
- Refrigerator: 150W
- Freezer: 200W
- Sump Pump: 800W
- Furnace Fan: 400W
- Lights (6 x 10W LED): 60W
- Television: 100W
- Computer: 150W
- Charging Phones: 50W
- Microwave (Medium): 1000W (often listed as "cooking power")
- Coffee Maker: 800W

Step 3: Identify the Starting Watts for Motorized Items

Now, for any item with a motor (refrigerator, freezer, sump pump, furnace fan, power tools, air conditioners), find its starting wattage. This is often 2-3 times (or even more) higher than the running wattage. Again, look at the label, manual, or online resources.

Example (Finding Starting Watts):
- Refrigerator: 150W running / 600W starting
- Freezer: 200W running / 800W starting
- Sump Pump: 800W running / 1500W starting
- Furnace Fan: 400W running / 1000W starting
- Lights, TV, Computer, Chargers: Usually 0W starting (or negligible)

Step 4: Calculate Your Total Running Watts Needed

Add up the running watts for all the items on your list that you anticipate running simultaneously. This gives you the minimum continuous power the generator needs to supply.

Example (Total Running Watts):
- Refrigerator (150W) + Freezer (200W) + Sump Pump (800W) + Furnace Fan (400W) + Lights (60W) + TV (100W) + Computer (150W) + Chargers (50W) = 1910 Running Watts
Note: You wouldn’t run the Microwave (1000W) and Coffee Maker (800W) continuously, but you might use them for a few minutes. For sizing the continuous load, you’d typically exclude short-duration appliances like these unless you planned to run them alongside everything else.

Step 5: Calculate Your Total Starting Watts (The Critical Step)

This is where many people make a mistake. You don’t add up the starting watts of every motorized item. You need to account for the fact that only one motor will typically start at the exact same second while everything else is already running.

To calculate your total starting watts requirement:

Take your Total Running Watts from Step 4.

Identify the item on your list with the single highest starting wattage.

Subtract that item’s running wattage from its starting wattage to find the extra surge it needs to start.

Add this extra surge to your Total Running Watts.

This final number is the maximum surge wattage your generator must be able to handle for a brief moment to start that highest-demand motor while everything else is running.

Example (Total Starting Watts Calculation):
- Total Running Watts: 1910 W
- Motorized Items & Starting Watts: Refrigerator (600W), Freezer (800W), Sump Pump (1500W), Furnace Fan (1000W).
- Highest Starting Wattage: Sump Pump (1500W)
- Sump Pump Running Wattage: 800W
- Sump Pump Extra Surge: 1500W (Start) – 800W (Run) = 700W
- Required Generator Starting Watts: Total Running Watts (1910W) + Sump Pump Extra Surge (700W) = 2610 Starting Watts

Step 6: Add a Buffer (Safety Margin)

It’s always wise to add a buffer to your calculated running and starting wattage requirements. A 10-20% buffer is typical.

Why add a buffer?
- Appliances might draw slightly more power than listed, especially as they age.
- Generators are most efficient when not running at their absolute maximum capacity continuously.
- It provides flexibility if you underestimated a need or want to add a small item later.
- It can extend the generator’s lifespan.

Example (Adding Buffer – 15%):
- Required Running Watts: 1910W + 15% (approx 287W) = approx. 2200 Running Watts
- Required Starting Watts: 2610W + 15% (approx 392W) = approx. 3000 Starting Watts

Conclusion: Choosing Your Generator Size

Based on our example, you would look for a generator rated with:

A Running Wattage capacity of at least 2200 W (or 2.2 kW).

A Starting Wattage capacity of at least 3000 W (or 3 kW).

Generator specifications usually list both numbers clearly. Find a model whose ratings meet or exceed both your calculated running and starting wattage needs, including your chosen buffer.

Remember, this process is specific to the items you intend to power. Your list and calculations will differ depending on whether you’re powering a critical circuit panel, an RV, or just a few essential appliances.

FAQs: Your Generator Sizing Questions Answered

Q: Can I just guess the size I need?
A: No, absolutely not. Guessing is the most common cause of dissatisfaction. You’ll likely end up with a generator that’s either inadequate for your needs or unnecessarily large and expensive.

Q: What if an appliance only lists Amps (A)?
A: You can convert Amps to Watts using the formula: Watts = Volts Amps. For most standard household items in North America, the voltage is 120V. So, W = A 120. For larger appliances on 240V circuits (like central AC or electric dryers), use 240V.

Q: Do I need to add up the starting watts for all my appliances?
A: No, this is a crucial mistake. You only need to add the highest single extra surge required by one appliance to your total running watts. Generators can handle the running load of multiple items simultaneously, but typically only surge for one motor starting at a time.

Q: Should I always buy a generator larger than I calculated?
A: Adding a buffer (10-20%) is recommended for efficiency and safety margin. However, buying significantly larger than your calculated needs means higher upfront cost, increased fuel consumption, more noise, and heavier weight, without a proportional benefit unless you anticipate substantial future power needs.

Q: Where is the best place to find appliance wattage information?
A: Start with the appliance’s physical label, usually near the power cord or on the back/bottom. The owner’s manual is also a reliable source. If you can’t find it, search online for the specific model number or consult online appliance wattage charts (using these as estimates).

Q: What if I have appliances that use 240V?
A: If you plan to power 240V appliances (like a well pump, central AC unit, or electric water heater), ensure the generator supports 240V output and include their running and starting wattage requirements in your calculations. Remember W = A * 240 for these items.

Q: What’s the difference between portable and standby generator sizing?
A: The process of listing appliances and calculating watts is the same. However, standby generators are often sized to power a significant portion or all of a home automatically, involving a transfer switch. Portable generators are typically sized for essentials or specific plug-in needs. The scale of the wattages calculated will usually be much higher for a whole-house standby system.

Conclusion

Buying a generator is a significant investment in preparedness and comfort. Don’t let that investment go to waste by skipping the vital step of proper sizing. By understanding the difference between running and starting watts, listing your specific power needs, performing the calculation carefully, and adding a buffer, you can confidently choose a generator that is perfectly suited to your requirements.

Taking the time to do this homework now will save you frustration, expense, and potentially even damage down the road. So, before you make that purchase, grab a pen and paper (or open a spreadsheet), list your needs, do the math, and buy until you’ve read and understood this essential guide to generator sizing!

3 days ago