Your Step-by-Step Guide to Sizing a Generator

Powering Up Preparedness: Your Step-by-Step Guide to Sizing a Generator

Power outages can strike unexpectedly, leaving homes and businesses in the dark. Whether it’s a severe storm, grid overload, or local issue, losing electricity disrupts daily life, halts work, and can even lead to costly losses like spoiled food or damaged equipment. A reliable generator provides peace of mind, keeping essential appliances running and maintaining a degree of normalcy.

However, simply buying a generator isn’t enough. Choosing the right size is crucial. An undersized generator won’t be able to power the equipment you need, potentially stalling or tripping breakers. An oversized generator costs more upfront, consumes excessive fuel, is often louder, and can even suffer from reduced lifespan due to "wet stacking" (unburned fuel residue) if consistently run below its optimal load.

This guide breaks down the process into simple, manageable steps, ensuring you select a generator perfectly tailored to your specific power needs.

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

Before we dive into the steps, let’s clarify some key terms:

• Watts (W): The standard unit of electrical power. Think of it as the "strength" or capacity needed to operate an appliance. Generators are rated in Watts or Kilowatts (kW), where 1 kW = 1000 W.
• Running Watts (Rated Watts): This is the continuous power an appliance needs to operate after it has started. It’s the power requirement for steady use.
• Starting Watts (Surge Watts): Many appliances, especially those with electric motors (like refrigerators, air conditioners, pumps, power tools), require a significant burst of extra power for a fraction of a second when they first start up. This surge can be 2-3 times, or even more, than the appliance’s running wattage. The generator needs to handle this brief peak.

The Step-by-Step Generator Sizing Process

Follow these steps to accurately determine the generator size you need:

Step 1: Identify Your Essential Power Needs

Start by making a list of everything you absolutely must have running during a power outage. Don’t list every single item in your house; focus on necessities for safety, comfort, and preserving perishables. Common essential items include:

• Refrigerator / Freezer (to preserve food)
• Lights (strategic lighting for key areas)
• Furnace fan / Boiler circulator pump (for heat in cold weather)
• Well pump (if you rely on well water)
• Medical equipment (oxygen concentrator, CPAP, etc.)
• Sump pump (to prevent flooding)
• Communication devices (charging phones, running a router)
• A few key outlets for small electronics

Consider your typical usage during an outage. Will you need the furnace and the well pump running simultaneously? Think about scenarios.

Step 2: Find the Running Wattage for Each Item

Once you have your list, find the running wattage for each appliance. The best places to find this information are:

• Appliance Labels or Data Plates: Look for a sticker or plate on the appliance itself, often near the power cord connection. It should list Voltage (V), Amps (A), and sometimes Watts (W) or Kilowatts (kW). If it only lists Volts and Amps, you can estimate Watts using the formula: Watts = Volts x Amps. (For a rough estimate on standard household items using 120V, simply multiply Amps by 120).
• Owner’s Manual: The manual often contains power consumption details.
• Manufacturer’s Website: Check the product specifications online.
• Online Search: A quick web search for "[Appliance Name/Model] running wattage" can often yield good estimates, but use these with caution and prefer manufacturer data when available.

List each item and its running wattage.

Example List:

• Refrigerator: 150 W
• Lights (5 x 60W bulbs): 300 W
• Furnace Fan: 400 W
• Well Pump: 1000 W
• Sump Pump: 800 W
• Phone Chargers/Router: 100 W

Step 3: Calculate Your Total Continuous (Running) Wattage

Add up the running wattage of all the items you listed in Step 2. This gives you the minimum continuous power your generator needs to supply if everything on your essential list were running at the same time (which isn’t always the case, but it’s a baseline).

Example Calculation (from list above):
150W (Fridge) + 300W (Lights) + 400W (Furnace) + 1000W (Well Pump) + 800W (Sump Pump) + 100W (Electronics) = 2750 W

This 2750 W is your minimum running wattage requirement.

Step 4: Identify the Starting Wattage (Surge) for Motor-Driven Appliances

Now, identify the appliances on your list that have electric motors. These are the ones that will have a higher starting wattage requirement. Using the same sources as in Step 2 (labels, manuals, online), find the starting or surge wattage for these items.

• If the starting wattage isn’t listed, you’ll need to estimate. A common rule of thumb is 2-3 times the running wattage, but some motors (like older or heavily loaded ones) can draw 4-5 times their running wattage on startup. Use the higher end of the estimate if you’re unsure.
• Prioritize finding accurate data for your highest-wattage motor items (like pumps, well pumps, or refrigerators).

Example List with Estimated Starting Watts:

• Refrigerator: 150 W Running / ~800 W Starting (estimate ~5x running)
• Lights: 300 W Running / 300 W Starting (no motor surge)
• Furnace Fan: 400 W Running / ~800 W Starting (estimate ~2x running)
• Well Pump: 1000 W Running / ~2000 W Starting (estimate ~2x running)
• Sump Pump: 800 W Running / ~1600 W Starting (estimate ~2x running)
• Phone Chargers/Router: 100 W Running / 100 W Starting (no motor surge)

Step 5: Calculate Your Maximum Starting Wattage Requirement

This is arguably the most critical and sometimes confusing step. You do not need to add up the starting watts for all items. You only need to account for the highest starting surge you anticipate happening while other essential items are already running.

Think about which motor-driven appliance is most likely to start up while other loads are active. For most homes, this might be the refrigerator, a sump pump kicking on, or potentially a well pump.

The calculation is: (Highest Single Starting Wattage) + (Running Wattage of ALL Other Essential Items Running Concurrently)

Let’s use our example list:

• Total Running Load (from Step 3): 2750 W
• Highest Starting Load Item: Well Pump at 2000 W

Imagine the worst-case startup scenario: Everything on your list is running except the well pump, and then the well pump kicks on.
Calculation: (Well Pump Starting Watts) + (Running Watts of All Other Items)
Calculation: 2000 W (Well Pump Start) + (150W Fridge + 300W Lights + 400W Furnace + 800W Sump Pump + 100W Electronics) Running Watts
Calculation: 2000 W + 1750 W = 3750 W

Alternatively, imagine the sump pump kicks on while everything but the sump pump is running:
Calculation: 1600 W (Sump Pump Start) + (150W Fridge + 300W Lights + 400W Furnace + 1000W Well Pump + 100W Electronics) Running Watts
Calculation: 1600 W + 1950 W = 3550 W

In this example, the Well Pump starting while everything else is running creates the highest peak demand (3750 W). This 3750 W is your maximum starting wattage requirement.

Your generator needs to have a "Starting Watts" or "Surge Watts" rating at least as high as your calculated maximum starting wattage requirement (3750 W in this example). It also needs a "Running Watts" rating at least as high as your total continuous running load (2750 W in this example).

Step 6: Add a Safety Buffer

It’s always wise to add a 10-20% buffer to your calculated running and starting wattage requirements. This accounts for:

• Potential inaccuracies in wattage estimates.
• Aging appliances that might draw more power over time.
• Adding a future essential item to your list.
• Ensuring the generator isn’t constantly running at its absolute maximum capacity, which is better for its longevity and performance.

Example Calculation (using a 15% buffer):

• Required Running Watts: 2750 W + (2750 W * 0.15) = 2750 W + 412.5 W = 3162.5 W
• Required Starting Watts: 3750 W + (3750 W * 0.15) = 3750 W + 562.5 W = 4312.5 W

Step 7: Choose Your Generator Size

Based on your calculations, you now know the minimum wattage ratings your generator needs:

• Required Running Watts: Approximately 3200 W (rounding up from 3162.5 W)
• Required Starting Watts: Approximately 4400 W (rounding up from 4312.5 W)

Look for generators with a "Running Watts" rating equal to or greater than 3200 W and a "Starting Watts" rating equal to or greater than 4400 W. Generator specifications clearly list these two ratings.

Important Considerations Beyond Wattage:

• Voltage: Most household items run on 120V. Larger appliances like central air conditioning, well pumps, or electric water heaters may require 240V. Ensure the generator provides the necessary voltage(s) and outlets.
• Fuel Type: Gasoline, propane, natural gas, or diesel. Consider availability, storage, and run time.
• Inverter Generators: These are more fuel-efficient, quieter, and produce "cleaner" power (pure sine wave), which is safer for sensitive electronics like computers and medical equipment. If you plan to power electronics, an inverter generator is highly recommended, even if it costs more.
• Noise Level: Generator noise is measured in decibels (dB). Lower dB ratings mean quieter operation. This is especially important in residential areas.
• Portability vs. Standby: Portable generators are manually set up and connected (often via extension cords or a transfer switch). Standby generators are permanently installed, connect directly to your home’s electrical system via an automatic transfer switch, and turn on automatically when power is lost. Standby units are more expensive but offer maximum convenience.

Why Proper Sizing Matters (Again)

Getting the size right is critical for several reasons:

• Functionality: An undersized generator simply won’t power your essential items. Motor loads might fail to start, or the generator could shut down or damage connected appliances due to voltage drops.
• Cost: Oversizing wastes money on the purchase price and fuel during operation.
• Generator Lifespan: Running a generator consistently below about 40-50% of its capacity can lead to engine problems ("wet stacking"). Running it constantly at maximum capacity also reduces its lifespan.

FAQs

• What’s the difference between Watts and Kilowatts? A Kilowatt (kW) is simply 1000 Watts (W). It’s just a larger unit of measurement for the same thing (power).
• What about Voltage (V) and Amps (A)? Voltage is the electrical "pressure," Amps are the "current" or flow rate. Watts are the total power consumed (W = V x A). Generator sizing is primarily done in Watts because that represents the total power load, regardless of the voltage used (though you need to ensure the generator provides the correct voltage).
• Can’t I just guess the size I need? No, guessing is a recipe for frustration. You’ll likely end up with a generator that’s either too small to be useful or much larger and more expensive than necessary.
• What if I only need to power one or two small things? If you only need to run a few lights and charge phones, a very small portable inverter generator (1000-2000 W range) might be sufficient. Always perform the wattage calculation for even minimal needs.
• Should I size a generator to run my entire house? For most residential situations, sizing a generator to run everything (central AC, electric heat, all lights, all outlets simultaneously) is cost-prohibitive and results in a very large, fuel-hungry generator. Focusing on essential items is the practical approach for backup power.
• What if I can’t find the exact wattage of an appliance? Look for Amps and Volts and calculate Watts (W = V x A). If you still can’t find data, search online forums or consult an electrician, but be aware that online estimates can vary. When in doubt, use a slightly higher estimate.

Conclusion

Sizing a generator correctly is not a step to be skipped or rushed. It’s a foundational part of ensuring your investment provides reliable power when you need it most. By carefully listing your essential items, determining their running and starting wattages, and performing the simple calculations outlined in this guide, you can confidently choose a generator that meets your specific requirements. Taking the time to plan ensures you get the peace of mind and functionality you expect from your backup power source, keeping your lights on and your essentials running through any outage.