A Beginner’s Guide to Generator Sizing

A Beginner’s Guide to Generator Sizing: Power Up Safely and Efficiently

Power outages can range from minor annoyances to significant disruptions, potentially causing financial loss (spoiled food, damaged electronics) and discomfort. A portable or standby generator is a fantastic solution to keep essential appliances and systems running when the grid goes down. However, simply buying the biggest or cheapest generator you find is a common mistake. The key to reliable backup power is proper generator sizing.

Getting the size right isn’t just about convenience; it’s crucial for the longevity of your generator, the safety of your appliances, and the efficiency of your fuel use. A generator that’s too small will be overloaded, potentially damaging itself or tripping breakers constantly. One that’s too large wastes fuel, costs more upfront, and might not even run efficiently under light loads.

This guide is designed for beginners, breaking down the process of generator sizing into simple, actionable steps.

Why Does Generator Sizing Matter?

Think of a generator like the engine of your emergency power system. Just as a small car engine can’t haul a large truck’s load, a small generator can’t power a whole house full of appliances.

• Too Small: An undersized generator attempting to power too many items will struggle. This can lead to:

  • Overloading: The generator shuts down or trips its internal breaker.
  • Voltage Drops: The power output is unstable, potentially harming sensitive electronics.
  • Reduced Lifespan: The generator’s engine and components are constantly stressed.
  • Failure to Start: Appliances with high starting demands might not turn on at all.

• Too Large: While seemingly safer, an oversized generator has drawbacks:

  • Higher Cost: You pay more initially and for ongoing maintenance.
  • Increased Fuel Consumption: A larger engine burns more fuel, even under light loads.
  • "Wet Stacking": Running a generator significantly below its capacity can lead to incomplete fuel combustion, residue build-up, and potential engine damage over time.
  • Increased Noise: Larger generators are typically louder.

Proper sizing ensures you have enough power for your needs without unnecessary expense or potential harm.

Understanding the Power Numbers: Watts, Amps, and Volts

Before we size, let’s quickly define the key electrical terms you’ll encounter:

• Volts (V): This is the electrical "pressure" or potential difference. In North America, standard household circuits are typically 120 Volts, while larger appliances like air conditioners or well pumps might use 240 Volts.
• Amps (A): This is the electrical "current" or the flow rate of electricity.
• Watts (W): This is the measure of electrical power consumption or production. It’s the most common unit used for generator sizing because it represents the actual work being done (W = V x A). Generator capacity is typically rated in Watts (kW = 1000 W).

Running Watts vs. Starting Watts: The Critical Difference

This is perhaps the most important concept in generator sizing for beginners.

• Running Watts (also called Rated or Continuous Watts): This is the power an appliance needs to run continuously after it has started. A light bulb, for example, only needs its running watts.
• Starting Watts (also called Surge Watts): Many appliances, particularly those with electric motors (like refrigerators, air conditioners, sump pumps, power tools), require a significantly higher surge of power momentarily to start the motor turning. This surge can be 2 to 6 times their running wattage. Once the motor is running, the power requirement drops back down to the running watts.

Your generator needs enough Running Watts capacity to handle the total power demand of all the appliances you want to run simultaneously. It also needs enough Starting Watts (Surge Capacity) to handle the largest single starting surge that might occur while other items are already running.

The Step-by-Step Generator Sizing Process

Now, let’s get practical. Follow these steps to figure out what size generator you need:

Step 1: Identify Your Essential Loads

During a power outage, you likely don’t need to run everything in your house. Decide what’s truly essential. Create a list of appliances and systems you absolutely need access to. Common essentials include:

• Refrigerator / Freezer (to preserve food)
• Lights (at least in key areas)
• Furnace Fan (for heating)
• Sump Pump (if you have one)
• Well Pump (if you rely on a well)
• Medical Equipment (oxygen concentrator, CPAP, etc.)
• Essential Electronics (router, TV, phone chargers)
• Maybe a microwave or coffee maker for short periods.

Step 2: Find the Power Requirements (Running and Starting Watts)

Go through your list and find the power requirements for each item.

• Look at the Appliance Label: Most appliances have a label or nameplate listing voltage (V), amperage (A), and sometimes wattage (W). It might list both Running Watts and Starting Watts.
• Check the Manual: The appliance manual is another good source.
• Use the Formula W = V x A: If only Volts and Amps are listed, multiply them to get watts (W). If both Running and Starting Amps are listed, calculate both wattages.
• Consult Online Charts: Many websites offer charts of typical running and starting watts for common household appliances. These are good estimates if you can’t find the specific label, but the label is always best.
• Appliances with Motors: Pay special attention to motor-driven appliances. You must find their Starting Watts. Lights and standard electronics typically have no significant starting surge (Starting Watts = Running Watts).

Create a simple table like this:

Appliance	Running Watts (W)	Starting Watts (W)	Notes
Refrigerator	800	2400
Living Room Lights	500	0	(e.g., 5 x 100W bulbs)
Furnace Fan	600	1800	Check your specific unit
TV	200	0
Internet Router	50	0
Well Pump	1000	3000	Often 240V – check voltage

Step 3: Calculate Your Total Running Watts

Add up the Running Watts of all the appliances you listed that you might want to run simultaneously. Be realistic about what needs to be on at the exact same time.

• Example (from table above): Refrigerator (800W) + Lights (500W) + Furnace Fan (600W) + TV (200W) + Router (50W) = 2150 Running Watts.

Step 4: Identify the Single Largest Starting Watt Requirement

Look at the Starting Watts column in your table. Find the appliance with the highest starting wattage.

• Example: Refrigerator (2400W), Furnace Fan (1800W), Well Pump (3000W). The Well Pump has the largest starting wattage at 3000W.

Step 5: Calculate Your Minimum Generator Size

This is where you combine your calculations. Your generator needs enough Running Watts capacity to meet your Total Running Watts from Step 3. It also needs enough Starting Watts (Surge) capacity to handle the surge from the single largest appliance (Step 4) while everything else is still running.

Here’s the calculation for the required Surge capacity:

• Required Surge Watts = (Total Running Watts) – (Running Watts of the appliance with the largest starting surge) + (That appliance’s full Starting Watts)

Let’s use our example list: Total Running = 2150W. The appliance with the largest starting surge is the Well Pump (3000W Starting, 1000W Running).

• Required Surge Watts = (2150W) – (1000W) + (3000W) = 1150W + 3000W = 4150 Watts.

So, based on this example, you would need a generator with a Running Wattage capacity of at least 2150 Watts and a Starting Wattage (Surge) capacity of at least 4150 Watts.

Generator specifications typically list both a "Running Watts" (or "Continuous Watts") and a "Starting Watts" (or "Surge Watts"). Look for a generator whose specifications meet or exceed both your calculated minimum running and surge requirements. In our example, you might look for a generator rated around 2200-2500 Running Watts and 4200-4500 Starting Watts.

Step 6: Add a Buffer

It’s always wise to add a safety margin. Appliance ratings can vary slightly, your future needs might change, and a generator running consistently at its absolute maximum capacity isn’t ideal. Add 10-20% to your calculated minimums.

• Example: With calculated minimums of 2150 Running W and 4150 Surge W, adding a 15% buffer:

  • Buffered Running Watts = 2150W * 1.15 = 2472.5 W
  • Buffered Surge Watts = 4150W * 1.15 = 4772.5 W

Now you’d be looking for a generator with roughly 2500+ Running Watts and 4800+ Starting Watts.

Step 7: Consider Voltage and Phase

• Voltage: Ensure the generator provides the voltage(s) you need (120V and/or 240V). Most portable generators offer both.
• Phase: Standard residential power is single-phase. Most portable and smaller standby generators are single-phase, which is appropriate for typical homes. Don’t worry about three-phase unless you have specialized equipment requiring it.

Important Considerations:

• Prioritization: You don’t have to run everything at once. If you can cycle some loads (e.g., run the microwave only when the furnace fan is off), you might get away with a slightly smaller generator.
• Transfer Switch: For connecting a generator directly to your home’s electrical panel, a transfer switch is required for safety and compliance. This allows you to select specific circuits to power. This simplifies sizing as you only calculate the loads on those chosen circuits.
• Inverter Generators: If you need to power sensitive electronics (computers, newer TVs, medical equipment), consider an inverter generator. They produce "cleaner" power (stable sine wave) which is safer for these devices than the "dirtier" power from traditional generators. Inverter generators are often quieter and more fuel-efficient but can be more expensive per watt.

FAQs

• Q: Where can I find the watts of my appliances?

  • A: Look for a metal plate or sticker on the appliance itself, check the owner’s manual, or search online charts for typical wattages of that appliance type and size.

• Q: What happens if I buy a generator that’s slightly too small?

  • A: It may constantly trip its breaker, fail to start some appliances with high surges, or run inefficiently under overload, potentially reducing its lifespan.

• Q: What happens if I buy a generator that’s too large?

  • A: You’ll pay more upfront, use more fuel, it will be louder, and running it consistently at a very low load can cause engine problems ("wet stacking").

• Q: Do I need to calculate the starting watts for every appliance?

  • A: You need to know the starting watts for all appliances with motors (refrigerators, pumps, fans, etc.). However, you only add the single largest starting watt requirement (minus its running watts, added to the total running watts) to your total running watts to determine the required surge capacity. You don’t add up all the starting watts.

• Q: Can I run sensitive electronics with any generator?

  • A: It’s recommended to use an inverter generator for sensitive electronics. Traditional generators can produce power fluctuations that may harm computers and other delicate devices.

• Q: What’s the difference between Running Watts and Peak/Surge Watts on a generator’s specification?

  • A: Running/Continuous Watts is the maximum power the generator can output continuously. Peak/Surge/Starting Watts is the maximum power it can output for a short period (like when starting a motor). Your calculation needs to ensure your total simultaneous running load is below the generator’s Running Watts, and your largest potential starting surge (while other items are running) is below the generator’s Peak/Surge Watts.

Conclusion

Generator sizing might seem a bit daunting at first with all the numbers, but by breaking it down into listing your needs, finding appliance wattages, and performing a few simple calculations, you can determine the right size generator for your situation.

Taking the time to size correctly ensures you get the most out of your investment. You’ll have reliable power when you need it most, protect your valuable appliances, and operate your generator safely and efficiently. Don’t guess – calculate! Your peace of mind during the next outage is worth the effort.