Free Guide to Calculating Your Backup Generator Size Needs

Understanding Generator Sizing Fundamentals

A backup generator must produce enough electrical power to run the devices and appliances you want to operate during an outage. Generator size is measured in watts, and selecting the right size requires understanding two key electrical concepts: starting watts and running watts.

Running watts, also called rated watts, represent the continuous power a generator can produce. This is the amount of electricity needed to keep appliances running steadily. Starting watts, sometimes called surge watts, are the peak amounts of power required at the moment an appliance first turns on. Many devices—especially those with motors like air conditioners, refrigerators, and well pumps—require significantly more electricity to start than they do to run continuously. For example, a 5,000-watt air conditioning unit might need 15,000 watts of starting power but only 5,000 watts to continue running.

According to the U.S. Department of Energy, the average American household uses between 877 and 2,000 kilowatt-hours of electricity per month, though this varies significantly by location and usage patterns. However, during an outage, most people don't need to run everything at once—just the essentials.

Understanding this difference between starting and running watts prevents undersizing your generator, which could cause it to shut down when high-demand appliances turn on. It also prevents oversizing, which wastes money on unnecessary capacity and fuel consumption.

• Starting watts are typically 1.5 to 3 times higher than running watts for motor-driven devices
• Resistive loads like heaters and lights use roughly equal starting and running watts
• Generator nameplates always list both starting and running wattage capabilities
• Your generator must handle the highest combined starting wattage of devices you plan to run simultaneously

Practical takeaway: Gather the product manuals or specification sheets for appliances you want to power. Write down both the running watts and starting watts for each device. This list becomes your foundation for all sizing calculations.

Calculating Your Essential Loads

The first step in determining generator size is identifying which appliances and systems are truly essential during an outage. Most homes cannot run everything at once—nor should the generator be sized for maximum theoretical load. Instead, focus on what you actually need to maintain safety, health, and basic functionality.

Essential loads typically fall into these categories: climate control (heating or air conditioning), refrigeration, water systems, lighting, and medical equipment. Many people also want to power communications devices like phones and internet routers, as well as some cooking capability. Secondary priorities might include laundry, entertainment systems, or workshop equipment.

The National Electrical Manufacturers Association (NEMA) provides standard wattage estimates for common household devices. A typical kitchen refrigerator runs at 600-800 watts continuously but may need 1,200-2,000 watts to start. A sump pump operates at 800-1,500 running watts with 2,000-4,500 starting watts. A window air conditioning unit uses 1,000-1,500 running watts but requires 2,500-3,500 starting watts. These numbers vary based on the specific model, age, and efficiency rating of equipment.

To calculate your essential loads, create a detailed inventory. Note that you won't run all devices simultaneously. For instance, you might power the refrigerator, some lights, and a few outlets all at once, but probably not the water heater, air conditioner, and oven simultaneously. The generator size should handle your realistic simultaneous usage pattern, not theoretical peak capacity.

• Refrigerator/freezer: 150-800 running watts (600-2,000 starting watts)
• Sump pump: 800-1,500 running watts (2,000-4,500 starting watts)
• Well pump: 500-2,000 running watts (1,500-5,000 starting watts)
• Air conditioning window unit: 1,000-1,500 running watts (2,500-3,500 starting watts)
• Central air conditioning: 3,500-5,000 running watts (10,000-15,000 starting watts)
• Electric water heater: 4,000-5,500 watts (typically no surge)
• Furnace with blower: 750-1,500 running watts (1,200-2,000 starting watts)
• Well lights and outlets: 500-2,000 watts combined
• Television and computer setup: 100-500 watts
• Microwave: 1,000-1,500 running watts

Practical takeaway: Create a spreadsheet listing each essential appliance with its running watts and starting watts. Then identify which devices would realistically run at the same time (refrigerator plus lights plus phone charger, for example). Add up the starting watts of simultaneous devices to determine your minimum required generator capacity.

Step-by-Step Calculation Method

Now that you have identified essential loads and their wattage requirements, you can perform the actual sizing calculation. This straightforward process involves adding numbers and comparing totals to generator capacity options.

Start by listing all devices you want to power simultaneously. For most households, this realistic scenario includes a refrigerator, some lighting, several outlets for chargers and devices, a furnace or air conditioning unit, and possibly a sump pump or well pump. Write down the starting watts for each device in this simultaneous-use scenario.

Add all the starting wattages together. This total represents the peak power your generator must deliver at the moment all these devices are running and any motor-driven equipment is starting up. For example, if you're running a refrigerator (starting 1,800W) plus a furnace (starting 2,000W) plus lights and outlets (500W), your peak starting wattage need is 4,300 watts.

Next, add up the running watts for the same devices. This number shows what the generator must sustain continuously once everything is running. Using the same example: refrigerator (700W) plus furnace (1,000W) plus lights and outlets (400W) equals 2,100 watts running.

Your generator must meet both requirements. It must be rated for at least the highest starting wattage (4,300W in the example) and must sustain at least the running wattage (2,100W). You would select a generator rated for at least 4,300 starting watts and 2,100 running watts or higher. In practice, generators are sold in standard sizes like 5,000W, 7,500W, 10,000W, and so on.

A useful guideline: add 20-25% to your calculated starting watts to provide a safety margin for unexpected loads or future additions. In the example above, 4,300W plus 25% equals 5,375W, making a 6,000-watt or larger generator a practical choice.

• List all devices for simultaneous operation
• Record starting watts (surge watts) for each device
• Add all starting watts together for your peak requirement
• Record running watts for each device
• Add all running watts together for your continuous requirement
• Select a generator rated for both requirements
• Add 20-25% margin to account for variability and future needs
• Round up to the next available generator size

Practical takeaway: Use this calculation template: Write down 5-7 essential devices you'll run together. Find their starting watts from manuals or online specification sheets. Add those numbers. If the total is 4,500W, you need at least a 5,500-6,000W generator. If it's 8,000W, you need at least a 9,500-10,000W generator. Match this number to available generator sizes from manufacturers.

Common Sizing Mistakes and How to Avoid Them

Many people make predictable errors when selecting generator size, leading to either inadequ