Standby Generator Sizing Guide for Beginners

Posted on July 22, 2025

When the lights go out and everything grinds to a halt, the last thing you want is a generator that can’t keep up.

Let’s be honest, standby generator sizing isn’t exactly thrilling dinner table conversation. But if you’ve ever lost power mid-summer with a fridge full of groceries and two kids melting down faster than your AC unit, you know exactly how important it is. Choosing the right generator isn’t about guessing or crossing your fingers. It’s about understanding your needs, your loads, and, yes, maybe doing a little math (don’t worry, we’ll keep it light).

Whether you're outfitting a home or a small business, knowing how to choose generator size is the first step to keeping your power stable, your devices safe, and your life uninterrupted.

Why Standby Generator Sizing Isn’t Optional

Think of your generator like a backup quarterback. It’s not playing every down, but when the starter goes out, you better believe you need someone who can handle the job. If your generator’s too small, it’ll get overwhelmed trying to start your HVAC or sump pump. Too large, and you’re spending extra money on fuel and equipment you’ll never fully use. And no, a bigger generator isn’t always better. It's just more expensive and often less efficient.

When you properly size your standby generator, you extend its life, reduce wear and tear, and save money over time. You’re not just buying peace of mind; you’re making a smart, strategic investment.

First, a Quick Lesson in Wattage

Before you choose generator size, it helps to understand a few basics:

Running watts are what your appliances need to keep operating.
Starting watts are what they need just to turn on. Think of this as the short burst of energy your fridge needs to kick into gear.
Total load is the combined wattage of everything you want to run at once.
kW (kilowatts) is the unit of measure for your generator’s capacity; to get this, divide your total watts by 1,000.

Some items (like lights or TVs) don’t draw much. Others (like air conditioners, water heaters, and well pumps) are greedy. You’ll need to factor them all in.

Step-by-Step: How to Choose Generator Size the Right Way

This isn’t rocket science, but it does require a bit of planning. Let’s break it down:

1. Make a List of What You Can’t Live Without

Start with your essentials. For a home, this might be your refrigerator, some lighting, a few outlets, your furnace or AC, and your sump pump. For a small business, think about servers, POS systems, security equipment, and HVAC.

2. Find the Wattage

Check appliance labels or manuals for running and starting watts. If you’re not into digging behind your fridge, there are plenty of online calculators to estimate common appliances.

3. Add It All Up (Then Add a Little More)

Once you total your running watts and starting watts, divide by 1,000 to get your total kW. But don’t stop there. Add 20–25% more capacity to cover any power spikes or future additions. This buffer helps your generator run without strain.

4. Match the Load to the Right Generator

If your total need is around 10kW, a 12kW standby unit will do nicely. Got a business with 45kW needs? You’ll need a commercial-grade generator that can keep up. Residential units generally run from 10kW to 22kW. Commercial? Anywhere from 25kW to 150kW or more, depending on the industry.

Real-World Examples You’ll Actually Relate To

Say you’ve got a 2,400-square-foot home in a warm climate. You want to power the essentials: fridge, lighting, outlets, a 3-ton AC unit, and your Wi-Fi. You’re probably looking at around 14kW minimum, maybe 18kW if your AC is particularly power-hungry.

Now imagine a small office with four workstations, a server rack, climate control, and a few essential machines. Even without industrial-grade equipment, that business might need a 30–40kW unit to avoid downtime.

Choosing the wrong size isn’t just annoying; it can shut you down. And in a business setting, downtime means dollars lost.

Pitfalls to Avoid (Because Mistakes Here Are Expensive)

Let’s save you some pain:

Don’t forget to factor in startup watts; they can be double or triple the running wattage.
Resist the urge to "just get the biggest one" because oversizing can be just as problematic, especially for fuel efficiency.
Don’t ignore your HVAC system. It’s often the largest single load and easy to underestimate.
Don’t DIY your way through load analysis if you’re unsure. Electrical load planning is where a lot of well-meaning folks go wrong.

We’ve seen customers blow fuses, trip breakers, and even damage appliances by pushing too much load through too small a generator. It’s avoidable if you know how to size it right.

When in Doubt, Bring in the Pros

If you’re looking at your list of devices and scratching your head, don’t sweat it. You’re not alone. A professional load analysis doesn’t just help you choose generator size more accurately; it ensures code compliance, prevents installation mishaps, and helps you sleep better at night. Hatton Power & Propulsion has guided hundreds of homeowners and business owners through this exact process. We’ll walk your property, map out your loads, and deliver a clear, custom recommendation.

Power loss doesn’t have to mean total shutdown. When you get standby generator sizing right, you can weather any outage with confidence. If you're still unsure how to choose a generator size for your home or business, Hatton Power & Propulsion is here to help. Contact us today for a personalized generator sizing consultation that takes the guesswork and stress out of staying powered.