What Size Generator Does My House Need?
If you have ever lost power during a Kansas City storm and sat in the dark wondering whether your sump pump was still running, you already understand why backup power matters. But when it comes to choosing a home standby generator, the most common question homeowners ask is simple: “What size do I need?”
The answer is more straightforward than most online calculators make it seem. For the majority of Kansas City homes with standard 200-amp electrical service, an 18kW Generac home standby generator provides reliable whole-home backup when paired with the right transfer switch and load management technology. Getting the size right matters. An undersized generator causes breakers to trip during outages. An oversized unit means spending more than necessary on equipment you will never fully use.
This guide walks you through how generator sizing works, what factors determine the right fit for your home, and why professional sizing from a qualified local team like Helio GreenTech is the most reliable way to get it right.
How Generator Sizing Works: Running Watts, Starting Watts, and Your Electrical Panel
Generator sizing is based on how much electrical power your home actually draws during an outage, not the maximum capacity printed on your electrical panel. This distinction trips up many homeowners and even some online sizing tools.
Your 200-Amp Panel Does Not Draw 200 Amps
A 200-amp main breaker panel is designed to safely handle up to 200 amps of electrical current at 240 volts, a theoretical maximum of about 48,000 watts. However, no typical home comes close to that at any given time. Most homes with 200-amp service actually draw between 80 and 90 amps at peak usage, putting real-world demand in the range of 19,000 to 21,600 watts.
You do not need a 48kW generator to back up a 200-amp home. You need a generator that can handle your actual peak demand, with smart load management to handle the moments when multiple high-draw appliances start simultaneously.
Running Watts vs. Starting Watts
Every appliance has two wattage ratings that matter for generator sizing.
Running Watts: The continuous power an appliance needs to operate. A central A/C unit might draw 3,500 to 5,000 running watts.
Starting Watts: Motor-driven appliances like A/C compressors, sump pumps, and refrigerators need a brief surge of two to three times their running watts to start up, lasting only a few seconds.
A 3-ton A/C unit running at 3,500 watts may need 7,000 watts to kick on. If your generator cannot handle that surge, the unit will not start, or the generator trips its overload protection. This is why load management technology matters, and why the U.S. Department of Energy recommends working with qualified professionals before purchasing a standby generator.
Why 18kW Generators Work for Most Kansas City Homes
For the majority of homes in the Kansas City metro with standard 200-amp electrical service, an 18kW Generac Guardian Series home standby generator is the right fit.
The 18kW Generac delivers 18,000 watts on liquid propane and 17,000 watts on natural gas. Since most Kansas City homes are connected to municipal natural gas lines, the practical output for local homeowners is 17kW of continuous power. Paired with Generac's load management technology, that is more than enough for a typical 200-amp home.
What an 18kW Generator Can Power
In a standard Kansas City home, an 18kW Generac with a 200-amp automatic transfer switch (ATS) handles:
Central air conditioning: One standard A/C unit (up to 4 tons) with managed starting loads
Refrigerator and freezer: Typically 100 to 800 running watts combined
Electric water heater: Approximately 4,500 watts, managed through load cycling
Sump pump: Critical for KC homes with basements, typically 800 to 1,500 watts
Furnace blower motor: Usually 300 to 800 watts
Lighting throughout the home: LED lighting pulls minimal power per circuit
Home office and kitchen appliances: Computers, routers, microwave, dishwasher, and more
The Technology That Makes 18kW Work
The reason an 18kW unit can back up a full 200-amp panel comes down to Generac's integrated load management technology in their 200-amp automatic transfer switches. The Smart A/C Module (SACM) manages up to four HVAC loads with no additional hardware. Optional Smart Management Modules (SMMs) extend control to eight additional loads.
When your A/C compressor kicks on, the controller temporarily pauses other high-draw appliances, such as the water heater or dryer, for a few seconds. Once the compressor is running, those loads are restored automatically. This prevents multiple high-draw appliances from surging simultaneously, which is the only scenario where a typical 200-amp home would briefly exceed 18kW.
Considering Whole-Home Backup Power?
Helio GreenTech can evaluate your home's electrical system and recommend the right generator size. Every consultation is free, educational, and completely no-pressure.
Key Factors That Affect Your Generator Size
While the 18kW recommendation covers most homes, several variables influence whether you need more or less capacity. A qualified installer will evaluate each of these during a professional site assessment.
#1 Electrical Service Size
Most Kansas City homes built or updated in the last 20 or 30 years have 200-amp service. Some older homes may still have 100-amp or 150-amp panels, which generally means lower total demand and potentially a smaller generator. If you are considering a main service panel upgrade, discuss generator sizing at the same time, since both decisions impact the same electrical infrastructure.
#2 Central Air Conditioning
Your A/C system is almost always the single largest electrical draw in the home. A 3-ton unit draws roughly 18 to 30 amps at 240 volts; a 5-ton unit can pull up to 50 amps. One A/C unit of 4 tons or less is handled comfortably by the 18kW with load management. Homes with dual A/C systems or oversized units are where 22kW or 24kW options come into play.
#3 Essential vs. Whole-Home Backup
Some homeowners only want critical circuits powered during an outage. Others want everything running as if the power never went out. A 200-amp ATS gives you whole-home coverage, and the 18kW unit with load management makes that possible without oversizing.
#4 Major Electrical Loads
Beyond A/C, your installer will evaluate every high-draw appliance in the home. Electric water heaters typically pull 18 to 25 amps at 240V, while an electric range or oven can draw 30 to 50 amps at 240V. Sump pumps are especially common in Kansas City due to clay soil and flood-prone basements, drawing 10 to 15 amps at 120V with starting surges two to three times higher. Homes outside municipal water systems may have well pumps pulling 10 to 20 amps. An electric clothes dryer draws 20 to 30 amps at 240V, and your furnace blower motor typically runs at 3 to 8 amps at 120V.
#5 Fuel Type
Generac generators are rated at peak output on liquid propane (LP). When running on natural gas, output is slightly lower because natural gas has a lower energy density. An 18kW generator produces 18,000 watts on LP but 17,000 watts on natural gas. A 22kW unit on propane produces roughly 19.5kW on natural gas.
#6 Future Electrical Needs
If you are planning to add an EV charger, battery storage, a hot tub, a pool pump, or a home workshop in the near future, your installer should factor those loads into the sizing calculation now. It is far more cost-effective to size correctly upfront than to replace the generator later.
The Helio GreenTech Difference: The Value of Professional Generator Sizing
Online sizing calculators provide a rough starting point, but they cannot replace a professional evaluation.
Every home's electrical system is different. Wiring age, HVAC efficiency, panel layout, code requirements, and the distance to the gas meter all affect the sizing and installation plan. A qualified installer performs a load calculation based on your actual panel, not on generic square-footage estimates.
Helio GreenTech approaches generator installations the same way we approach every service we provide: with precision, transparency, and a commitment to getting it right the first time. As a NABCEP-certified, BBB A-rated company voted Best Solar Company in Kansas City four years running, our team handles the full process from sizing consultation through professional installation and ongoing service.
We also understand how generators fit into the bigger energy picture. Homeowners who pair a Generac standby generator with solar panels and battery storage get the most complete energy resilience available. Solar reduces daily utility costs, battery storage covers shorter outages, and a standby generator ensures whole-home protection during extended grid failures. It is a layered approach that covers every scenario.
Generac: Built for Reliability
Seven out of ten homeowners who purchase a home standby generator choose Generac, making it the number one selling brand in America.
Generac's True Power Technology delivers clean electricity with less than 5% total harmonic distortion (THD), protecting sensitive electronics. The G-Force engine features pressure lubrication, cast-iron cylinder sleeves, and hydraulic lifters for extended life. Quiet-Test mode runs weekly diagnostics at reduced RPM. Free Mobile Link Wi-Fi monitoring lets you check generator status from any device. Every Guardian Series unit is designed, engineered, and assembled in the USA with a 5-year limited warranty and 24/7/365 customer support.
Generac Generator Sizing Questions
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Square footage alone does not determine generator size. What matters is your electrical service size, the appliances you want to power, and whether you use gas or electric for heating, water, and cooking. Most 2,000 square foot Kansas City homes with 200-amp service and a single A/C unit are well served by an 18kW Generac with load management.
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Yes. An 18kW Generac paired with a 200-amp automatic transfer switch and integrated load management can start and run a standard central A/C unit up to approximately 4 tons. The load management technology handles the starting surge by briefly pausing other high-draw appliances until the compressor is running.
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Running watts are the continuous power an appliance needs to operate. Starting watts are the brief burst of extra power needed to start motor-driven appliances like A/C compressors and sump pumps. Starting watts are typically two to three times higher than running watts and last only a few seconds.
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A whole-house generator with a 200-amp ATS powers every circuit so everything runs normally. A partial-house setup powers only selected critical circuits at lower cost. For homeowners in storm-prone areas like Kansas City, whole-home backup provides the best peace of mind.
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On natural gas, a Generac standby generator runs continuously for as long as the outage lasts, since municipal gas supply is independent of the electrical grid. On propane, runtime depends on tank size. A 500-gallon tank can typically power an 18kW generator for several days of normal use.
Protect Your Home with the Right Generator Size
Kansas City’s storm season isn’t letting up. Across the area, severe wind events, ice storms, and grid failures are happening more often, and multi-day outages are no longer unusual. The right standby generator, properly sized and professionally installed, can turn a stressful outage into a non-event.
Helio GreenTech is here to help you understand exactly what your home needs. No guesswork, no overselling, and no pressure. Whether you’re looking for a standalone generator or a complete energy-resilience system with solar, battery storage, and backup power, we handle everything from initial assessment to final installation.
One Call Fixes It All
From new solar installations and orphaned system rescues to storm damage repairs and complete roof replacements, Helio GreenTech is Kansas City’s premier, fully certified solar and roofing team. Our rapid response, skilled crews, and unwavering commitment mean we treat your home or business as if it were our own.
Choose Helio GreenTech with complete confidence for all your solar and roofing needs.
