How Do You Know What Size Backup Generator Your Home Needs?
- 5 days ago
- 4 min read
At a Glance: Generator sizing depends on what you need to power during an outage. A 7,000 to 10,000-watt portable generator handles the basics: refrigerator, sump pump, lights, and phone charging. A whole-home standby generator requires a load calculation based on your panel and what circuits you want covered. The average whole-home standby generator installation in Colorado runs $10,000 to $20,000 installed. Undersizing a generator is one of the most common and preventable mistakes homeowners make.
Picking the wrong generator size causes one of two problems. Too small, and you cannot run the appliances you need without tripping the generator's overload protection. Too large, and you paid for capacity you never use, and have a unit that short-cycles at low load, which damages the engine over time.
That's why a professional load assessment is an important part of the planning process. PH Electric provides Generator Installation Windsor CO services that begin with a detailed load calculation, helping homeowners determine the appropriate generator size based on their home's actual electrical demand. Spending a little extra time on this assessment helps prevent one of the most common and costly generator installation mistakes: choosing equipment that is either oversized or unable to meet the home's backup power needs.
What Is a Load Calculation and How Does It Work?
A load calculation adds up the starting wattage and running wattage of every appliance and system you want to power during an outage. Starting wattage is the surge a motor draws when it first starts, which is typically 2 to 3 times the running wattage. Running wattage is what the appliance draws during normal operation.
A central air conditioner with a 5-ton compressor draws approximately 7,500 watts running and 22,500 watts at startup. A sump pump draws 1,500 watts running and 3,000 at startup. A refrigerator draws 150 watts running and 600 watts at startup.
The generator must handle the largest simultaneous starting surge that will occur in operation. If the AC starts while the refrigerator is running, the generator must supply the AC startup surge plus the refrigerator running load simultaneously.
What Size Generator Handles the Basics?
A 7,000 to 10,000-watt generator handles the critical circuits in most homes without powering everything simultaneously.
Critical circuits typically include the refrigerator, the sump pump, one or two window AC units or fans, lighting circuits, the garage door opener, and device charging. Running these simultaneously requires approximately 5,000 to 8,000 watts of running capacity with adequate starting surge reserve.
A portable generator in this range costs $800 to $2,500 for the unit itself. It requires manual connection through a transfer switch to prevent backfeed into the utility grid. National Electrical Code Section 702.6 and Colorado state electrical code require a transfer switch for any generator connected to the home's wiring.
What Does a Whole-Home Standby Generator Cover?
A whole-home standby generator connects permanently to the home's electrical panel and starts automatically within seconds of a utility outage. It runs on natural gas or propane and does not require refueling during an extended outage.
Standby generators for average homes range from 14 kW to 24 kW. A 14 kW unit handles most homes that do not have electric heat. A 20 kW unit handles homes with electric heat, multiple HVAC systems, or large kitchen appliances on the emergency circuit.
Generator sizing calculators can provide a preliminary estimate, but they should not be the final basis for selecting equipment. Every home's electrical demand is different. A professional load calculation evaluates your actual panel, circuits, and anticipated power usage to recommend a generator that is appropriately sized for your property.
What Does Installation Cost in Colorado?
A whole-home standby generator installation in Windsor in 2024 runs $10,000 to $20,000 complete. That range includes the generator unit, the automatic transfer switch, the gas line extension (if needed), the concrete pad, electrical permits from Weld County, and the labor for a licensed electrical and gas contractor.
The generator unit itself costs $3,000 to $8,000, depending on capacity. Generac's 18 kW Guardian series lists at approximately $5,000. Kohler's 20RESAL lists at approximately $6,000. Installation labor accounts for the rest.
Colorado requires permits and inspections for standby generator installation. The permit triggers an inspection that verifies the transfer switch, grounding, and gas connections meet NEC and Colorado-specific requirements.
What Maintenance Does a Standby Generator Need?
A standby generator that sits idle between outages still requires regular maintenance. The engine oil, spark plugs, and air filter need replacement on an annual schedule or after every 100 to 200 hours of operation. Most standby generators run a weekly self-test cycle for 5 to 10 minutes to keep the engine lubricated and verify function.
An annual professional service visit costs $150 to $300 and includes oil change, filter replacement, battery test, and load test to verify the generator performs at rated output.
Key Takeaways
A load calculation that adds starting and running wattages for each appliance is the only accurate method for generator sizing; square footage estimates consistently produce undersized or oversized results
National Electrical Code Section 702.6 requires a transfer switch for any generator connected to a home's wiring, making DIY direct connections both dangerous and code-violating
A 7,000 to 10,000-watt portable generator handles critical circuits in most homes; whole-home standby units require 14 kW to 24 kW depending on HVAC configuration
Whole-home standby generator installation in Colorado runs $10,000 to $20,000 complete including permits, transfer switch, pad, and all gas and electrical connections
Annual professional maintenance at $150 to $300 per visit and a weekly self-test cycle are the minimum upkeep requirements to ensure a standby generator starts reliably during an actual outage
Weld County requires electrical permits for generator installation; the required inspection verifies code compliance that protects the homeowner's insurance coverage and resale disclosure
A generator that starts reliably during an outage is the product of a correct size calculation and proper installation. A generator that almost works is worse than no generator, because it creates false confidence while failing to cover the loads you actually need.
