Heating and cooling (the big loads)
Kitchen and laundry
Lighting and electronics
8 Tips for Sizing a Home Backup Generator Correctly
Tip 1: Calculate Starting Watts, Not Just Running Watts
Most appliances require 2-3 times their running wattage to start up. A refrigerator rated at 600 running watts might need 1,800 watts to start the compressor. Your generator must handle starting watts for critical appliances. Add all starting watts for items you want to run simultaneously.
Tip 2: Prioritize Essential Loads, Not Maximum Loads
You don’t need to run everything simultaneously. Identify must-haves: refrigerator, furnace/AC, water heater, lights, and medical equipment. Size for these essentials, then add secondary loads if you have capacity. A 10-15 kW generator for essentials is more practical than a 30 kW generator to run everything.
Tip 3: Know Your Transfer Switch Amperage Limits
Your home’s main service panel and transfer switch have amperage ratings that limit generator size. A 100-amp panel typically supports a 20 kW generator maximum. A 200-amp panel supports 40+ kW. Check before buying an oversized generator that your panel can’t handle.
Tip 4: Propane vs. Natural Gas vs. Diesel Affects Sizing
Propane generators typically max out at 15-20 kW for home use. Natural gas (if available) can support larger generators but supply is fixed by meter size. Diesel generators are powerful but expensive and louder. Choose fuel based on availability and your needs, as fuel type limits practical generator size.
Tip 5: Climate Affects Performance and Altitude Affects Capacity
Generators perform at lower output in extreme heat or cold. A 20 kW generator at sea level might produce only 18 kW at 5,000 feet elevation. If you live in a hot climate or high altitude, size up slightly to account for performance loss.
Tip 6: Installation Location Affects Sizing and Cost
Outdoor concrete pad installation is cheapest. Indoor installation in a garage requires larger clearances and ventilation, increasing cost. Underground installation protects the generator but costs significantly more. Consider installation complexity when budgeting.
Tip 7: Calculate Your Actual Power Needs, Not Dealer Recommendations
Dealers often recommend oversized generators for profit. Use a calculator based on your actual appliances and peak simultaneous use. An 8-10 kW generator handles most home essentials. A 15-20 kW handles most homes including AC or electric heat. Don’t buy 30 kW unless you’ve calculated and verified the need.
Tip 8: Automatic Transfer Switches Ensure Safety and Convenience
A manual transfer switch is cheaper but requires you to physically switch power (dangerous if done wrong). An automatic transfer switch detects power loss and switches in seconds (essential for medical equipment or sump pumps). The extra cost is worth the safety and convenience.
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Where to Go From Here
Now that you understand starting watts, essential loads, transfer switch limits, fuel types, climate and altitude effects, installation locations, actual power needs, and automatic transfer switches, use our Generator Sizing Calculator to determine the right generator for your home. Enter your essential appliances and their starting wattages, and the calculator will show your recommended generator size.
Frequently Asked Questions About Generator Sizing
What if I size my generator too small?
It won’t start high-demand appliances or will shut down if you try to run too many things simultaneously. You’ll be frustrated and regret not sizing larger. Undersizing is the most common mistake.
Can I upgrade my generator later if I need more power?
You can add a second generator, but it’s expensive and complex. Better to size right initially. If you anticipate future expansion (like adding an electric vehicle charger), size for that now.
How often should I run my generator?
Run it under load for 30 minutes monthly to keep fuel fresh and ensure it starts during emergencies. Most failures are generators that won’t start because they haven’t been exercised. Monthly maintenance is essential.
What’s the difference between portable and standby generators?
Portable generators (5-15 kW) are cheaper and mobile but require manual setup and fuel management. Standby generators (10-40+ kW) are permanent, automatic, and expensive but provide true home backup. For whole-home protection, standby is better despite higher cost.
How much does it cost to install a home generator?
Portable: 500-2,000 including extension cords. Standby: 3,000-8,000+ installed, depending on fuel type and transfer switch complexity. Expect the installation cost to rival or exceed the generator cost.