Generator Sizing Guide: How to Choose the Right Backup Power
An undersized generator is worse than no generator at all — it creates a false sense of security, overloads under critical loads, and can damage both the generator and connected equipment. An oversized generator wastes money on capacity you will never use and runs inefficiently at low loads. Proper generator sizing requires calculating your essential loads, understanding starting wattage requirements, and matching the right generator type and fuel source to your needs. This guide walks through the sizing process for residential and small commercial applications.
Calculating Your Essential Load
Start by listing every appliance and system you need during a power outage. Essentials typically include the refrigerator (600-800 watts running, 2,000-2,400 starting), well pump or sump pump (800-1,500 running, 2,000-4,500 starting), heating system (500-2,500 watts depending on type), lights (100-500 watts for essential rooms), and medical equipment if applicable. A basic essential-load setup totals 3,000-5,000 watts running.
Add comfort loads based on your priorities: central air conditioning (3,500-5,000 running, 7,000-10,000 starting), electric range (2,000-5,000 watts), clothes dryer (5,000-5,500 watts), and hot water heater (4,500 watts). A whole-house generator that powers everything requires 15,000-25,000 watts for most homes. The gap between essential loads (3,000-5,000W) and whole-house loads (15,000-25,000W) explains why generator pricing varies so dramatically.
- Refrigerator: 600-800W running, 2,000-2,400W starting
- Sump pump: 800-1,500W running, 2,000-4,500W starting
- Furnace blower: 500-800W running, 1,500-2,400W starting
- Central AC (3-ton): 3,500W running, 7,000-10,000W starting
- Well pump: 1,000-2,000W running, 3,000-6,000W starting
- Essential lighting: 100-500W running
Portable vs Standby Generators
Portable generators (3,000-12,000 watts, $500-3,000) provide backup for essential loads during short outages. They require manual setup, fueling, and connection through a transfer switch or individual extension cords. Gas-powered portables run 8-12 hours per tank. They are economical for infrequent, short outages but impractical for multi-day events or whole-house power.
Standby generators (10,000-25,000+ watts, $3,000-15,000 plus installation) are permanently installed outside the home with an automatic transfer switch that activates within seconds of a power outage. They run on natural gas or propane (no refueling needed) and can power the entire home indefinitely. Installation costs $3,000-7,000 on top of the unit price. Standby generators are the right choice for frequent outages, medical needs, or whole-house backup.
Fuel Types and Runtime
Gasoline generators are the cheapest and most widely available but require manual refueling every 8-12 hours and gasoline storage (which degrades over 3-6 months). Dual-fuel generators ($800-3,000) run on gasoline or propane, providing flexibility — propane stores indefinitely and eliminates fuel degradation concerns. Diesel generators are more fuel-efficient and durable but cost more upfront and are primarily used for commercial applications.
Natural gas generators connect to your home gas supply for unlimited runtime without refueling — the ideal setup for standby generators in areas with natural gas service. Propane generators use a dedicated propane tank (250-1,000 gallons for standby units) that provides days to weeks of runtime depending on load and tank size. A 500-gallon propane tank powers a 20kW generator at half load for approximately 10 days.
Transfer Switches: Essential Safety Equipment
A transfer switch prevents backfeed — the deadly situation where generator power flows backward through the panel into utility lines, electrocuting lineworkers repairing the outage. NEC 702.6 requires a transfer switch for all permanent generator installations. Manual transfer switches ($200-500 plus installation) require you to physically switch from utility to generator power. Automatic transfer switches ($500-2,000 plus installation) detect the outage and switch automatically within 10-30 seconds.
Never connect a generator to your home electrical system through a standard outlet (backfeeding through a dryer outlet or male-to-male cord). This practice is illegal, violates NEC, voids insurance, and can kill utility workers. A proper transfer switch installation costs $500-1,500 for a manual switch or $1,000-3,000 for an automatic switch — a modest cost for safety and code compliance.
Installation Costs and Considerations
Portable generator with manual transfer switch: $1,500-4,000 total (generator $500-2,000, transfer switch $300-800, installation $500-1,200). This setup powers essential circuits during outages with manual startup and switching. It is the most cost-effective backup power solution for homes with infrequent outages.
Whole-house standby generator: $8,000-20,000 total (generator $3,000-10,000, automatic transfer switch $1,000-2,000, installation $3,000-7,000, permits $200-500). This provides seamless, automatic backup for the entire home. Natural gas units cost less to operate than propane; propane units work where natural gas is unavailable. Typical installation takes 1-2 days after the concrete pad is poured.
Frequently Asked Questions
What size generator do I need for my house?
For essential loads only (refrigerator, lights, furnace, sump pump): 5,000-7,500 watts. For essential loads plus central AC: 10,000-14,000 watts. For whole-house backup: 16,000-25,000 watts. The correct size depends on your specific loads — use a load calculator or have an electrician perform a load study.
How much does a whole-house generator cost?
Total installed cost for a whole-house standby generator is $8,000-20,000. The generator unit costs $3,000-10,000, the automatic transfer switch costs $1,000-2,000, and professional installation adds $3,000-7,000. Natural gas units have lower operating costs than propane. Annual maintenance costs are $200-400.
Do I need a transfer switch for a portable generator?
Yes, if you are connecting the generator to your home electrical system. NEC requires a transfer switch to prevent dangerous backfeed. Never use a male-to-male cord to connect a generator to an outlet — this is illegal and can electrocute utility workers. A manual transfer switch for a portable generator costs $500-1,500 installed.
How long will a generator run on a tank of gas?
A typical portable generator runs 8-12 hours on a full tank at half load. A 500-gallon propane tank powers a 20kW standby generator at half load for approximately 10 days. Natural gas standby generators run indefinitely on the utility gas supply. Runtime depends on load — running more appliances consumes fuel faster.