Generator Size Calculator

Add up wattage of all appliances/devices you want to run simultaneously. Check appliance labels or manuals for wattage. If only amps listed: Watts = Amps × Volts (120V for most appliances, 240V for large appliances). Account for starting watts (surge): Motors require 2-3× running watts to start (refrigerator, AC, well pump, sump pump). LED/lights use rated watts only. Add 20-25% safety margin to prevent overload. Example: Refrigerator (700W running, 2100W starting) + lights (300W) + TV (200W) = 2600W surge, 1200W running. Need 3500W+ generator.

Running watts (rated watts): Continuous power needed once device is operating. Starting watts (surge watts): Brief high power needed to start motors/compressors. Devices with motors need surge power: Refrigerator: 3× running watts, air conditioner: 2-3× running watts, well pump: 3-4× running watts, furnace blower: 2-3× running watts, sump pump: 2-3× running watts. Devices without surge: LED lights, TV, computer, phone chargers use only running watts. Generator must handle highest surge wattage of any single device, not total of all surges (they don't all start simultaneously).

Essential (safety/health): Refrigerator/freezer (food preservation), furnace/boiler (winter heating), well pump (water supply), sump pump (flood prevention), medical equipment (life safety). Important (comfort): Lights (basic illumination), phone chargers (communication), garage door opener (vehicle access). Nice to have: Air conditioner (summer comfort), TV/internet (entertainment), microwave (convenience), coffee maker (sanity). Most homes can't run everything simultaneously - prioritize based on outage season and needs. Rotate non-essential loads.

Whole house generators sized by home size and systems: Small home (1500 sq ft): 10-12 kW handles essentials, 15-20 kW handles most loads including AC. Medium home (2500 sq ft): 15-20 kW for essentials + some comfort, 20-25 kW for full house including AC. Large home (4000+ sq ft): 25-30 kW for essentials, 30-48 kW for full loads. Factors increasing size: Central AC (5-7 kW), electric water heater (4-5 kW), electric range (5-8 kW), well pump (1-2 kW). Natural gas/propane whole house generators: automatic transfer switch, runs indefinitely with fuel supply, professional installation required, $3000-15,000 installed.

Portable generators ($400-2000): Gasoline powered, manual start, extension cords or manual transfer switch, store when not in use, 2000-10,000 watts typical. Pros: Lower cost, versatile (camping, job sites), no installation needed. Cons: Manual setup, fuel storage issues, noise, carbon monoxide danger (outdoor only). Standby generators ($3000-15,000): Natural gas/propane, automatic start during outage, automatic transfer switch, permanently installed. Pros: Automatic operation, runs indefinitely, whole house power, quieter. Cons: High cost, professional installation, maintenance. Choose portable for occasional outages, standby for frequent outages or critical medical needs.

Safety is critical - carbon monoxide kills: NEVER run indoors, garage, or enclosed space - CO is deadly and odorless. Place 20+ feet from house, point exhaust away from windows/doors. Use in dry location, never in rain (electrocution risk). Ground generator properly if required. Use heavy-duty outdoor extension cords rated for wattage. Don't overload - respect wattage limits. Turn off generator before refueling (fire risk). Use transfer switch (prevents backfeed to power lines - kills utility workers). Install CO detectors in home. Let cool before storing. Run monthly to keep operational. These rules save lives.

Runtime depends on fuel capacity and load: Portable gasoline generators (typical): 2000W inverter: 6-8 hours on 1 gallon at 25% load. 5000W conventional: 8-10 hours on 5 gallons at 50% load. 7500W: 8-10 hours on 7 gallons at 50% load. Higher loads = less runtime (often 50% shorter at full load). Standby natural gas/propane: Runs indefinitely with continuous fuel supply. Propane tanks: 250 gallon tank = 2-3 days continuous at 50% load, 500 gallon tank = 5-7 days. Natural gas from utility = unlimited. Fuel stabilizer extends stored gasoline life to 1-2 years.

Inverter generators produce clean, stable power vs conventional generators: Clean power (low THD): Safe for sensitive electronics (computers, phones, TVs). Conventional generators: Power fluctuations can damage electronics. Quieter operation: 50-60 dB vs 70-80 dB for conventional. Fuel efficient: Auto-throttles based on load, 25-50% better fuel economy. Lighter weight: Easier to move and store. More expensive: 50-100% price premium. Best for: Tailgating, camping, RVs, powering electronics. Unnecessary for: Basic lights, refrigerator, well pump (use cheaper conventional). Worth premium if running computers, medical equipment, or value quiet operation.