Generator Sizing Calculator

Watts: starting vs running

Every appliance has TWO wattage ratings:

• Running watts (continuous): What it draws once running steady.
• Starting watts (surge): The 2-5 second spike when an electric motor first starts. Typically 2-3× running watts.

Your generator's peak (surge) rating must cover the largest single starting load PLUS running loads already on. Its continuous rating must cover the sum of running watts. Generators advertise both numbers; the smaller one is what matters for sustained use.

Typical wattage by appliance

• Refrigerator (full-size): 700 running / 2,200 starting
• Window AC (10,000 BTU): 1,200 running / 3,600 starting
• Central AC (3 ton): 3,500 running / 5,000-7,500 starting (often the biggest surge in a house)
• Furnace blower: 600 running / 1,800 starting
• Well pump (1/2 HP): 1,000 running / 2,500 starting
• Sump pump (1/2 HP): 1,050 running / 2,200 starting
• Microwave (1,000W cooking): 1,500 running / no surge
• Coffee maker: 600-1,500 running
• Lights (LED): 50-100 watts per room
• Laptop, phone charger, router: 100-300 running total
• Space heater (1,500W): 1,500 running / 0 starting
• Electric water heater (40 gal): 4,500 running / 0 starting
• Electric range (full): 5,000+ running

By scenario: minimum generator size

• Camping / tailgate: 1,000-2,000W inverter. Lights, fans, phones, small fridge.
• RV hookup (30-amp service): 3,500-4,000W to run a 13,500 BTU AC + microwave + lights simultaneously.
• Power outage essentials (fridge + lights + furnace + phones): 3,000-4,000W continuous, 5,000-7,000W surge.
• Whole-home backup with central AC: 9,000-14,000W continuous. Standby generators.
• Construction / job site: 5,000-7,500W for compressors, miter saws, lights.

Inverter vs conventional generators

• Conventional: Cheaper per watt. Heavier, louder (typically 72+ dB). Engine runs at fixed RPM regardless of load — burns more fuel at light loads. Lower-quality "noisy" power can damage electronics without surge protection.
• Inverter: Quieter (~50-58 dB), lighter, more fuel-efficient at low loads. Clean sine-wave output safe for laptops/phones direct. 30-50% more expensive per watt. Most are 1,000-4,000W range; bigger inverter generators exist but cost climbs steeply.

For home backup running for hours at light load (lights + fridge cycling), inverter saves 30-40% fuel vs conventional. For job sites running heavy tools continuously, conventional is cheaper to buy and just as efficient.

Fuel options compared

• Gasoline: Cheapest to buy, easiest to find. 6-month shelf life (longer with stabilizer). Most common choice.
• Propane: Indefinite shelf life. Slightly less power output than gas (10-15% less). Cleaner burn. Refill anywhere (BBQ tanks).
• Dual-fuel (propane + gas): Versatility. Default to propane for storage; switch to gas if propane runs out.
• Diesel: Most fuel-efficient and longest-running engines. Heavier, more expensive, harder to find at consumer level. Standby/industrial use.
• Natural gas: Permanent home installation only. Unlimited runtime as long as gas main is on. Standby units only.

Connection and safety

• Never use extension cords through walls. Get a transfer switch ($300-700 installed) for safety and legality.
• Carbon monoxide kills. Run generators outside, 20+ feet from any window or door. CO buildup in attached garages — even with doors open — has killed people.
• GFCI outlets on generator are required for outdoor use in most code jurisdictions.
• Grounding rod: Hardwired installations require a code-compliant grounding rod.