Generator Sizing Calculator

How to Size a Generator

A generator must handle both the steady running load (everything consuming power at the same time) and the peak surge (starting inrush from the biggest motor). Resistive loads like LED bulbs and electric heaters draw a constant amount. Induction motors — refrigerator compressors, well pumps, AC compressors, furnace blowers — draw 2-3× their running watts for the first second or two as they overcome inertia. The generator must be sized so that when the biggest motor starts while everything else is running, voltage does not sag enough to drop out sensitive electronics.

Typical Generator Sizes by Coverage

• 2,000-3,000 W (portable inverter): lights, phone chargers, small TV, fan. No fridge or well pump.
• 3,500-5,000 W: fridge + lights + small loads + sump pump. Essentials only, no AC or electric cooking.
• 5,500-7,500 W: fridge + lights + well pump + furnace blower + microwave. Typical "essential circuits" backup.
• 7,500-10,000 W: above + 1-ton AC or one electric range burner. Comfortable but not luxurious.
• 10,000-15,000 W: whole-house with central AC up to 2 tons, electric cooking, and full lighting.
• 17,000-22,000 W (standby): whole-house with 3-ton AC, water heater, electric dryer. Typical standby generator size.
• 22,000-27,000 W: whole-house with heat pump + electric water heater + EV charger. Larger standby units.

Starting Surge Rules

Only count the biggest motor starting surge once, not the sum of every motor's surge. Motors do not all start simultaneously — controls stagger them, and even if they start at similar moments, the surge duration is under 2 seconds so the generator recovers quickly. The calculator uses this rule: it sums all running watts plus the largest single (starting − running) delta. This matches how generator manufacturers and NEC 702.4 size residential standby systems.

Transfer Switch and Code

NEC Article 702 requires a transfer switch to disconnect the house from the utility before a generator feeds it. Manual transfer switches for 6-10 essential circuits cost $300-600. Automatic transfer switches for whole-house standby generators cost $1,500-3,000. Backfeeding through a dryer outlet with a double-male cord ("suicide cord") is illegal and deadly — it can electrocute utility lineworkers who believe the line is dead. Always use a proper transfer switch.

Fuel Runtime

A typical 7,500 W portable generator burns about 0.75 gallons of gasoline per hour at half load. A full 6.6-gallon tank runs about 8-9 hours. Standby generators on natural gas run indefinitely (as long as gas service is up). Propane standby units run 100-200 hours on a 500-gallon tank, depending on load. For long outages consider a dual-fuel portable or a propane standby.

Common Appliance Watts Reference

Typical running and starting (surge) watts for household appliances. Starting watts apply to motor loads only; resistive loads have no surge.

Typical packages: "essentials only" (fridge + furnace blower + lights + small loads + sump pump) = ~2,500 running / ~4,900 surge → 5,000-6,000 W generator. "Essentials + well pump" = ~3,700 running / ~6,100 surge → 7,500 W. "Whole-house with 3-ton AC" = ~8,000 running / ~17,000 surge → 17-22 kW standby.

Frequently Asked Questions

Can I run a fridge on a 2,000 W inverter generator?

Barely — a modern fridge with 2,200 W starting and 700 W running fits a 2,000-2,500 W generator only if nothing else is drawing power during the start.

What does "dual fuel" mean?

A generator that can run on both gasoline and propane. Gas for portability, propane for storage (propane doesn't go bad).

How long do standby generators last?

Standby units (Generac, Kohler, Cummins) are rated for 3,000-5,000 hours of life. At 50-100 hours of annual use that's 30-100 years of life.

Do I need a 240 V generator?

Yes if you want to run a well pump, electric dryer, electric water heater, or central AC — they all require 240 V. 120 V-only generators can power only 120 V appliances.

Inverter vs conventional generator — which is better?

Inverter units produce cleaner power (low THD) and run quieter at partial loads, but cost more per watt. Use inverter for sensitive electronics; conventional for construction or high-wattage loads.