How do I size a generator for my house?

Add the running watts of all appliances you want to power. Then find the one with the highest starting surge and add that extra starting surge to your running total. Round up to the next standard generator size.

What is the difference between running watts and starting watts?

Running watts is the continuous power a device needs during normal operation. Starting watts (surge watts) is the brief extra power required to start motors (refrigerators, AC compressors, pumps). Starting watts can be 2-3x the running watts.

What size generator do I need for a 2000 sq ft house?

A typical 2,000 sq ft home needs 7,500–12,000 watts to run essential systems including central AC (3-ton), refrigerator, lights, and kitchen appliances. A whole-home standby generator is typically 15,000–20,000 watts.

Generator Sizing Calculator — How Many Watts Do You Need?

Generator	Cord AWG	Max Length
Up to 1,800W	16 AWG	25 ft
Up to 2,400W	14 AWG	50 ft
Up to 3,600W	12 AWG	100 ft
Up to 6,000W	10 AWG	100 ft

Generator

Cord AWG

Max Length

Up to 1,800W

16 AWG

25 ft

Up to 2,400W

14 AWG

50 ft

Up to 3,600W

12 AWG

100 ft

Up to 6,000W

10 AWG

100 ft

AWG	Ampacity	Use Case
#14	15A	15A circuits
#12	20A	20A circuits, small gen
#10	30A	30A transfer, med gen
#8	40A	40A circuits
#6	55A	50-60A gen/transfer
#4	70A	Large gen feeder
#2	95A	100A+ standby gen

AWG

Ampacity

Use Case

#14

15A

15A circuits

#12

20A

20A circuits, small gen

#10

30A

30A transfer, med gen

40A

40A circuits

55A

50-60A gen/transfer

70A

Large gen feeder

95A

100A+ standby gen

How to Size a Generator in 3 Steps

List Your Loads

Check every appliance you want to run during an outage. Note both the running watts (labeled on the device) and starting watts (2-3× for motors).

Calculate Total Watts

Sum all running watts. Then add the single largest starting surge (e.g., central AC) to the running total of everything else to get your starting peak.

Choose the Right Size

Pick the next standard generator size (3500 / 5000 / 7500 / 10000 / 12000 / 15000 W) above your starting watt total. Add 20% headroom for long-term reliability.

Key Terms

Running Watts

The continuous wattage a device consumes during normal operation. Shown on the nameplate or specification label.

Starting Watts

The brief surge of power required to start motors (refrigerators, ACs, pumps). Typically 2–3× running watts for the first few seconds.

kVA

Kilo-volt-amperes — the apparent power rating of a generator. kW = kVA × power factor. Most residential generators are rated in kW.

Transfer Switch

A device that safely disconnects your home from utility power before connecting the generator, preventing dangerous backfeed onto utility lines.

Power Factor

The ratio of real power (kW) to apparent power (kVA). Resistive loads = 1.0; motor loads = 0.8. A generator's kW rating assumes PF = 0.8.

Load Shedding

Strategically turning off non-essential loads when generator capacity is limited, to ensure critical loads like medical equipment or the refrigerator stay on.

Real-World Examples

Example 1

Essential Backup (Weekend Cabin)

Loads: Refrigerator (700W run/2200W start), 5 LED lights (50W), Router (50W), TV (150W), Window AC (1200W run/3600W start)

Running total: 2,150W | Starting peak: 3,600W (AC surge) + 950W = 4,550W

Result: 5,000W generator recommended. Propane or dual-fuel preferred for storage.

Example 2

Partial Home (Power Outage)

Loads: Central AC 3-ton (3500W/7000W start), Refrigerator, Sump pump (800W/2400W start), Lights, Microwave

Running total: 6,350W | Starting peak: 7,000W + 2,850W = 9,850W

Result: 12,000W generator. 240V outlet required, transfer switch mandatory.

Example 3

Whole Home Standby

Loads: All circuits — AC 4-ton, dryer, range, water heater, all lights and electronics

Running total: ~16,000W | Starting peak: ~22,000W

Result: 20,000W+ standby generator with automatic transfer switch. LP or natural gas connection.

Generator Sizing: The Complete Guide

Why Getting the Size Right Matters

Undersizing a generator is dangerous — overloading causes voltage sag, which can damage sensitive electronics and overheat the generator's windings. Oversizing wastes money and runs inefficiently, burning more fuel per useful watt. The goal is to size 15–25% above your peak starting demand.

Motor Starting Surge: The Most Misunderstood Factor

Most homeowners calculate running watts and stop there. The critical oversight is motor starting surge. An induction motor (refrigerator compressor, AC compressor, well pump, sump pump) draws 2–3× its running current for 0.5–3 seconds at startup. Your generator must handle this surge without stalling. Modern inverter generators handle surges better than conventional generators of the same rated output.

Fuel Type Comparison

Gasoline is the most common choice for portable generators. It's widely available but has a 1-year shelf life (treated with stabilizer). Propane stores indefinitely and burns cleaner but costs more per BTU. Diesel generators are most fuel-efficient and last longer under continuous load, making them ideal for standby applications. Dual-fuel generators (gas + propane) offer flexibility during fuel shortages.

Inverter vs. Conventional Generator

Conventional generators produce power at a fixed RPM (3600 RPM = 60Hz). They're less expensive but louder and less fuel-efficient at partial load. Inverter generators vary engine speed to match load, producing cleaner power (lower THD), ideal for electronics. For whole-home backup, conventional generators typically offer better value per watt. For camping or sensitive electronics, inverter generators are superior.

Transfer Switch Requirements

The National Electrical Code (NEC Article 702) requires a means to prevent backfeed when connecting a generator to home wiring. A manual transfer switch (interlocked breaker or dedicated panel) is required for any generator connected to house wiring. Automatic transfer switches (ATS) are required for standby generators. Running a generator without a transfer switch is illegal and dangerous to utility workers.

Frequently Asked Questions

What size generator do I need to run a central air conditioner?

A 3-ton central AC requires a generator of at least 10,000–12,000 watts to handle the 7,000-watt starting surge plus other household loads. A 4-ton unit needs 12,000–15,000 watts. Always account for other simultaneous loads.

Can I run a well pump on a portable generator?

Yes. A 1/2 HP well pump needs about 1,000 running watts and 2,100 starting watts. A 5,000-watt generator can handle it alongside a refrigerator and lights. Make sure your generator has a 240V outlet for well pumps requiring 240V.

How long will a generator run on 5 gallons of gas?

At 50% load, most 7,500-watt generators consume 0.5–0.6 gallons per hour. Five gallons provides roughly 8–10 hours of runtime. Fuel consumption rises sharply at higher loads — at 75% load expect 0.75–0.9 gal/hr.

Do I need a transfer switch for a portable generator?

Yes, if you're connecting to your home's wiring. Using extension cords directly from the generator doesn't require a transfer switch, but connecting to your panel does — it's required by the NEC and prevents dangerous backfeed to the utility grid.

What is the difference between kW and kVA for generators?

kW (kilowatts) is real power — what your appliances actually consume. kVA (kilovolt-amperes) is apparent power, which includes reactive components. For generators, kW = kVA × power factor. Most residential loads have a PF of 0.8, so a 10 kVA generator delivers 8 kW of real power.

Can a generator be too big for my needs?

Generators run most efficiently at 50–75% of rated capacity. Running a large generator at very low loads (10–20%) causes "wet stacking" in diesel generators and carbon buildup. It also burns more fuel per useful watt. Size your generator to run at 50–75% load under normal conditions.

How do I calculate the fuel cost to run a generator?

Fuel cost/hr = consumption rate (gal/hr) × fuel price ($/gal). A 7,500W generator at 50% load uses ~0.5 gal/hr. At $3.50/gal, that's $1.75/hr or $42/day. Propane and diesel rates differ — use this calculator's Fuel tab for your specific scenario.

What appliances use the most starting watts?

The biggest starting loads are air conditioners (2–3× running watts), sump pumps, well pumps, refrigerator compressors, and chest freezers. Always add the single largest starting surge to your running total to size correctly.

Should I get a standby or portable generator?

Portable generators (3,500–12,500W) are lower cost ($500–$3,000) and good for occasional outages. Standby generators (7,500–20,000W+) start automatically on utility failure, run on natural gas or LP, and cost $3,000–$15,000 installed. If you experience outages more than 2–3 times per year or have medical equipment, standby is worth the investment.

What gauge wire do I need for a 7,500-watt generator?

A 7,500W generator at 240V draws 31 amps (7500/240). For a 30-amp transfer switch feeder, use #10 AWG copper minimum. If run is over 50 feet, upsize to #8 AWG to limit voltage drop. Always match wire gauge to the transfer switch amperage.

Can I power my electric dryer or water heater with a generator?

Yes, but they're high-draw loads. An electric dryer uses 5,000W and an electric water heater uses 4,000W. Running both simultaneously requires a 12,000W+ generator. During an outage, consider skipping these loads to conserve fuel for critical systems.

Is it safe to use a generator in the rain?

Generators must be kept dry during operation. Use a generator tent or canopy (never fully enclosed) to protect it from rain while maintaining ventilation. NEVER operate inside a garage, basement, or enclosed space — carbon monoxide is deadly.

How often should I run a standby generator?

Standby generators should run under load for 20–30 minutes weekly or monthly (most have an automatic exercise mode). This keeps fuel fresh, lubricates seals, and ensures the generator is ready when needed. Log each exercise run.

What is a dual-fuel generator?

Dual-fuel generators run on either gasoline or propane via a switch. Propane stores indefinitely, making dual-fuel ideal for emergency preparedness. Most dual-fuel generators produce slightly less power on propane than gasoline due to energy density differences.

Do I need a permit to install a standby generator?

In most jurisdictions, yes. Standby generator installation requires an electrical permit (and sometimes a gas permit for the fuel line). The transfer switch installation must be inspected. Always pull permits — this protects your home's insurance coverage and ensures code compliance.

Generator Sizing Calculator