How do I calculate what size generator I need?

To calculate generator size, list all electrical appliances you want to run simultaneously. Sum up their running (continuous) wattage. Next, identify the single highest starting (surge) wattage requirement among motorized appliances and add that surge difference to your total running watts. Finally, multiply the total by 1.20 (adding a 20% safety margin per NEC 80% continuous duty rating) to determine your recommended generator size.

What is the difference between running watts and starting surge watts?

Running (continuous) watts is the ongoing electrical power required to keep an appliance operating. Starting (surge) watts is the extra power needed for 2 to 3 seconds to ignite electric motors (compressors, pumps, blowers).

What size generator do I need to power a whole house?

For basic emergency home backup (refrigerator, lights, TV, Wi-Fi, microwave, sump pump), a 5,000 to 7,500 Watt generator is sufficient. To power central AC, electric water heaters, and well pumps via a transfer switch, an 8,500 to 12,000 Watt portable or 14,000 to 24,000 Watt whole-house standby generator is recommended.

How many watts does a central air conditioner require?

A typical 3-ton (36,000 BTU) central AC requires approximately 3,000 to 3,500 running watts and 9,500 to 10,500 starting (LRA) watts. Installing a soft-start kit can reduce starting surge by up to 50-70%.

How do I convert Generator Watts to kW, kVA, and Amps?

To convert Watts to kW, divide by 1,000 (e.g., 7,500 W = 7.5 kW). To calculate single-phase Amps at 240V, divide Watts by Volts (e.g., 7,500W / 240V = 31.25 Amps). To calculate kVA for 3-phase commercial generators, divide kW by Power Factor 0.8.

Why should I add a 20% safety margin to my generator wattage?

Adding a 20% safety buffer prevents running your generator at 100% capacity continuously, avoiding overheating, excessive fuel burn, and voltage spikes that can damage sensitive electronics.

Generator Size Calculator

Generator Size Calculator | What Size Generator Do I Need?

Don't guess on your generator size. Use our free generator size calculator to calculate exact running watts, starting surge wattage, kW, kVA, and amps for home backup, RVs, emergency power, or standby generators.

1. Select Household Appliances

Choose from refrigerators, central air conditioners, sump pumps, lights, and heaters to calculate total continuous running watts.

2. Calculate Starting Surge Watts

Motor-driven appliances require 2x to 4x their running wattage during startup. We automatically add the highest starting surge requirement.

3. Recommended Generator Size

Includes a recommended 20% safety capacity margin to protect engine efficiency, prevent overload trips, and ensure clean voltage output.

Whole House Generator Sizing by Square Footage

Home Size	Essential Circuits Only	Essentials + Central AC (with Soft Start)	Full Whole-House Standby
1,000 – 1,500 sq ft (2 Bed)	3,500 – 5,000 W	6,500 – 8,000 W	10,000 – 14,000 W
1,600 – 2,400 sq ft (3-4 Bed)	5,500 – 7,500 W	8,500 – 11,500 W	16,000 – 20,000 W
2,500 – 3,500 sq ft (4-5 Bed)	7,500 – 9,500 W	12,000 – 15,000 W	22,000 – 24,000 W
3,600+ sq ft (5+ Bed / Dual AC)	9,500 – 12,000 W	18,000 – 22,000 W	26,000 – 48,000+ W

Standard Generator Sizing Guidelines

2,000 – 3,000 Watts: Ideal for RV camping, tailgating, small electronics, TV, mini-fridge, and LED lighting.
3,000 – 5,000 Watts: Small home emergency backup. Powers refrigerator, freezer, sump pump, lights, and microwave.
5,000 – 8,500 Watts: Medium home backup. Powers well pump, furnace blower, refrigerator, lights, and small window AC unit.
8,500 – 12,000 Watts: Heavy-duty portable generator with transfer switch. Powers central AC (up to 3-ton with soft start), well pump, and essential home circuits.
14,000 – 24,000+ Watts: Whole house standby automatic generator. Powers entire home seamlessly including central air, electric water heater, stove, and clothes dryer.

Common Household Appliance Wattage Chart

Appliance	Running (Continuous) Watts	Starting (Surge) Watts
Refrigerator / Freezer	700 W	2,200 W
Central AC (3-Ton / 36,000 BTU)	3,500 W	10,500 W
Sump Pump (1/2 HP)	1,050 W	2,150 W
Well Pump (1/2 HP)	1,000 W	2,100 W
Furnace Blower Fan (1/2 HP)	800 W	2,300 W
Microwave Oven	1,000 W	1,000 W
Electric Water Heater	4,500 W	4,500 W
Portable Space Heater	1,500 W	1,500 W
Wi-Fi Router & Modem	20 W	20 W
LED Lights (10 Bulbs)	100 W	100 W

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