The Short Answer: What Size Generator Do You Need?

Most homes between 1,500 and 3,000 square feet need a 20–22 kW standby generator to run all essential systems including central air conditioning. Smaller homes under 1,500 sq ft can often get by with a 14–17 kW unit. Larger homes over 3,000 sq ft or homes with electric vehicle chargers, pools, or electric ranges typically need 26–36 kW.

The single most common sizing mistake is choosing a generator based on home square footage alone. The real driver of generator size is your peak electrical load — the maximum wattage your home draws at any given moment. This guide shows you exactly how to calculate that number.

Step 1: Identify Your Critical Loads

Start by listing every appliance and system you want to run during a power outage. Most homeowners fall into one of two categories: those who want to run essential loads only (lights, refrigerator, well pump, a few outlets) and those who want whole home power (everything including central AC, electric range, and EV charger).

Appliance	Running Watts	Starting Surge
Central AC (3-ton)	3,500W	7,000–10,000W
Central AC (4-ton)	4,800W	9,600–14,000W
Well pump (1 HP)	750W	2,000–3,000W
Refrigerator	150–400W	800–1,200W
Electric range (one burner)	1,200–2,400W	Same
Electric water heater	4,000–5,500W	Same
EV charger (Level 2)	7,200W	Same
Lighting (whole home)	500–1,500W	Same
Sump pump (1/2 HP)	800W	1,300W
Furnace blower	400–800W	1,200–2,400W

Step 2: Calculate Your Peak Load

Add up the running wattage of all appliances you want to run simultaneously, then add the highest single starting surge in your list. The starting surge is the extra power a motor-driven appliance needs to start — typically 2–3x its running wattage.

Example for a 2,500 sq ft home with central AC, well pump, refrigerator, and lighting:

• Central AC (3-ton): 3,500W running
• Well pump: 750W running
• Refrigerator: 200W running
• Lighting: 800W running
• Total running: 5,250W
• Highest starting surge (AC): +7,000W
• Peak load: ~12,250W (12.25 kW)

This home needs a generator rated for at least 14 kW to handle the peak load with headroom. Most installers recommend sizing up 20–25% above your calculated peak load, making a 16–20 kW unit appropriate here.

Step 3: Choose Your Generator Size

Home Size	Recommended kW	Typical Models	Installed Cost
Under 1,500 sq ft	14–17 kW	Kohler 14RESAL, Generac 16kW	$7,000–$11,000
1,500–2,500 sq ft	20–22 kW	Kohler 20RESCL, Generac 22kW	$10,000–$16,000
2,500–4,000 sq ft	22–26 kW	Kohler 26RCL, Generac 26kW	$13,000–$20,000
Over 4,000 sq ft	36–48 kW	Kohler 38RCLB, Generac 48kW	$18,000–$30,000
With EV charger	Add 8–10 kW	Size up one tier	+$2,000–$4,000

The Well Pump Rule

If your home uses a well pump, this single appliance often determines your minimum generator size. A 1 HP well pump draws 750 watts running but surges to 2,000–3,000 watts at startup. If your AC also starts at the same time, the combined surge can exceed a smaller generator's capacity, causing it to shut down. Always confirm your generator's surge rating — not just its running wattage rating — before purchasing.

Natural Gas vs Propane: Does Fuel Type Affect Sizing?

Yes — propane generators typically produce 5–10% less power than the same generator running on natural gas. This is because propane has a lower energy density per cubic foot. If you are sizing a propane generator, add 10% to your calculated load to compensate. For example, if your peak load calculation suggests a 20 kW unit on natural gas, choose a 22 kW unit for propane.

Frequently Asked Questions

What size generator do I need for a 2,000 sq ft house?

A 20 kW generator is appropriate for most 2,000 sq ft homes with central AC. If you have a well pump or large appliances, size up to 22 kW.

Is a 22kW generator enough for a 2,500 sq ft house?

Yes, a 22 kW generator is sufficient for most 2,500 sq ft homes. It can run central AC (up to 4-ton), a well pump, refrigerator, lighting, and most other household loads simultaneously.

Can a generator be too big for my house?

Technically yes — an oversized generator runs at low load, which can cause carbon buildup (wet stacking) in diesel units. For natural gas and propane generators, oversizing is less problematic, but you will pay more upfront for capacity you rarely use. Aim to size within 20–30% above your peak load calculation.