Whole-House Generator Sizing Guide

A power outage lasting hours is an inconvenience. One lasting days — from a hurricane, ice storm, or grid failure — can mean spoiled food, frozen pipes, a non-functional well pump, or lost access to life-sustaining medical equipment. A properly sized whole-house generator eliminates all of that.

But sizing a generator incorrectly is an expensive mistake. Too small and it can’t support your loads — or worse, runs under load and damages equipment. Too large and you’ve paid for capacity you’ll never use while burning more fuel than necessary. This guide walks through the sizing process step by step.

Standby Generator vs Portable Generator

Before sizing, decide which type of generator fits your situation.

Standby Generators (Home Generators)

Standby generators are permanently installed, connected to natural gas or propane, and start automatically when the utility power fails — typically within 10–30 seconds.

Advantages:

Automatic operation — no action needed during outage
Fuel supply is essentially unlimited (connected to gas line or large propane tank)
Weather-protected in a permanent enclosure
Sized for full-house loads — can power everything simultaneously
Higher output — typically 7.5 kW to 22 kW for residential

Disadvantages:

Significant installation cost ($3,000–$15,000+ installed)
Requires a licensed electrician and often a gas contractor
Annual maintenance required

Portable Generators

Portable generators run on gasoline (or dual-fuel with propane). You roll them out during an outage, connect loads via extension cords or a manual transfer switch, and manually fuel them.

Advantages:

Much lower purchase price ($500–$3,000)
Can be used at job sites and remote locations
No installation required (though a transfer switch is strongly recommended)

Disadvantages:

Must be operated outdoors — carbon monoxide risk
Limited run time (6–12 hours per tank)
Loud and inconvenient during extended outages
Cannot run all loads simultaneously — requires load management

Bottom line: If you want reliable, automatic, whole-house backup power, a standby generator is the right choice. If you need emergency backup for essential circuits only and want to minimize cost, a properly connected portable generator works.

Step 1: Inventory Your Electrical Loads

The foundation of generator sizing is a load inventory. List every electrical device you want to run during an outage, and find the wattage for each.

Essential Load Categories

HVAC:

Central air conditioning — typically 2,500–5,000 watts running, 7,500–10,000 watts starting surge
Heat pump — similar to AC
Gas furnace (blower only) — 400–800 watts running
Electric heat — extremely high demand; often excluded from generator loads

Refrigeration:

Refrigerator — 150–400 watts running, 800–1,200 watts starting
Chest or upright freezer — 100–300 watts running

Kitchen:

Microwave — 1,000–1,500 watts
Electric range or oven — 1,000–5,000 watts per burner/element (usually excluded from generator sizing)
Dishwasher — 1,200–2,400 watts

Water:

Well pump — 750–2,000 watts running, 2,000–5,000 watts starting surge
Sump pump — 400–1,000 watts running, 1,000–2,000 watts starting

Lighting:

LED lighting loads are minimal — budget 500–1,000 watts for whole-house basic lighting

Electronics and Communication:

Computers, TVs, routers — 100–500 watts total typical household

Medical Equipment:

CPAP/BiPAP — 100–400 watts
Oxygen concentrator — 150–600 watts
Home dialysis — check specific equipment specs

Step 2: Understand Running Watts vs Starting Watts

Every motor-driven device (refrigerators, air conditioners, well pumps, sump pumps) requires significantly more power to start than to run. This starting surge (also called inrush current) typically lasts less than a second but can be 2–3x the running wattage.

Your generator must handle the highest starting surge of any single device, added to the total running watts of everything else operating at that moment.

Example:

Central AC running: 3,500 watts
Refrigerator running: 200 watts
Lights: 400 watts
Total running load: 4,100 watts
AC starting surge (AC starts while everything else is running): 8,000 watts
Peak demand when AC starts: ~12,000 watts

This is why a generator that covers your “running” loads may fail when the AC cycles on — you need to account for the starting surge.

Soft-Start Kits

Modern inverter generators and some standby generators can power loads they couldn’t otherwise handle because they more gently ramp up voltage. Soft-start kits (such as the MicroAir EasyStart for mini-splits and window AC units) dramatically reduce starting current — down to 60% or more. If motor starting surge is limiting your generator choice, a soft-start kit for the AC is worth investigating.

Step 3: Prioritize Your Loads

For most homes, running everything simultaneously during an outage is neither necessary nor practical. Prioritize:

Tier 1 — Life and Safety:

Well pump or sump pump
Medical equipment
Basic lighting
Communication (phones, internet)

Tier 2 — Comfort and Food Safety:

Refrigerator and freezer
Furnace/heat pump
One bathroom and kitchen

Tier 3 — Full Comfort:

Central AC
Multiple appliances
All rooms lit

A whole-house standby generator is sized for Tier 2 or Tier 3 operation. A portable generator is typically sized for Tier 1 or partial Tier 2.

Step 4: Calculate the Required Generator Size

For Essential Circuits Only (Partial Load)

Add up the running watts of your Tier 1 and Tier 2 loads, then add the starting surge of the largest single motor:

Example (essential loads):

Well pump running: 1,000 W
Refrigerator: 200 W
Furnace blower: 600 W
Lights: 400 W
Total running: 2,200 W
Well pump starting surge: 3,000 W additional
Peak demand: 5,200 W

A 5,500–6,500 watt generator handles these loads comfortably.

For Whole-House Operation (All Loads)

Add your total running load, then add the largest starting surge (typically the central AC):

Example (whole-house):

AC running: 4,000 W
Refrigerator: 200 W
Well pump: 800 W
Furnace fan: 600 W
Lights and electronics: 800 W
Total running: 6,400 W
AC starting surge: add 5,000 W
Peak demand: 11,400 W

A 12–14 kW standby generator handles this load. For most medium-sized homes (1,500–2,500 sq ft) with central AC, a 14 kW standby generator is the most common choice.

Standard Sizing by Home Size

Home Size	Recommended Standby Generator Size
Under 1,500 sq ft	7.5–10 kW
1,500–2,500 sq ft	10–16 kW
2,500–3,500 sq ft	16–20 kW
3,500+ sq ft	20–22 kW +
Large appliances (EV charger, hot tub)	Add 3–5 kW per large load

Popular Standby Generator Models

Generac 7043 22kW (Guardian Series)

Generac is the most widely installed standby generator brand in the US. The 22kW Guardian is their flagship residential unit, powered by a 999cc Generac-built engine running on natural gas or LP. It includes WiFi monitoring via the Mobile Link app, automatic weekly self-testing, and a 5-year limited warranty.

Rated power: 22 kW (natural gas), 19.5 kW (LP) Best for: Large homes, those who want Generac’s nationwide service network

Generac 7031 11kW

For homes that don’t need the full 22kW, the 11kW Guardian is a more economical option that still handles most whole-house loads except simultaneous high-demand appliances. The same auto-transfer, WiFi monitoring, and warranty as the larger units.

Best for: Medium homes with manageable AC loads

Kohler 14RESAL

Kohler’s residential standby generators are known for their build quality and the onboard Decision-Maker controller that manages utility monitoring and transfer. The 14kW model handles most residential loads and uses a fast 10-second transfer time.

Best for: Homeowners who prioritize build quality and fast transfer time

Briggs & Stratton 040626

Briggs & Stratton’s standby line offers competitive pricing and solid reliability. The 10kW model is a value leader for essential-circuit coverage in smaller homes.

Best for: Budget-conscious buyers who want a reputable brand

Installation Requirements

A standby generator installation involves multiple trades and several permits:

Electrical: Transfer switch installation (either a manual transfer switch panel or automatic transfer switch), generator interconnect wiring. Requires a licensed electrician and typically an electrical permit.

Gas/Plumbing: Natural gas line sizing and extension to the generator location, or LP tank installation. Requires a licensed plumber or gas contractor.

Mechanical: Generator pad and enclosure installation. Some jurisdictions require specific setbacks from windows, doors, and gas meters.

Permits: Most jurisdictions require a permit for generator installation. Expect inspections of both electrical and gas work.

Utility notification: Some utilities require notification when a generator with automatic transfer is installed, to protect line workers from backfeed during maintenance. An approved transfer switch prevents backfeed automatically.

Generator Maintenance

Standby generators require regular maintenance to be reliable when you need them:

Weekly self-test — built into modern generators; run under a light load to keep the engine ready
Annual oil and filter change — typically every 200 hours or annually, whichever comes first
Spark plug replacement — every 2 years or per manufacturer schedule
Air filter inspection — annually or when operating in dusty conditions
Load bank test — every 1–2 years for larger commercial units; optional but useful for residential

Most generator installers offer annual maintenance contracts — worth considering if you’re not mechanically inclined.

Sizing a whole-house generator correctly is a meaningful investment that protects your family and property during extended outages. Get multiple installer quotes, verify the installer is licensed and familiar with your generator brand, and make sure all permits and utility notifications are completed properly.