Whole-House Generator Sizing Guide

A power outage lasting hours is an inconvenience. One lasting days — from a hurricane, ice storm, wildfire shutoff, or grid failure — can mean spoiled food, frozen pipes, a non-functional well pump, or lost access to life-sustaining medical equipment. A generator can reduce those risks when it is sized, installed, ventilated, fueled, and transferred correctly.

Sizing a generator incorrectly is an expensive mistake. Too small and it cannot support the loads you care about. Too large and you may pay for capacity, fuel, gas-piping work, and maintenance you do not need. This guide walks through the sizing process as a planning worksheet, not as a substitute for a licensed installer.

Generator Planning Decision Matrix

Goal or constraint	Better fit	Why
Keep refrigerator, lights, internet, sump pump, and furnace blower running	Critical-load panel or managed standby setup	Lower kW, lower fuel demand, clearer priorities
Run central AC, well pump, kitchen loads, and most outlets	Larger standby generator with load management	Motor starting surge and simultaneous loads matter
Occasional storm backup with manual setup	Portable generator plus listed transfer equipment or direct cords	Lower cost but requires safe outdoor operation and fuel handling
Medical equipment, mobility needs, or frequent outages	Standby generator and maintenance contract	Automatic transfer and service reliability matter
Dense lot, close windows, limited gas capacity	Professional site/gas review before shopping	Clearance, exhaust, gas meter, and code can decide feasibility

Do not backfeed a house through a dryer outlet or improvised cord. Portable and standby generators create carbon monoxide, fire, and electrocution risks when placed or connected incorrectly. CDC’s generator safety fact sheet and FEMA’s portable generator safety guidance both emphasize outdoor operation away from enclosed or partially enclosed spaces. NFPA 37 covers fire-safety criteria for stationary combustion engines, and CPSC’s stationary-generator hazard report is another reason placement and installer review matter.

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
• Can power selected cord-connected loads without panel work when used outdoors with correctly rated cords

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
• Needs listed transfer equipment installed by a qualified electrician if it will feed household circuits

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.

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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

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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.

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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.

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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

Use square footage only as an early screening tool. Actual sizing should come from load inventory, HVAC starting surge, fuel type, transfer strategy, and installer calculation.

Home Size	Early screening range
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

Generac’s home standby sizing workflow is a useful comparison point because it asks whether you want whole-home or selected-circuit backup and whether large loads like EV charging matter during an outage.

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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

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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.

Load Inventory Worksheet

Bring this to installer quotes:

Load	Running watts or amps	Starting/surge note	Must run?	Managed/optional?	Notes
Refrigerator/freezer		Motor surge	Yes	No	Food safety
Furnace blower or heat pump		Motor/compressor surge	Yes	Maybe	Heat source and auxiliary heat type
Sump pump		Motor surge	Yes	No	Basement flooding risk
Well pump		High motor surge	Yes	Maybe	Water pressure and pump HP
Medical device	Manufacturer label	Ask equipment supplier	Yes	No	Battery backup plan
Central AC	Nameplate/MCA/LRA	Usually largest surge	Maybe	Yes	Soft start/load management
Internet/phones		Low load	Maybe	Yes	Communications
Electric range/dryer/EV charger	High load	Usually excluded	No	Yes	Discuss lockout/load shed

Ask each installer to show which loads are included, which are managed or locked out, what transfer switch is proposed, whether gas piping is adequate, and what permit/inspection steps are included.

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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.

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Sizing a generator correctly is a planning exercise across electrical load, fuel supply, exhaust clearance, transfer equipment, maintenance, and local code. Get multiple installer quotes, verify licensing and brand familiarity, ask for the included-load worksheet in writing, and make sure permits, inspections, and utility notifications are completed properly.