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Portable vs. Standby Home Generators: Sizing, Cost, and Safety

Published 8/13/2026

The short answer: A portable generator is the low-cost, flexible choice for keeping a few essentials running during an outage, but it must be run outdoors, well away from the house, and started and refueled by hand. A standby (whole-home) generator is permanently installed, starts automatically the moment the power fails, and can run your whole house — at several times the price. Whichever you choose, the single most important thing to get right is carbon monoxide safety: portable generators kill more than 80 people a year in the U.S., almost always because they were run too close to or inside a home.^[1]

The three types, in plain terms

Portable generators are gas-powered units, typically in the 3,500–10,000 running-watt range, that you roll out during an outage and either plug appliances into directly or connect to your home's circuits through a transfer switch. They are the least expensive option and must always be operated outdoors.^[8]

Inverter generators are a refined type of portable unit. They produce cleaner, more stable power that is safer for sensitive electronics, and they run noticeably quieter — a good inverter model can run at around 51 decibels, versus 70–100 dB for a conventional gas generator (about the difference between a conversation and a vacuum cleaner). The tradeoff is that they are generally smaller in output and cost more per watt.^[8]

Standby (whole-home) generators are permanently installed on a concrete pad and wired into the house through an automatic transfer switch. When the utility drops, the switch detects it and the generator "start[s] supplying power to your home seconds after an outage begins," whether or not anyone is home. They run on natural gas from the utility line or on-site liquid propane, so they can supply power continuously as long as fuel is available.^[7]

Carbon monoxide: the safety rules that matter most

Portable generator engines produce large amounts of carbon monoxide (CO) — an invisible, odorless gas. The Consumer Product Safety Commission reports that more than 80 people die each year from CO poisoning caused by portable generators.^[1] The rules that prevent these deaths are simple and non-negotiable:

Run a portable generator outside only — never inside a home, garage, basement, crawlspace, shed, or on a porch, even with doors and windows open.^[1]^[2]
Keep it at least 20 feet from the house, with the exhaust pointed away from doors, windows, and vents.^[1]^[2]^[3]
Install CO alarms — battery-powered or with battery backup — outside each sleeping area and on every level of the home.^[1]
Get to fresh air immediately if anyone feels headache, dizziness, weakness, nausea, chest pain, or confusion; these are CO symptoms.^[2]

Across all sources, unintentional CO poisoning kills more than 400 Americans a year.^[2] Newer portable generators may include a CO sensor that shuts the engine off automatically as gas builds up — models built to the ANSI/PGMA G300 or the stricter ANSI/UL 2201 standard — and these features save lives. But testing shows they are not a substitute for the 20-foot rule: with a generator running in a garage doorway, safety testers found units where the shutoff did not prevent CO from reaching unsafe levels indoors.^[5]

Never backfeed your home's wiring

A dangerous shortcut worth naming: never plug a portable generator into a wall or dryer outlet to power your house (a so-called "suicide cord"). OSHA is blunt about it — "never attach a generator directly to the electrical system of a structure... unless a qualified electrician has properly installed the generator with a transfer switch," because an improper connection "creates a risk of electrocution for utility workers and others."^[4] Backfed power can energize the utility lines a lineworker believes are dead. Use a manual transfer switch (or interlock) for a portable generator, and plug appliances directly into the unit with proper grounded cords otherwise.^[4] Standby units use an automatic transfer switch installed by a licensed electrician.^[7]

How to size a generator

Sizing comes down to watts. Every device has a running wattage (the continuous power it needs) and, for anything with a motor, a higher starting wattage (a brief surge at startup, often one to two times the running figure). The method: add up the running watts of everything you want to power, then add the single largest starting wattage on top.^[6]

Appliance	Running watts	Starting watts
Refrigerator	150–400	800–1,200
Sump pump (1/2 HP)	800–1,050	1,300–2,150
Furnace blower (1/2 HP)	300–800	800–1,600
Window AC (10,000 BTU)	900–1,200	1,800–2,400
Microwave	600–1,200	—
LED light bulb	8–15	—

Representative wattages; check the nameplate on your own appliances.^[6]

For backing up a few essentials — refrigerator, sump pump, furnace blower, and some lights — many households land in the 5,000–7,500-watt portable range. For whole-home coverage, standby units are commonly 10–18 kW for smaller homes and 22–28 kW for larger ones; as a rough guide, a 17–20 kW unit suits a home of about 1,500–3,000 square feet, while 22–25 kW covers roughly 3,000–5,000 square feet.^[7]^[9]

Fuel, runtime, and storage

Portable generators typically run on gasoline (some on propane), while standby units run on natural gas or propane.^[10] Two practical differences stand out:

Runtime. A portable unit runs from roughly an hour to about 14 hours on a tank depending on size and load, then needs refueling.^[8] A natural-gas standby unit can run as long as fuel is supplied, though air-cooled models should be shut down and inspected after every 24 hours of continuous operation.^[7]^[13]
Fuel storage. Gasoline does not keep indefinitely — regular gas lasts about 3–6 months stored, ethanol blends (E10) closer to 3 months, and fuel with stabilizer added can last 1–3 years.^[11] If you rely on a gasoline portable, rotate your stored fuel and keep stabilizer on hand. A natural-gas standby unit sidesteps fuel storage entirely.

What each costs

Type	Typical price range
Portable (gasoline)	~$500–$3,200^[9]
Portable (liquid propane)	~$600–$2,800^[9]
Standby unit + installation	~$5,000–$9,000 typical, up to $13,500+^[10]^[9]

A standby generator is as much an installation project as a purchase. HomeAdvisor puts the average installed cost around $5,159, with the unit itself running from about $2,000 for a small model to $6,500–$15,000 for the largest, plus labor of roughly $1,500–$5,000, permits, a concrete pad (around $1,000), and a transfer switch.^[10] Forbes cites an average total nearer $9,000 installed.^[9] Installation requires a licensed electrician and usually a gas hookup and permit — it is not a DIY job.

Maintenance and lifespan

Standby units need a professional service at least once a year, plus attention every 100–200 hours of runtime for oil, filters, and spark plugs.^[12] They also run a weekly self-test ("exercise") cycle; if you notice a missed cycle or rough running, have it checked.^[13] Well-maintained, a home standby generator can last 25–30 years, while a light-duty portable is generally good for a few hundred hours of use over 5–7 years.^[13]

Which should you buy?

Choose a portable (or inverter) generator if you want the lowest cost, only need to keep essentials running through occasional outages, and are willing to set it up outdoors and refuel it by hand. An inverter model is worth the premium if you will power computers or other sensitive electronics or want quieter operation.

Choose a standby generator if you experience frequent or long outages, want whole-home coverage that starts automatically (important if you travel or have medical equipment), and can invest several thousand dollars in professional installation.

Frequently asked questions

Can I run a portable generator in my garage if the door is open?

No. The CDC and CPSC are explicit that a generator must never run inside a garage, even with the door open — run it outside, at least 20 feet from the house, with exhaust pointed away.^[1]^[2]

How big a generator do I need?

Add the running watts of everything you want to power, then add the largest single starting wattage. Essentials often fit a 5,000–7,500-watt portable; whole-home standby units commonly run 10–28 kW depending on house size.^[6]^[7]

Do I need a transfer switch?

To power your home's built-in circuits safely, yes. Portables use a manual transfer switch; standby units use an automatic one. Never backfeed the panel through an outlet.^[4]

How long does stored gasoline last for my portable?

Roughly 3–6 months for regular gas, about 3 months for E10, and 1–3 years if you add fuel stabilizer. Rotate your supply so it is fresh when you need it.^[11]