Backup Generators: Portable vs. Standby, Sizing, and Safe Operation

Portable vs. Standby Generators

Portable generators ($500-2,000): gasoline or dual-fuel (gasoline + propane) units that you roll into position during an outage. You start them manually, connect appliances with extension cords or through a transfer switch, and refuel them every 8-12 hours. Output: 3,000-12,000 watts.

Standby generators ($3,000-15,000 installed): permanently mounted on a concrete pad outside the house, connected to the electrical panel through an automatic transfer switch, and fueled by natural gas or propane. They start automatically within seconds of a power outage and run as long as fuel is available. Output: 10,000-40,000 watts.

A standby generator is the right choice if you have medical equipment that cannot lose power, a sump pump protecting a finished basement, or you experience frequent extended outages. A portable generator is sufficient for keeping the fridge cold, a few lights on, and a phone charger running during occasional short outages.

Sizing: How Much Power Do You Need?

List the appliances you need to run during an outage. Add up their running watts and note the starting watts for anything with a motor (refrigerators, sump pumps, AC compressors). A motor draws 2-3 times its running wattage for the first few seconds when starting.

Essential loads for most homes: refrigerator (600W running / 1,800W starting), sump pump (800W / 2,400W), furnace blower (500W / 1,500W), lights and electronics (500-1,000W), well pump if applicable (1,000W / 3,000W).

A 5,000-7,500 watt portable generator handles the essential loads for most homes. A 10,000-watt portable can add a window AC unit. A 20,000+ watt standby generator powers an entire house including central AC.

Inverter generators produce cleaner power (pure sine wave) that is safe for sensitive electronics. Standard generators produce modified sine wave power that can damage some electronics. If you are powering computers, TVs, or medical equipment directly, use an inverter generator.

Transfer Switches

Never connect a generator to the house by plugging it into a wall outlet or wiring it directly to the electrical panel without a transfer switch. This backfeeds power onto the utility lines, which can electrocute utility workers repairing the lines and damage the generator when utility power is restored.

A manual transfer switch ($200-500 installed) is a panel mounted next to your main electrical panel. It has a lever that disconnects the house from the utility and connects it to the generator. You start the generator, then flip the transfer switch. Selected circuits (the ones you want powered) are wired through the transfer switch.

An automatic transfer switch (ATS, $500-2,000 installed) senses the utility power loss, starts the standby generator, and transfers the load automatically. When utility power returns, the ATS transfers the load back and shuts down the generator. No manual intervention needed.

An interlock kit ($50-100, installed by an electrician) is a less expensive alternative to a transfer switch for portable generators. It is a mechanical device that prevents the main breaker and the generator backfeed breaker from being on simultaneously. You turn off the main breaker, turn on the generator breaker, and selected circuits receive power from the generator.

Carbon Monoxide Safety

Generators produce CO in lethal concentrations. More people die from generator CO poisoning during power outages than from the storms themselves. This is not an exaggeration — the CDC documents dozens of deaths every major hurricane and ice storm.

Never run a generator indoors, in a garage (even with the door open), in a basement, in a crawlspace, or within 20 feet of any window, door, or vent. Place it outdoors in an open area with the exhaust pointing away from the house.

Have working CO detectors on every floor of the house, especially near sleeping areas. Battery-powered or battery-backup detectors work during outages when hardwired detectors may not.

If the CO detector alarms: leave the house immediately, call 911, do not re-enter until emergency services clear the building. CO is odorless and colorless — by the time you feel symptoms (headache, dizziness, nausea), you may be unable to get out.

Fuel and Runtime

Gasoline: most common for portable generators. Store in approved containers, add fuel stabilizer if storing longer than 30 days, and never refuel a running generator (the fuel vapors can ignite on the hot engine). A typical portable generator runs 8-12 hours on a full tank at half load.

Propane: cleaner burning, stores indefinitely, and available from a permanently installed tank for standby generators. Slightly lower energy content per gallon than gasoline, so runtime per gallon is about 10% less.

Natural gas: unlimited fuel supply for standby generators connected to a gas line. No fuel storage or refueling. Lower energy content per unit than propane or gasoline, so the generator is derated (a 20kW generator on natural gas may produce only 18kW).

Diesel: available on larger standby generators (20kW+). Most fuel-efficient option. Diesel stores longer than gasoline without stabilizer. Less common in residential applications.

Maintenance

Portable generators: change the oil after the first 20 hours, then every 50-100 hours. Check and clean the air filter monthly during use. Run the generator for 30 minutes once a month with a load to keep it in operating condition. Drain or stabilize the fuel if storing for more than 30 days.

Standby generators: most run an automatic weekly exercise cycle (5-15 minutes under no load) to keep the engine lubricated and the battery charged. Change oil and filter per the manufacturer schedule (typically annually or every 200 hours). Have a professional service the unit annually — they check the coolant, spark plugs, battery, and transfer switch.

Keep the area around any generator clear of debris, vegetation, and combustible materials. A running generator gets hot and throws sparks from the exhaust.

Frequently Asked Questions