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

A backup generator keeps critical systems running during power outages - refrigerators, sump pumps, furnace blowers, well pumps, and medical equipment. The choice between portable and standby comes down to budget, convenience, and how much of the house you need to power. Either way, the safety rules are absolute: generators produce carbon monoxide in lethal concentrations, and improper connection to the house wiring can electrocute utility workers restoring power on the lines outside your home.

Portable vs. Standby Generators

Portable generators range from $500 to $2,000 and run on gasoline or dual fuel (gasoline plus propane). You wheel them into position during an outage, start them manually with a pull cord or electric start, and connect appliances either with heavy-duty extension cords or through a transfer switch wired to your electrical panel. Output ranges from 3,000 to 12,000 watts depending on the model, and you will need to refuel every 8 to 12 hours during continuous operation.

Standby generators cost $3,000 to $15,000 installed and represent a fundamentally different approach. They mount permanently on a concrete pad outside the house, connect to the electrical panel through an automatic transfer switch, and run on natural gas or propane from a dedicated supply. When utility power drops, the automatic transfer switch detects the loss and signals the generator to start - typically within 10 to 30 seconds. When power is restored, the system transfers the load back to utility power and shuts the generator down. No manual intervention at any point.

The output range for standby units is 10,000 to 40,000 watts, which covers anything from essential circuits to the entire house including central air conditioning. Popular residential brands include Generac, Kohler, and Briggs and Stratton, with Generac holding the largest market share in the home standby category.

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 lasting more than a few hours. A portable generator handles the basics for occasional short outages - keeping the fridge cold, a few lights on, phones charged, and a space heater or fan running depending on the season.

Sizing: How Much Power Do You Need?

Generator sizing starts with a load calculation. List every appliance you need to run during an outage and add up their running wattage. Then note the starting wattage for anything with a motor, because motors draw two to three times their running wattage for the first few seconds when starting up. This surge demand is what trips an undersized generator.

Essential loads for most homes break down as follows: refrigerator at 600 watts running and 1,800 watts starting, sump pump at 800 watts running and 2,400 watts starting, furnace blower at 500 watts running and 1,500 watts starting, lights and small electronics at 500 to 1,000 watts, and a well pump (if applicable) at 1,000 watts running and 3,000 watts starting.

A 5,000 to 7,500 watt portable generator handles these essential loads for most homes, provided you manage the startup sequence carefully. Start the highest-surge appliance first (usually the well pump or sump pump), let it stabilize, then add the next load. Trying to start everything simultaneously will overload the generator even if the running watts are within capacity.

A 10,000-watt portable generator adds enough headroom for a window air conditioning unit or a few additional circuits. A 20,000-watt or larger standby generator powers the entire house including central AC, which alone draws 3,000 to 5,000 watts running and up to 7,000 watts on startup.

Inverter generators deserve a specific mention. They produce clean, pure sine wave power that is safe for sensitive electronics like computers, televisions, and medical devices. Standard open-frame generators produce a modified sine wave that can damage sensitive equipment or cause it to malfunction. If you are powering electronics directly from the generator (not through a UPS or power conditioner), an inverter generator is the safer choice. Honda and Yamaha make some of the most reliable inverter models, though they cost more per watt than conventional generators.

Transfer Switches

Never connect a generator to your house by plugging it into a wall outlet (known as backfeeding) or wiring it directly to the electrical panel without a transfer switch. Backfeeding sends power onto the utility lines outside your home. Utility workers repairing downed lines assume those lines are dead. Backfed power can electrocute them. It also damages your generator when utility power is restored unexpectedly.

A manual transfer switch costs $200 to $500 installed and mounts next to your main electrical panel. It contains a lever or series of switches that disconnect selected circuits from the utility and connect them to the generator input. You start the generator, confirm it is running and producing stable voltage, then flip the transfer switch to move those circuits to generator power. Selected circuits - typically the refrigerator, sump pump, furnace, a few lighting circuits, and a couple of outlet circuits - are wired through the transfer switch during installation.

An automatic transfer switch (ATS) costs $500 to $2,000 installed and is standard equipment with standby generators. It monitors utility voltage continuously. When power drops below a threshold, the ATS sends a start signal to the generator, waits for the generator to reach stable output, then transfers the load. When utility power returns and holds stable for a set period (usually 5 to 10 minutes), the ATS transfers the load back and signals the generator to shut down.

An interlock kit is a less expensive alternative to a transfer switch for portable generators, costing $50 to $100 plus electrician installation. It is a mechanical plate that mounts on the panel cover and physically prevents the main breaker and the generator backfeed breaker from being turned on at the same time. You flip the main breaker off, flip the generator breaker on, and power from the generator feeds selected circuits in the panel. The interlock ensures you cannot have both sources on simultaneously, which prevents backfeeding.

Carbon Monoxide Safety

Generators produce carbon monoxide in concentrations that can be lethal within minutes in an enclosed space. More people die from generator-related CO poisoning during power outages than from the storms that caused the outages. The CDC documents dozens of deaths following every major hurricane, ice storm, and widespread power event. This is not a theoretical risk.

The placement rules are simple and non-negotiable: never run a generator indoors, in a garage (even with the door open), in a basement, in a crawlspace, under a covered porch, 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 and away from any neighboring structures.

Have working CO detectors on every floor of the house, especially near sleeping areas. During an outage, hardwired CO detectors without battery backup are useless. Battery-powered or battery-backup CO detectors are the only ones you can count on when the power is out. Test them before storm season.

If a CO detector alarms, leave the house immediately with everyone including pets. Call 911 from outside. Do not re-enter the building until emergency services have tested the air and cleared it. Carbon monoxide is odorless and colorless. By the time you feel symptoms - headache, dizziness, nausea, confusion - your ability to get yourself out may already be compromised.

Newer portable generators from most major manufacturers include a CO safety shutoff feature that automatically stops the engine if CO levels near the unit reach a dangerous threshold. This is a valuable safety feature but it is not a substitute for proper placement outdoors.

Fuel and Runtime

Gasoline is the most common fuel for portable generators. A typical portable unit holds 4 to 8 gallons and runs 8 to 12 hours at 50% load on a full tank. The practical challenges with gasoline are storage and availability: gas stations need electricity to pump fuel, so during a widespread outage the fuel supply dries up quickly. Store gasoline in approved containers (no more than 5 gallons each), add fuel stabilizer (like Sta-Bil) if storing longer than 30 days, and rotate your supply every 6 months. Never refuel a running generator. Shut it down, let it cool for a few minutes, then refuel. Gasoline vapors ignite easily on hot engine surfaces.

Propane burns cleaner than gasoline and stores indefinitely without stabilizer. A 20-pound propane tank (the standard barbecue size) runs a portable generator for roughly 5 to 8 hours at half load. Larger permanently installed tanks (100 to 1,000 gallons) supply standby generators. Propane has about 10% less energy per gallon than gasoline, so runtime per gallon is slightly shorter, but the storage advantages more than compensate.

Natural gas provides an unlimited fuel supply for standby generators connected to a gas utility line. No tanks to fill, no fuel to store, no refueling during an outage. The tradeoff is lower energy content per unit compared to propane or gasoline, which means a generator is slightly derated on natural gas - a unit rated at 20kW on propane may produce only 18kW on natural gas.

Diesel is available on larger standby generators (typically 20kW and above) and is the most fuel-efficient option. Diesel stores longer than gasoline without treatment and diesel generators tend to have longer service lives. However, diesel generators are less common in residential installations and generally cost more upfront.

Maintenance

A generator that will not start during an outage is worse than no generator at all, because you planned around having it. Maintenance is straightforward but it has to actually happen.

For portable generators, change the oil after the first 20 hours of operation (break-in period) and then every 50 to 100 hours thereafter. Check and clean the air filter monthly during periods of use. Inspect the spark plug annually and replace if worn or fouled. Most importantly, run the generator for 30 minutes once a month with an electrical load connected - a few lamps or a space heater. This circulates oil through the engine, charges the battery (if equipped with electric start), and burns off moisture in the fuel system. If you are storing the generator for more than 30 days without running it, either drain the fuel system completely or fill the tank and add fuel stabilizer.

Standby generators handle most of their own maintenance scheduling. They run an automatic weekly exercise cycle, typically 5 to 15 minutes under no load, to keep the engine lubricated and the battery charged. You will hear it kick on at the same time each week. Change the oil and filter per the manufacturer's schedule, which is typically annually or every 200 hours of operation. Have a qualified technician service the unit once a year for a full inspection covering coolant levels, spark plugs, battery condition, air filter, fuel connections, and the transfer switch.

Keep the area around any generator clear of debris, leaves, vegetation, and combustible materials. A running generator gets hot and can throw sparks from the exhaust. Maintain at least 3 feet of clearance on all sides for ventilation and service access.

Frequently Asked Questions

Can I Run My Whole House on a Portable Generator?

Not practically. A large portable generator at 10,000 watts can handle essential loads but not the entire house simultaneously. Central air conditioning alone draws 3,000 to 5,000 watts running. A portable generator is designed for selective, essential loads - you choose which circuits get power and manage the total draw. If you need whole-house backup, a standby generator sized to your main panel (typically 20,000 to 40,000 watts) is the appropriate solution.

How Long Can a Standby Generator Run Continuously?

Natural gas standby generators can run indefinitely as long as the gas supply remains active. Propane standby generators run until the tank is empty - a 500-gallon tank powering a 20kW generator at half load lasts roughly 5 to 7 days. All generators need periodic maintenance during extended outages. Most manufacturers recommend an oil change every 100 to 200 hours of continuous runtime. During major events like week-long ice storms, this maintenance becomes part of the operating routine.

Do I Need a Permit for a Standby Generator?

In most jurisdictions, yes. The installation involves electrical work (transfer switch connection to the panel), potentially gas line work (running a new gas line or tapping an existing one), and a concrete pad. Each component may require a separate permit and inspection. The electrical permit is the safety-critical one - it ensures the transfer switch is properly installed and that no backfeed condition can occur. Your generator installer should handle the entire permitting process as part of the installation contract.

Related Reading

• Gas Line Safety
• Sump Pump Maintenance