Generator Interlock Kit vs Transfer Switch: Cost, Safety and Which One a Grid-Down Plan Needs

Short answer: both an interlock kit and a transfer switch let a portable generator power house circuits without sending electricity back onto the utility line. An interlock is usually cheaper and lets you choose any circuit in your panel; a manual transfer switch costs more but is simpler to use and limits you to pre-wired circuits. Either should be installed with a power inlet box and, in most places, a permit and licensed electrician.

Why you need either one

Plugging a generator into a wall or dryer outlet with a "suicide cord" (male-to-male) backfeeds the house. If the main breaker is on, power can flow onto the grid and the step-up transformer can energise lines that utility workers believe are dead. It can also fry the generator when grid power returns. The US Consumer Product Safety Commission and utilities warn against it. The National Electrical Code requires transfer equipment for connecting a generator to premises wiring.

How an interlock kit works

An interlock is a mechanical plate or slide bolted to your panel cover. It physically prevents the main breaker and a dedicated generator backfeed breaker from being on at the same time. Power comes in from an outdoor inlet box via a generator cord.

• Typical cost: kit often under $200; with inlet, breaker and electrician, many installs land somewhere in the $500–$1,500 range depending on region.
• Pros: run any circuit you choose; cheaper; whole-panel flexibility.
• Cons: you must manage load yourself by switching breakers off; kit must be listed for your specific panel brand.

How a manual transfer switch works

A transfer switch is a small subpanel wired to selected circuits (often 6 to 10). Each circuit has a switch to choose Line or Generator.

• Typical cost: the unit often a few hundred dollars; installed totals frequently $800–$2,000+.
• Pros: harder to overload; easy for family members to use; some include wattmeters.
• Cons: limited to pre-wired circuits; more wiring.

Automatic transfer switches

Standby generators use automatic transfer switches that sense outages and switch over. They are part of a standby system costing thousands to install and are a different tier from portable setups.

Which suits a grid-down plan?

For most households with a 5,000–9,000 W portable, an interlock gives the most flexibility at the lowest cost. If several people may need to run it, or your panel has no listed interlock, a transfer switch is the cleaner choice. Either way, run the generator outdoors, well away from doors and windows, to avoid carbon monoxide. Our rapid-deploy generator plan, fuel storage guide and well pump sizing guide cover the rest.

Solar generators

Some large portable power stations can also feed a transfer switch or interlock via an inlet if they provide 240 V or the right output. Many cannot. Check the manufacturer's documentation before connecting any battery unit to house wiring.

Sources

• CPSC portable generator safety: cpsc.gov
• OSHA on generator backfeed hazards: osha.gov
• NFPA 70, National Electrical Code, Article 702 (optional standby systems): nfpa.org

Frequently asked questions

Is a generator interlock kit legal?

Yes, when the kit is listed for your panel and installed to code, often with a permit. Rules vary by jurisdiction.

Is an interlock or transfer switch cheaper?

An interlock is usually cheaper, especially if you need many circuits.

Can I plug my generator into a dryer outlet?

Not safely. Backfeeding without transfer equipment can energise utility lines and is a code violation.

Do I need an electrician?

Many jurisdictions require a licensed electrician or permit for panel work. It is strongly recommended either way.