Generator Interlock vs. Transfer Switch: Which Is Right (and Legal)?
Both an interlock kit and a transfer switch safely connect a generator to your home’s panel — the interlock is cheaper and flexible, the transfer switch is more convenient (and required for automatic standby generators) — but whichever you choose, it must be a code-legal device installed to keep utility and generator power separated. What you must never do is “backfeed” by plugging a generator into an outlet — it can kill a lineworker and start a fire. Here’s how the two compare.
The Two Legal Options
| Feature | Interlock kit | Transfer switch |
|---|---|---|
| What it is | A sliding plate on the panel that prevents the main and generator breakers being on at once | A separate switch that transfers selected circuits to generator power |
| Cost (installed) | $150 – $500+ | $400 – $2,000+ (manual); more for automatic |
| Circuits powered | Any in the panel (you manage load) | Pre-selected circuits |
| Convenience | Manual; you manage what’s on | Cleaner switching; automatic with standby units |
| Required for | — | Automatic standby generators (whole-house generator cost) |
Both create a mechanical interlock so utility power and generator power can never be connected together — the core safety requirement.
Why You Must Never Backfeed
A “suicide cord” or plugging a generator into a wall outlet to power the house is illegal and deadly:
- It energizes the utility lines outside, which can electrocute lineworkers restoring power.
- It bypasses circuit protection — a fire risk.
- When the utility comes back, it can destroy your generator and equipment.
This is exactly why a code-approved interlock or transfer switch exists — it makes backfeeding physically impossible.
Which Should You Choose?
- Interlock kit — best value for a portable generator if you’re comfortable manually managing which breakers are on. Flexible (any circuit), cheap, but you balance the load yourself.
- Manual transfer switch — cleaner operation for a set of essential circuits; a bit more to install.
- Automatic transfer switch — required and bundled with a permanent standby generator that starts itself during an outage.
Local code and your panel type matter — some jurisdictions prefer one approach, and not every panel has a compatible interlock.
Installation, Permits, and Cost
This is panel work — have a licensed electrician install it with a permit and inspection. Beyond safety, an unpermitted install can void insurance and fail at home sale.
| Item | Typical cost |
|---|---|
| Interlock kit + install | $150 – $500+ |
| Manual transfer switch install | $400 – $2,000+ |
| Automatic transfer switch (with standby) | $2,000 – $5,000+ (plus generator) |
| Inlet box/wiring | $150 – $500 |
Related: whole-house generator cost, emergency electrician cost, and what to keep running in an outage (electrical emergency what to do). Verify the installer: questions to ask an electrician, verify a contractor’s license.
Frequently Asked Questions
What’s the difference between a generator interlock and a transfer switch? An interlock kit is a sliding plate on your existing panel that prevents the main breaker and the generator breaker from being on at the same time, letting you power any circuit while managing load manually. A transfer switch is a separate device that switches pre-selected circuits to generator power, and is required for automatic standby generators.
Is a generator interlock kit legal? Yes, a listed interlock kit installed to code is a legal, safe way to connect a portable generator — it’s recognized as long as it’s the correct kit for your panel and installed properly, usually with a permit. What’s illegal is backfeeding through an outlet without an interlock or transfer switch.
Is an interlock or transfer switch cheaper? An interlock kit is generally cheaper, often $150–$500 installed, versus $400–$2,000+ for a manual transfer switch. The interlock is also more flexible since it can power any circuit. The transfer switch is more convenient and is required for automatic standby generators.
Why can’t I just plug my generator into an outlet? Backfeeding through an outlet is illegal and deadly. It energizes the utility lines outside and can electrocute lineworkers, bypasses circuit protection and risks a fire, and can destroy your generator when utility power returns. A code-approved interlock or transfer switch prevents this by keeping the two power sources separated.
Do I need a permit and electrician for this? Yes. Connecting a generator to your panel is electrical work that should be done by a licensed electrician with a permit and inspection. An unpermitted or improper install is a safety hazard, can void your insurance, and may cause problems when you sell the home.
Last updated: June 15, 2026. Sources: ESFI and CPSC generator safety guidance (backfeeding hazards, transfer equipment); National Electrical Code transfer-equipment requirements; 2026 cost ranges per our electrical guides. Use a licensed electrician and pull a permit.