Once you own a generator big enough to run part of the house, the next question is how to connect it safely — and that comes down to a generator interlock kit vs a transfer switch. Both do the same critical job: they make sure your generator can never backfeed the utility grid, which is what protects line workers and keeps your setup legal. They just get there differently, and which one is right depends on your panel, your budget, and how hands-on you want to be. Here is how I lay it out for people.
First the safety rule that makes this whole topic matter, then the honest comparison.
The 30-second answer: Both an interlock kit and a transfer switch stop your generator from backfeeding the grid. An interlock kit is a mechanical plate on your main panel that blocks the main breaker and a backfed generator breaker from being on at the same time — cheaper, and it can power any circuit in the panel, but you switch loads manually. A transfer switch is a separate box that feeds pre-selected circuits and can be automatic on a standby unit — simpler to operate, more expensive, limited to its wired circuits. Either one must be installed by a licensed electrician. Never use a double-male “suicide cord.”
The rule underneath both of them
A generator plugged into your house without a proper switch can push power back out through your meter and onto the utility lines — energizing wires that a lineman believes are dead. That kills people. It can also destroy your generator when the utility power returns. Both the interlock and the transfer switch exist for exactly one reason: to make it physically impossible for your generator and the grid to be connected at the same time. That is why a plain double-male extension cord — a “suicide cord” — is illegal and genuinely deadly. Do not do it.
The interlock kit
An interlock is a listed metal plate that mounts on your existing main panel. You add a double-pole breaker for the generator (fed from an outdoor inlet box), and the plate slides so that either the main breaker is on or the generator breaker is on — never both. Its strengths are cost and flexibility: it is inexpensive hardware, and because it backfeeds the whole panel, you can run any circuit in the house, including big 240-volt loads like a well pump, just by turning individual breakers on and off. The trade-offs: your panel has to be a compatible type with an open breaker slot, you manage the load manually (keep your total under what the generator can handle), and it must be installed to code with a permit and inspection in most areas.
The transfer switch
A transfer switch is a separate switch or sub-panel wired to a specific set of circuits. A manual transfer switch is a small box with a switch per circuit — flip them to “generator” during an outage. An automatic transfer switch pairs with a permanently installed standby generator and does everything for you the moment the power drops. Its strengths are simplicity and clarity: labeled circuits, foolproof operation, and the automatic option for hands-off backup. The trade-offs are cost and flexibility — it is more expensive, and you are limited to the circuits it was wired for when installed.
Which one I steer people toward
If you want the most capability for the least money and you do not mind flipping breakers during an outage, an interlock kit is hard to beat — it turns your whole panel into a generator panel for the price of a plate and a breaker. If you want it simple, clearly labeled, or fully automatic with a standby unit, a transfer switch earns the extra cost. Both are correct answers; they fit different owners. And both require a licensed electrician and a code-compliant inlet — this is the one part of generator ownership I never tell people to DIY, because the failure mode is a house fire or a dead lineman.
Frequently asked questions
Is an interlock kit safe and legal?
Yes, when you use a listed kit made for your panel and it is installed to code by a licensed electrician with the required permit. The interlock is an accepted method for connecting a portable generator.
Can I install a transfer switch or interlock myself?
This is panel and service-entrance work — hire a licensed electrician. Most jurisdictions require a permit and inspection, and the safety stakes (backfeed, fire) are too high to guess at. I do not DIY this and I do not advise it.
Do I need an inlet box either way?
Yes. Both methods use a weatherproof power inlet on the outside of the house that your generator cord plugs into, wired to the panel. It is part of both installations.
What size generator works with these?
Enough to cover the loads you plan to run at once. With an interlock you manage that manually; with a transfer switch it is set by the wired circuits. Size to your real running load, not the surge peak.
Interlock vs transfer switch in one breath
- Both stop dangerous backfeed — that is the whole point, and a “suicide cord” is never an option.
- Interlock kit: cheaper, powers any panel circuit, but you switch loads manually and need a compatible panel.
- Transfer switch: simpler and available as an automatic standby setup, but pricier and limited to its wired circuits.
- Either way, use a licensed electrician and a proper inlet box — this is not a DIY job.
| Component | Typical price | Notes | Install |
|---|---|---|---|
| Interlock kit (panel-specific) | $50–150 | Plus a double-pole breaker | Electrician |
| Manual transfer switch | $300–600 | Pre-selected circuits | Electrician |
| Automatic transfer switch | $500–1,500+ | For standby generators | Electrician |
| Power inlet box | $40–120 | Needed for both methods | Electrician |
Pro service vs DIY math.
This is the rare generator topic where the pro is not optional. Electrician labor for an interlock and inlet typically runs a few hundred dollars on top of the hardware; a transfer switch install is more. It is real money, but it is the price of not backfeeding the grid or burning down a panel. Spend it, pull the permit, and get it inspected — the hardware is cheap compared to the consequences of doing it wrong.
Safety — quick reminders.
Never connect a generator to your house with a double-male cord — it energizes the grid and kills people. Both an interlock and a transfer switch must be installed by a licensed electrician to code. And never run the generator inside a garage or against the house — carbon monoxide is invisible and deadly. Full set in generator safety.
Once you know how you will connect it, size the machine with the what-size-generator guide so it covers your real load.