What both of them are for
There is exactly one reason both of these devices exist: to make backfeed impossible.
Backfeeding means energizing your house wiring from a generator in a way that also sends power out through the meter and up the utility line. The transformer on the pole steps that up to distribution voltage, which means a lineworker who has isolated your street and is working on what they believe is a dead conductor can be electrocuted by your generator.
It is illegal essentially everywhere, and both devices prevent it mechanically rather than by asking you to remember. That is the point: they are designed so the dangerous configuration cannot be selected, not so that it is discouraged.
The transfer switch
A separate box mounted beside your main panel, wired to a chosen group of circuits. Each circuit gets a three-position toggle: Line, Off, Generator. In normal use everything sits on Line. In an outage you start the generator, plug it into the inlet, and flip the toggles.
What it does well:
- Per-circuit control. You choose which circuits are on the generator at any moment, which makes load shedding trivial.
- Load metering. Watt meters show what you are drawing on each leg, so balancing is a reading rather than a guess.
- Foolproof. The switch physically cannot connect a circuit to both sources.
- Obvious to operate. Anyone can be shown how in two minutes, which matters if the person home is not the person who installed it.
What it costs you: more hardware, a longer installation, and a fixed circuit list. A ten-circuit switch backs ten circuits chosen at installation, and adding an eleventh later means an electrician.
The interlock kit
A metal plate fitted to your existing panel that physically prevents the main breaker and a dedicated generator breaker being on at the same time. Turn off the main, slide the plate, turn on the generator breaker — and the panel is now fed from the generator instead of the utility.
What it does well:
- Every circuit is available. The whole panel is fed, so you can use any breaker you like — subject to the generator's output.
- Cheaper hardware. A plate rather than a second panel.
- Neater. No extra box on the wall.
- Flexible. Priorities can change during the outage without any rewiring.
What it costs you: discipline. Because every circuit is available, nothing stops you turning on more than the generator can supply — it will simply overload and trip, possibly repeatedly, at the least convenient moment. There is no metering, so managing load is entirely on you.
Interlock kits must also be listed for your specific panel make and model. A generic plate is not acceptable and inspectors check. Some panels have no listed interlock available at all, which settles the question by itself.
Side by side
| Property | Transfer switch | Interlock kit |
|---|
| Prevents backfeed | Yes, mechanically | Yes, mechanically |
|---|
| Circuits available | The ones wired to it, typically 6–12 | Every circuit in the panel |
|---|
| Load metering | Usually included | None |
|---|
| Overload protection from yourself | Limited circuit list caps it | None — you must manage it |
|---|
| Hardware cost | Higher | Lower |
|---|
| Panel compatibility | Works with any panel | Must be listed for your exact panel |
|---|
| Ease for a non-technical user | Labeled toggles, very clear | A sequence that must be followed |
|---|
| Permit and electrician | Yes, in most jurisdictions | Yes, in most jurisdictions |
|---|
Which to choose
A transfer switch if you have a defined critical-load list, if the person most likely to be home is not the person who understands the panel, or if you want metering so you can use the generator's full output without tripping it. This is the right answer for most households.
An interlock kit if you have a large generator, you want access to circuits that are not on a fixed list, you are confident about managing load, and a listed kit exists for your panel. It is also the better fit for a well pump or an electric water heater, which are awkward to put on a small transfer switch.
Neither if you only intend to run extension cords to a fridge and a couple of lamps. That is perfectly legitimate, needs no installation, and is how most people start. The switch becomes necessary the moment you want a furnace, a well pump or a sump pump — all of which are hard-wired.
Two things that apply to both
You need an inlet box. Neither device includes the weatherproof exterior inlet the generator cord plugs into. It mounts on an outside wall, ideally at least twenty feet from where the generator will stand, so the carbon monoxide clearance is achievable with a 25-foot cord rather than by moving the machine closer.
Both need an electrician and usually a permit. This is live-panel work where a mistake is fatal, and an unpermitted alteration to your service can affect an insurance claim. The labor typically exceeds the hardware cost for either option — budget for it, and do it in the spring rather than the week a storm is forecast, because electricians are unavailable exactly when you want one.
The full connection guide, including what not to do.
