An interlock kit is a low-cost mechanical device that lets you safely power your whole electrical panel from a portable generator, one circuit at a time, by physically preventing the main breaker and generator breaker from both being on at once. A transfer switch is a dedicated device pre-wired to a fixed set of circuits, offering simpler, more consistent operation for the circuits it covers. Both are safe, code-recognized alternatives to the dangerous and illegal practice of “backfeeding” a generator into a wall outlet — the right choice depends on your budget and how much flexibility you want.
How an Interlock Kit Works
An interlock kit is a small sliding metal plate installed directly in your main breaker panel. It mechanically prevents the main utility breaker and a dedicated generator-input breaker from ever being switched on at the same time — so you can never accidentally backfeed the utility line. Once installed, you get access to your entire panel, letting you manually choose which circuits to power based on what the generator can handle, rather than being limited to a pre-selected subset.
How a Transfer Switch Works
A manual transfer switch is a separate small sub-panel, pre-wired to a fixed set of circuits you chose during installation (typically 6–10 circuits covering your real essentials). Flipping it disconnects those circuits from the utility line and connects them to the generator instead. An automatic transfer switch (ATS) does this same job without any manual action — sensing an outage and switching automatically — but this is generally paired with a permanently installed standby generator rather than a portable one; see our best home standby generators guide if that’s the setup you’re considering.
Side-by-Side Comparison
| Interlock Kit | Manual Transfer Switch | |
|---|---|---|
| Typical installed cost | Lower ($50-150 parts + labor) | Higher ($400-900+ installed) |
| Circuit access | Entire panel, you choose what to power | Fixed, pre-selected circuits only |
| Operation | Manual breaker management required | Simple flip of dedicated switches |
| Best for | Flexibility, lower cost, portable generators | A known, fixed set of essential circuits |
Both Require a Licensed Electrician and a Permit
Both interlock kits and transfer switches are National Electrical Code (NEC) recognized methods, but exact permit requirements and approved products vary by local jurisdiction. Always use a licensed electrician and pull the proper permit — this isn’t a safe DIY panel-modification project, and an improperly installed interlock or switch can defeat the entire safety purpose of the device.
Sizing Your Generator to Whichever Option You Choose
Neither device increases how much your generator can actually power — you still need to size correctly for your real essential-circuit load. See our how to size a generator for your home guide before finalizing which circuits to wire into either setup, and our best portable generators for home backup roundup for models suited to an interlock-kit setup specifically.
Frequently Asked Questions About Transfer Switches and Interlock Kits
Is an interlock kit as safe as a transfer switch?
Yes, when properly installed by a licensed electrician — both are recognized, code-compliant methods that prevent dangerous backfeeding. The difference is flexibility and cost, not safety.
Can I install an interlock kit myself?
It’s not recommended. Panel modifications should be done by a licensed electrician with the proper permit, since an improperly installed interlock can fail to prevent backfeeding — defeating its entire purpose.
Does a transfer switch let me power my whole house?
A manual transfer switch only powers the specific circuits it was wired to during installation, not your entire panel. An interlock kit gives access to the full panel instead, within your generator’s actual capacity.
Do I need an automatic transfer switch for a portable generator?
No — automatic transfer switches are typically paired with permanently installed standby generators. Portable generators are more commonly used with an interlock kit or a manual transfer switch.
Which option is cheaper overall?
An interlock kit is generally the lower-cost option for parts and installation while still providing full-panel flexibility, which is why it’s a popular choice specifically for portable-generator backup setups.
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