A generator is only half a backup power system. The other half is the device that decides whether your house is connected to the utility or to the generator, and never to both. Get that part wrong and you are not just risking your equipment, you are risking somebody's life.
Why you cannot just plug the generator into the wall
There is a widely circulated trick involving a cord with male plugs on both ends, run from a generator into a dryer outlet. It energises the house through the panel. It also energises the utility side of your service, where the transformer on the pole steps that voltage back up to distribution levels on lines that a repair crew believes are dead. This is called backfeeding, and it is why OSHA's electrical safety standards require crews to treat every line as energised until it is tested and grounded. It is illegal essentially everywhere, uninsurable, and the failure mode is a fatality several streets away.
The legitimate solutions all share one property: a mechanical guarantee that the utility connection and the generator connection cannot be closed at the same time.
Option 1: manual transfer switch
A small subpanel installed next to your main panel with a handful of circuits (commonly six or ten) moved into it. Each circuit gets a three-position switch: line, off, generator. You plug the generator into an exterior inlet box, start it, and flip the circuits you want one at a time.
Pros: the least expensive code-compliant option, straightforward to install, and it forces you to think about load one circuit at a time, which prevents overloading.
Cons: you choose your backed-up circuits at installation and changing them later means calling the electrician back. Only the circuits in the switch can ever be powered.
Option 2: interlock kit
A precisely machined plate that bolts onto your existing panel and physically prevents the main breaker and a back-fed generator breaker from being on simultaneously. Turn the main off and the plate slides so the generator breaker can be turned on; you cannot do it in the other order.
Pros: every circuit in the panel is available, so you are not locked into a list from installation day. Usually cheaper than a manual transfer switch, since it reuses the panel you already have.
Cons: it must be the kit listed for your exact panel brand and model. A generic plate is not code compliant and an inspector will fail it. Because every circuit is live, load management becomes your job: turn off the big breakers before you start, then bring them up one at a time.
Option 3: automatic transfer switch
An automatic transfer switch (ATS) monitors utility voltage continuously. When it drops out, it signals the generator to start, waits for the output to stabilise, then transfers the load. When utility power returns and holds for a set period, it transfers back and lets the engine run a cool-down before shutting off. Nobody has to be home.
Automatic switches come in two flavours. A service-rated switch sits ahead of your main panel and can carry the entire service, typically 100, 150 or 200 amps. An essential-loads switch feeds a subpanel with a chosen set of circuits, which lets you pair a smaller generator with a bigger house.
Pros: it is the only option that works while you are away or asleep, which is the whole point if you have a sump pump or a freezer.
Cons: the most expensive, and it is genuinely an electrician's job with a permit and an inspection.
The inlet box, the cord and the breaker
Whichever route you choose, a few details separate a clean installation from a nuisance:
- The inlet box is a weatherproof, recessed male connector on the outside wall. Mount it where the cord run is short but the generator is still far from doors and windows.
- The cord must be rated for the full generator output, not just the amperage you plan to use, and its length matters. Long runs of undersized cord cause voltage drop, and voltage drop cooks motors.
- The generator breaker in an interlock installation must be sized to the generator, not to the panel. A 30 amp inlet gets a 30 amp double-pole breaker.
- Neutral bonding catches people out. Some portable generators bond neutral to ground internally, which conflicts with a house that is already bonded at the service. Your electrician either selects a switch that transfers the neutral or has the generator's bonding modified. If a GFCI on the generator trips the moment you connect the house, this is usually why.
Permits are not optional, and they are cheap
Most jurisdictions treat any of these installations as electrical work requiring a permit and an inspection. That is a good thing: it is a second set of eyes on a system that only gets used under stress, at night, in bad weather. It also matters after a fire, when an insurer asks whether the work was permitted. Ask your installer to include the permit in the quote and to tell you who pulls it.
How to spec it in one paragraph
Write this down before you call anyone: the make and model of your panel, the amperage of your main breaker, the generator you own or plan to buy and its outlet type, the list of circuits you actually need, and whether anyone will be home when the power fails. An installer can quote a transfer solution from those five facts, and comparing quotes becomes easy because everyone is pricing the same job. For general background on running a portable safely once it is connected, Ready.gov's generator page is worth five minutes.