Portable Inlet vs Interlock vs Whole-House ATS: Backup Method Decision Matrix
Why this matters
A homeowner who wants generator backup has three legitimate ways to connect, and they sit on a ladder of cost, automation, and code complexity: a power-inlet box feeding a small manual transfer panel, a panel interlock kit that lets the main and a backfed generator breaker share the bus safely, or a full whole-house automatic transfer switch with a permanently installed standby unit. The wrong recommendation either overspends the customer or, far worse, leaves them with an unsafe backfeed. Every one of these methods exists to do the same critical thing: prevent the generator from feeding power back onto the utility line. This matrix lays out the three, what each requires, and how to match one to the customer.
The options
Method 1: Power-inlet box plus manual transfer panel (or inlet plus a small subpanel of transfer switches). A flanged inlet on the exterior wall lets the customer plug a portable generator into a dedicated, weather-rated inlet. Inside, a manual transfer panel (a small switch panel with individual transfer switches for selected circuits) lets the homeowner move chosen circuits from utility to generator one at a time. The generator is a portable the customer rolls out and connects per outage.
Method 2: Panel interlock kit. A listed mechanical interlock mounted on the load center physically prevents the main breaker and a backfed generator-inlet breaker from being ON at the same time. The customer rolls out a portable, plugs into an inlet feeding the backfed breaker, turns off the main, slides the interlock, and turns on the generator breaker. The whole panel is then available (load-managed by the customer turning off big loads). It is a manual method but covers the entire panel, not just a few circuits.
Method 3: Whole-house automatic transfer switch with a permanent standby generator. A permanently installed standby genset (air-cooled or liquid-cooled) plus an automatic transfer switch. On utility loss, the ATS senses it, starts the generator, and transfers the load with no human action. Returns to utility automatically when power restores. This is hands-off backup with the highest hardware and install cost.
Comparison
- Automation: inlet/manual and interlock are fully manual (the customer must be home, roll out the portable, fuel it, and switch). Whole-house ATS is automatic, works when nobody is home.
- Coverage: an inlet-plus-transfer-panel covers only the circuits wired to it; an interlock covers the entire panel (with the customer shedding big loads); a whole-house ATS covers the whole panel or a managed subset automatically.
- Fuel: portable methods run gasoline or portable propane the customer refills; whole-house standby runs on permanent natural gas or a large LP tank with no refueling during the outage.
- Cost and install complexity: inlet/transfer panel is lowest; interlock is low (least added hardware, but requires a listed kit specific to that panel); whole-house ATS is highest.
- Backfeed safety: ALL three, installed correctly, prevent backfeed. The interlock and the ATS do it with a listed device; the manual transfer panel does it because each circuit physically can only be on one source.
When it applies
- Recommend the inlet-plus-manual-transfer-panel when the customer wants to back up a defined short list of essential circuits (furnace, fridge, a few outlets), owns or will buy a modest portable, and is comfortable switching circuits by hand.
- Recommend the interlock kit when the customer wants whole-panel flexibility on a budget, has a portable big enough to be useful, is reliably present during outages, and the panel has a listed interlock kit available for it (not every panel does).
- Recommend the whole-house ATS with a permanent standby when the customer wants automatic, unattended backup, has critical loads (medical equipment, sump in a flood-prone basement, a home they leave for long stretches), and the budget and gas supply support a permanent unit.
Field decision flow
- Establish the customer's priority: lowest cost, whole-panel coverage, or hands-off automation. That single answer points to inlet panel, interlock, or ATS respectively.
- Confirm the load center. For an interlock, verify a listed kit exists for that exact panel make and model; without a listed kit, the interlock path is off the table and you fall back to an inlet/transfer panel or an ATS.
- Confirm presence and ability. Manual methods require someone home and physically able to roll out, fuel, and switch a portable. If the customer travels or has mobility limits, push toward the ATS.
- Confirm fuel. Portable methods need on-hand fuel and refueling; a permanent standby needs a natural-gas drop or an adequately sized LP tank with code-legal setback.
- Size the generator to the chosen coverage: a few circuits for an inlet panel, a managed whole-panel load for an interlock or ATS.
- Permit and inspect. Every method requires a permit and an inspection in most jurisdictions; the inlet, the interlock kit, and the ATS must all be listed equipment installed to code.
The entire purpose of all three methods is to prevent backfeed onto the utility. A "suicide cord" (double-male plug) or any connection without a listed inlet, interlock, or transfer switch can energize the utility line and kill a lineworker. Never connect a generator to a building's wiring without one of these listed transfer means, and always confirm the main is open before energizing a backfed breaker.
References
- NEC Article 702, Optional Standby Systems, transfer-equipment and interconnection requirements.
- NEC 702.4 and 702.5 for capacity, interlock, and transfer-switch provisions; NEC Article 705 where applicable to interconnection.
- UL 1008, Standard for Transfer Switch Equipment, for automatic and manual transfer switches.
- Manufacturer listing and installation instructions for the specific inlet box, panel interlock kit, or automatic transfer switch.