First Alert 9120BFF vs SMI110: Hardwired or Battery Smoke Detector in 2026

Price — The hardwired model is cheaper

The 9120BFF costs $21.29, while the SMI110 runs $24.59. That's a $3.30 difference in favor of the hardwired unit. Both are priced well under $30, making them affordable safety upgrades.

In daily use, the price gap is negligible. The SMI110's higher cost includes the sealed 10-year battery, eliminating battery replacements. Over a decade, the 9120BFF may need a few 9V battery changes (around $5 total).

Budget-minded shoppers should prioritize the 9120BFF if they have existing wiring. Renters or those avoiding installation may find the extra $3.30 worth it for the convenience of the SMI110.

Power Source — Hardwired vs battery

The 9120BFF runs on 120V AC with a 9V battery backup. The SMI110 uses a sealed 10-year lithium battery only. Both provide reliable power, but the hardwired model continues monitoring even if the battery is depleted.

For daily use, hardwired means you never worry about battery life. The SMI110 cannot be hardwired, so it relies entirely on the sealed battery — which lasts the alarm's 10-year lifespan but requires whole-unit replacement if it fails early.

Homeowners with existing wiring should stick with hardwired. Renters or those in temporary housing benefit from the flexibility of a battery-only unit.

Sensor Type — Both ionization

Both detectors use ionization sensors, which are fast at detecting flaming fires. Ionization sensors are slightly more prone to false alarms from cooking steam or smoke than photoelectric models, but they excel at rapid detection.

In a typical home, this sensor choice is adequate for most areas. For kitchen placement, consider a photoelectric or dual-sensor model to reduce nuisance alarms.

If false alarms are a major concern, look for alternatives with photoelectric sensors, like the Kidde 10SDR. For general use, ionization works fine.

Battery Backup — Replaceable vs sealed

The 9120BFF includes a 9V battery backup that can be replaced when low. The SMI110's sealed 10-year battery cannot be changed — you replace the whole unit. This affects long-term maintenance.

In daily use, the 9120BFF's replaceable battery means you can keep the alarm running with a simple $5 battery swap. The SMI110 requires buying a new alarm after 10 years, but you never hear low-battery chirps.

Homeowners who prefer user-serviceable parts should choose the 9120BFF. Those who want zero maintenance for a decade will prefer the SMI110.

Interconnectable — Only the hardwired model links alarms

The 9120BFF can be interconnected with up to 18 compatible BRK or First Alert units. When one alarm sounds, all connected alarms sound. The SMI110 is a standalone unit.

Interconnectivity is critical for whole-home safety. If a fire starts in the basement, alarms on every floor activate, giving you more time to escape. Standalone alarms only alert those nearby.

For multi-story homes or those wanting maximum protection, the 9120BFF's interconnectivity is essential. Renters in small apartments may not need this feature.

Nuisance Reduction — SMI110 excels

The SMI110 features Precision Detection technology that reduces false alarms from cooking smoke. The 9120BFF uses standard ionization without advanced filtering, so it may trigger near stoves.

This matters most in open-concept kitchens or small apartments where the kitchen is near sleeping areas. Frequent false alarms can cause residents to disable detectors — a dangerous habit.

Kitchen-heavy homes or those sensitive to nuisance alarms should lean toward the SMI110. The 9120BFF can be relocated away from the kitchen if necessary.

Warranty — 9120BFF offers 10-year coverage

The 9120BFF comes with a 10-year limited warranty. First Alert does not specify a warranty for the SMI110 in its product documentation. Coverage is a trust signal for long-term reliability.

In practice, a 10-year warranty means the manufacturer stands behind the product for its expected lifespan. The SMI110's lack of stated warranty may concern buyers who plan to keep it for a decade.

If warranty coverage is important to you, the 9120BFF is the safer bet. Both are UL listed, which ensures basic safety standards.

Installation — SMI110 is hassle-free

The SMI110 installs in minutes with just a screwdriver — no wiring needed. The 9120BFF requires connecting to 120V AC wiring, which may need an electrician if you're not comfortable with electrical work.

For daily use, easier installation means you can mount the SMI110 anywhere — hallway, bedroom, or basement — without planning around wiring. The 9120BFF is limited to locations with existing boxes.

Renters or DIY beginners should choose the SMI110 for its simplicity. Homeowners with existing boxes and basic electrical skills can handle the 9120BFF.