When someone types how does an emergency light work into a search bar, they usually expect one simple answer. I get it - it seems like it should be simple. Power goes out, light comes on. But after eight years coordinating emergency lighting projects - everything from single-unit replacements to multi-building IoT rollouts - I've learned the real answer is: it depends.
If you got here looking for spotlight bridal or spotlight calendar, you might be working on event lighting or maintenance planning. That's fine, but it's not the same as emergency lighting. Decorative fixtures and scheduled spot checks are both important, but they don't replace a fixture with its own battery and a transfer relay.
The short version: how does an emergency light work?
A standard emergency light - the kind with a battery inside - has four main parts: AC power input, a charger, a battery, and a relay that switches to battery power when the mains supply drops.
On normal power, the charger keeps the battery topped off. The lamp may be lit or stay off depending on the design. When AC power fails, the relay senses the loss and switches the load to the battery. That's the click you sometimes hear during a power cut.
It's actually pretty simple. The hard part is making sure it happens reliably every single time.
For commercial buildings in the U.S., the equipment should be UL 924-listed. The installation side is driven mainly by NFPA 101 Life Safety Code, which is where the emergency illumination requirement for egress paths comes from. I'm not a code expert, and anyone doing a permit-ready design should work with a licensed electrical engineer. But knowing those two references will help you ask better questions.
The problem with 'just recommend one for me'
I used to think I could recommend one good emergency fixture for everyone. I was wrong. It took one failed inspection - where a building owner had brand-new lights but none of them were on the emergency circuit - to stop assuming that a new fixture meant a compliant fixture. The wiring and switching setup matter just as much as the light itself.
Scenario 1: You're replacing an existing emergency light, one-to-one
This is the most common request, and honestly the easiest to get right.
If an existing emergency light is dead, failing its test, or you're upgrading old halogen units to LED, a standard self-contained emergency fixture is usually the right move. Look for UL 924-listed equipment with the same voltage as the existing circuit. Also check that the wiring includes an unswitched line. If someone wired the fixture into a switched circuit, the charger may lose power when the lights are turned off - which defeats the whole purpose.
One small product note: in control panels and status circuits, I really like using an ABB pilot light NSL44E 24 V to show whether the emergency circuit is live. It's not the emergency light itself. It's simply a 24-volt indicator lamp that lets you see at a glance if the control voltage is present. That little bit of visual confirmation has saved my team several late-night callbacks.
If you're in this scenario, you don't need central batteries or building automation. You need a reliable fixture, an unswitched feed, and a test log. Don't let anyone sell you an expensive IoT system for a one-for-one swap.
Scenario 2: You're responsible for multiple floors, multiple buildings, or remote sites
Here's where the game changes.
Testing emergency lights across a large portfolio is a pain. If you've ever walked 14 floors with a flashlight, pressing a test button on every fixture and scribbling notes - then transcribing them into an Excel spreadsheet - you know exactly what I mean. It's tedious, and it's easy to miss a unit.
This is when ABB's IoT connecting approach becomes genuinely valuable. I'm not going to pretend it's the cheapest option, because it isn't. But an IoT-enabled emergency lighting setup can give you remote status, battery health, and auto-testing data on a dashboard. Instead of asking whether a light is working, you can open a log and see the last test result, battery voltage, and fail count for every device.
In this scenario, the extra cost is usually worth it. If you have 50-plus units and a finite facilities team, an IoT-connected system pays for itself in hours saved. But if you only have five fixtures in a small office, don't feel pressured to make everything smart. A paper log works fine when the building is small.
Scenario 3: You're under inspection, dealing with code compliance, or retrofitting an older building
This is where things get serious, and it's also where I've seen the most panic.
A facility audit or fire marshal visit tends to surface emergency lighting issues that have been ignored for years. Common problems include batteries that no longer hold a charge, fixtures connected to switched circuits, and stairwell lights that don't turn on when the main breaker is cut.
If you're in this scenario, don't just buy fixtures. Start by reading the applicable code requirements - NFPA 101, the International Building Code, and any local amendments. Then hire someone who can trace the circuits and design compliant switching. Emergency lighting in new or renovated buildings is usually not a DIY project.
The good news: most issues are fixable. In 2023, I helped a retail client retroactively install backup lighting across three locations after an audit flagged their exits. It was not cheap, and the deadline was painful - I want to say 30 days, but don't quote me - but we got it done. The key was treating it as a compliance project, not a product purchase.
How to tell which scenario you're in
If you're still not sure, ask yourself these three questions:
- Is this a like-for-like replacement on an existing circuit? Go with Scenario 1.
- Do you need to monitor and test lights across a large area or multiple sites? Look at Scenario 2.
- Is there an inspection deadline, legal requirement, or occupancy permit involved? Treat it as Scenario 3.
A lot of projects actually start as one scenario and turn into another. For example, a property manager might call expecting Scenario 1, then mention that the last inspection report required central supervision. That's Scenario 3 with Scenario 2 elements. In that case, solve the code issue first, then consider IoT-enabled equipment so future testing becomes easier.
One thing I'd do differently
I still kick myself for an early mistake: assuming all emergency lights are self-contained.
About six years ago, I specified a bunch of individual battery units for a school corridor, and I didn't realize the design actually needed a central emergency lighting system to match the architectural panel schedule. If I'd asked more questions about how the building's emergency power was distributed, we'd have avoided a lot of rework. The lesson: understand the system before you recommend the product.
That's also why I don't like blanket statements like emergency lighting is just a light with a battery. In small buildings, sure. In larger facilities, it can involve central batteries, transfer switches, monitoring, and a whole web of control wiring.
Why testing still matters
I've seen enough green status icons on monitoring dashboards to know not to trust them blindly. Green icons don't mean the fixture will work when the power goes out.
NFPA 101 calls for emergency lighting to function for at least 90 minutes during a power failure, and the standard testing schedule is a monthly functional test - usually 30 seconds - plus an annual 90-minute test. Someone, or some system, has to actually interrupt the AC supply and prove the battery takes over. That's the part that gets skipped.
If you're setting up a maintenance routine, add a recurring spotlight calendar reminder on the first of every month. Test the units, note the results, and don't hide bad results. A failed monthly test is not fun, but finding out during a power outage or inspection is worse.
The honest bottom line
Emergency lights work by switching to battery power when AC fails. But how you choose one depends entirely on your situation.
- If it's a simple replacement: don't over-engineer. Buy a reliable UL 924-listed unit, make sure you have an unswitched feed, and use a 24 V pilot light if you want a quick status indicator.
- If you're managing many lights: an ABB IoT connecting system can really be a game-changer. It's not mandatory, but it makes ongoing compliance massively easier.
- If you're under an audit or code deadline: stop reading and call a licensed electrical professional. The product is only one piece of the compliance puzzle.
And if you originally landed here from a spotlight bridal search - well, now you know more about emergency lighting than you expected. That knowledge might come in handy someday.