A headlamp box says "100 hours." Another says "10 hours." Which one lasts longer in the field?
Probably not what you think, and the reason sits in a standard most people have never read.
What the standard measures
ANSI/PLATO FL1 is the flashlight performance standard. It defines how makers report light output, beam distance, peak beam intensity, runtime and a few durability ratings, so that numbers on different boxes mean the same thing.
Its runtime definition, as described by Streamlight and REI: the duration from the initial output until the light drops to 10 percent of the initial value. The initial value is taken 30 seconds after turn-on with fresh batteries.
Read that twice. The clock stops at 10 percent. Not at half. Not at "too dim to read a map." At a tenth of the starting brightness.
Why 10 percent is a generous finish line
Our eyes handle dim light better than a linear scale suggests, so 10 percent output still looks like something. It still isn't what you'd use to walk a trail or find a dropped key. For most tasks, a light has stopped doing its job well before it reaches that mark.
So when a box says "30 hours," a better question is, "30 hours of what?" The number is real and honestly measured. It describes the last glimmer.
Regulated vs unregulated
Two designs, two curves.
Regulated. The electronics hold output near constant as the battery drains, then output falls off steeply near the end. A runtime of 10 hours might mean nine hours at near-full brightness and one hour of dimming.
Unregulated. Output drifts down from the first minutes. A runtime of 10 hours might mean light that's half as bright by hour three.
Both hit "10 percent" and both can print the same number. Their usable time differs a lot. If the maker publishes a runtime graph, look at it. A flat line that dives at the end is a regulated light. A slope is not.
Brightness modes change everything
A single flashlight lists several runtimes, one per mode. High might be 90 minutes, low might be 40 hours. The headline number on the box is usually the one most flattering to the product, often the lowest mode.
So decide which mode you'll live in. A headlamp for reading in a tent lives on low. A search light lives on high. Compare runtime at the setting you'll actually use.
Convert it to watt-hours
If you know the battery's capacity, you can get a sense of how hard the light drives it. A cell rated 3.6 V and 3,000 mAh holds 3.6 x 3.0 = 10.8 Wh. A light that runs 10 hours on that cell averages about 1 watt. A light that runs 1 hour averages about 11 watts. The second one is a flood. The first is a glow.
This is the same arithmetic we use for vacuums and tools; see spec sheets in watt-hours. Watts and hours turn marketing into something you can compare.
Two lights, one runtime
Imagine two lights, each printing "10 hours" on the box, each running on that 10.8 Wh cell. They draw the same average power, about 1 watt. One is regulated and one isn't. At hour six the regulated light is still near its starting brightness and the other is visibly dimmer. Same energy, same claim, different night. That's the part the single number hides, and it's why a published graph is worth more than a headline.
Also remember that beam distance and peak intensity are separate FL1 figures. A light can run long and still throw a poor beam, so read each claim for what it covers.
A practical buying checklist
- Pick the mode you'll use, then read that mode's lumens and runtime.
- Look for a runtime graph. Shape tells you more than the number.
- Check the battery type. Rechargeable cells cost less over years; alkaline cells are easier to find in a pinch.
- Ask whether the claim assumes the battery supplied with the light.
- Treat "up to" figures as a ceiling.
Spare cells and cold nights
If a light runs 10 hours on one cell, a spare cell of the same size makes it 20. That's the cheapest runtime upgrade there is, and it beats buying a bigger light. Rechargeable cells make a spare affordable. Alkaline cells make one easy to find in a hardware store. Pick the one that matches how you use the light.
Cold also cuts into what a cell delivers, so a runtime measured in a warm lab shortens on a winter trail. We'd carry the spare inside a jacket, close to the body, not in a pack on the ground. Is that overkill for a camping trip? Perhaps. Dead lights tend to choose bad moments.
A strong opinion
For a headlamp or flashlight you rely on, we'd look at the hours at the brightness you need, not at the 10 percent figure at all. If the maker only publishes FL1 runtime, assume usable time is shorter and carry a spare cell.
Is that harsh? Maybe. But a headlamp that fails at hour six on a ten-hour claim is a bad surprise at the wrong moment.
Cost angle
Light is cheap to run. A 1-watt draw for an hour costs a fifth of a cent at the US residential average of 18.31 cents per kWh (EIA, July 2026). The money is in the batteries, which is why battery wear per hour is the number worth tracking for anything cordless.