Think in Systems
A headlamp is only one part of night strategy. You need light on the course, power through the night, and enough comfort that the lamp does not become another problem at 3 AM.
Product examples live in our headlamp guide. This page is about the race plan.
Battery Math
The whole calculation is one division:
runtime (h) = usable battery energy (Wh) ÷ average draw (W)
You rarely know the draw directly, so work backwards from the manufacturer's rated runtime at a stated output, which is measured under the ANSI/PLATO FL 1 protocol. Petzl lists the NAO RL with a 3200 mAh, 3.7 V lithium-ion battery — that is 3.2 Ah × 3.7 V = 11.84 Wh — and a STANDARD lighting mode of 250 lumens for 5 hours. So the implied average draw is 11.84 Wh ÷ 5 h = 2.37 W. At MAX POWER (900 lm, 2 h) the implied draw is 11.84 ÷ 2 = 5.92 W, two and a half times higher for 3.6 times the light.
Do the same for a second lamp and the numbers are directly comparable. BioLite lists the HeadLamp 800 Pro with a 3000 mAh lithium-ion battery. At a nominal 3.7 V that is 3.0 × 3.7 = 11.1 Wh, and the stated regulated runtimes are 7 hours at 500 lm and 8.5 hours at 250 lm — implying 11.1 ÷ 7 = 1.59 W and 11.1 ÷ 8.5 = 1.31 W. Note what that comparison does and does not show: it tells you how long each lamp lasts at its own stated settings, not that 250 lumens from one lamp lights the ground like 250 lumens from the other. Beam shape and regulation differ.
| Lamp and mode | Rated output | Rated runtime | Battery energy | Implied average draw |
|---|---|---|---|---|
| Petzl NAO RL — MAX BURN TIME | 10 lm | 80 h | 11.84 Wh | 0.15 W |
| Petzl NAO RL — STANDARD | 250 lm | 5 h | 11.84 Wh | 2.37 W |
| Petzl NAO RL — MAX POWER | 900 lm | 2 h | 11.84 Wh | 5.92 W |
| BioLite 800 Pro — Medium | 250 lm | 8.5 h | 11.1 Wh | 1.31 W |
| BioLite 800 Pro — High | 500 lm | 7 h | 11.1 Wh | 1.59 W |
| BioLite 800 Pro — Burst | 800 lm | 30 s per burst | 11.1 Wh | Not a sustainable setting |
A Worked 24-Hour Night Plan
This is an illustrative example, not a prescription. Assumptions: a flat 1-mile loop with crew access every lap, darkness from 20:00 to 06:00 — 10 hours — and a mixed output plan of 6 hours at a low setting where the loop is lit and other runners are close, plus 4 hours at a higher setting after the field spreads out and again through the pre-dawn hours. Take the Petzl NAO RL figures above and use MAX BURN TIME (10 lm, 80 h) as the low setting and STANDARD (250 lm, 5 h) as the higher one.
| Block | Hours | Setting | Draw | Energy used |
|---|---|---|---|---|
| 20:00–23:00 | 3.0 | 10 lm | 0.15 W | 0.45 Wh |
| 23:00–01:00 | 2.0 | 250 lm | 2.37 W | 4.74 Wh |
| 01:00–04:00 | 3.0 | 10 lm | 0.15 W | 0.45 Wh |
| 04:00–06:00 | 2.0 | 250 lm | 2.37 W | 4.74 Wh |
| Night total | 10.0 | Mixed | — | 10.38 Wh |
10.38 Wh against an 11.84 Wh battery is 88% of one charge. That is a plan that works on paper and fails in practice, because it assumes you hit your setting schedule exactly, the cells are new, and the night is not cold. Apply a margin instead.
The Margin Rule
Carry at least 1.5× the energy your plan says you need, and never fewer than two independent power sources. Here that is 10.38 × 1.5 = 15.6 Wh, which one 11.84 Wh battery does not cover, so the loadout is two NAO RL batteries (23.68 Wh, or 2.3× the plan) plus a separate backup lamp. Two reasons the margin is not padding:
- Cold reduces usable capacity. Manufacturer runtimes are measured at room temperature. A battery that spends the night at ambient does not deliver its rated energy.
- Plans drift upward, never down.Fog, rain, a dropped bottle, or a runner who wants to see their feet at 04:00 all push you to the higher setting. One extra hour at 250 lm costs 2.37 Wh — 23% of the whole night's budget.
Swap times. With two batteries and a 10-hour night, swap at the crew table at the end of the first high-output block rather than when the lamp warns you: in this example, at about 01:00, after 5.19 Wh of the first battery is gone. A swap in a lit crew area with warm hands takes under a minute; the same swap on course at 04:00 with cold fingers does not. If your lamp has a sealed battery, the equivalent is a second complete lamp, not a power bank you have to run a cable to while moving.
Scale the same arithmetic to other formats. A 48-hour race has two darkness blocks, so budget each separately and confirm you have a charging window and the right cable between them. A backyard ultra has an unknown number of dark loops, so budget the full darkness period and check charge state before every loop, not every few hours. A 6-day race needs the calculation repeated per night, with the weather forecast applied to each.
Testing
Test the lamp on the settings and terrain you expect to use, not only the brightest setting on the product page. Manufacturer runtime can be measured under conditions that differ from your weather and use.
- Test long enough to observe output changes and pressure or bounce.
- Wear the same hat, buff, or glasses you expect to use.
- Check bounce, forehead pressure, and whether buttons work with gloves.
- Rehearse charging or battery swaps in a safe, well-lit place.
- Compare settings without reducing light below what safe footing requires.
Backup Plan
If a light fails, stop at a safe place before troubleshooting or changing batteries. Do not continue over technical or unlit ground with inadequate visibility. Any required backup must stay with you as the rules specify; otherwise, decide where to carry it from the distance to the next staffed aid point or safe exit, not merely the distance to crew.
Add lights, cables, and batteries to the gear checklist generator before race week.
Sources
- Petzl — official explanation of brightness, beam shape, and burn-time tradeoffs.
- Petzl — ANSI/PLATO FL 1 lighting-performance measurements.
- Eiger Ultra Trail by UTMB — official mandatory-equipment example; check your own event's current rules.
- Petzl — NAO RL product specifications: 145 g, 1,500 lm maximum, 3200 mAh / 3.7 V (11.84 Wh) battery, and standard-lighting runtimes of 10 lm for 80 h, 250 lm for 5 h, and 900 lm for 2 h (read 14 September 2026).
- BioLite — HeadLamp 800 Pro product specifications: 150 g, 3000 mAh battery, 5 lm for 150 h, 250 lm for 8.5 h, 500 lm for 7 h, and an 800 lm burst lasting 30 seconds (read 14 September 2026).
Wh figures and implied draws on this page are our arithmetic from the manufacturers' published capacity and runtime numbers, using a nominal 3.7 V cell voltage. Neither manufacturer publishes a wattage figure, and real draw varies with regulation, temperature, and cell age.
Frequently Asked Questions
How many headlamps do you need for a 24-hour race?
Carry every light and power source required by the current race rules. Where the rules do not specify a number, choose redundancy from the hours of darkness, terrain, remoteness, aid access, weather, and the consequence of a failure; test the complete system before race day.
How many lumens do you need for an ultra?
There is no universal lumen target. Required visibility depends on speed, terrain, markings, weather, other runners, and individual vision. Follow any event minimum and test beam shape and usable output on comparable ground rather than relying on peak lumens.
Should you use reactive lighting?
Reactive lighting can save battery and reduce fiddling, but test it first. Some runners prefer predictable manual settings in fog, rain, groups, or reflective environments.