Battery claims, and what they mean on a real long run

A GPS battery figure is measured in the cheapest satellite mode the watch offers, with the display off between glances, no music playing, and the optical heart-rate sensor doing the minimum. It is a real measurement. It is just not a measurement of how you will use the watch.

That matters because the three things you are most likely to turn on are the three things that cost the most: multiband GPS, an always-on screen, and music. Each one alone is significant. Together they can halve the figure.

There is no published conversion, because it depends on how often you look at your wrist and how cold it is. What follows is a working margin rather than a measurement, and it is deliberately pessimistic — the failure mode is asymmetric. A watch with spare battery costs you nothing; a watch that dies costs you the data for the day and, on a route you were navigating, rather more than that.

1. 1

   Start from the mode you will actually use

   If you bought the watch for multiband accuracy, the multiband figure is your figure. Using the GPS-only number for a watch you will run in multiband is how people end up short.

2. 2

   Take off a fifth for the screen

   An always-on AMOLED is the single biggest drain after the GPS chip itself. Transflective screens cost almost nothing to leave on, which is most of why they still exist.

3. 3

   Take off another fifth for cold

   Lithium cells lose capacity below freezing. A winter ultra is not the day to find out how much.

4. 4

   Then add your margin

   Finish time plus the time you might lose to a bad patch, a wrong turn, or weather. For a first attempt at a distance, that margin should be generous.

Work that through for a 100 km attempt at 20 hours: a watch quoting 26 hours in multiband is comfortable. One quoting 21 hours in multiband is not, once the screen and the cold are paid for.

Everyday battery — days rather than hours — decides what the watch is like to own rather than what it is like to race. A three-day watch lives on a charger beside the bed and is an appointment you keep. A thirty-day watch is something you forget you own until it buzzes.

This is the axis where the smartwatches and the running watches genuinely diverge, and no amount of feature comparison closes it. It is worth deciding which side you are on before comparing anything else, because it eliminates half the market in one answer.

It is also the axis the wrist-size question interacts with most: the watches with the biggest batteries are physically the biggest watches, and a small wrist rules out the easy answer.

• Find the multiband figure, not the headline one, if you will run in multiband — see multiband GPS for whether you need it.
• Check whether the quoted smartwatch days assume an always-on screen. Usually they do not.
• Treat solar figures as a bonus rather than a base. They assume hours of strong sun.
• Ignore "up to" battery in power-saving modes: those reduce GPS fix frequency and change what the watch is.

And when two watches quote the same hours, the tie-breaker is usually the screen — a transflective display costs almost nothing to leave on, which is most of what MIP or AMOLED is about.