On the way up you're fine. You're making heat faster than the hill can take it off you. Then you stop at the top, sit down on cold rock, and within half an hour it's gone — the insulation that was plenty while you were climbing has nothing left to keep, because you stopped producing and that's all it ever does.

That's the problem this gilet actually solves. It doesn't hold your warmth. It makes its own.

Here's how it works.

Carbon fibre elements run through four zones — the back of your neck, your upper back, your abdomen and your waist. The neck one matters most, because that's where you feel the cold going through first when you're sitting still. One button on the chest cycles three levels, 30, 40 and 50 degrees, and it's large enough to use with gloves on.

Power comes from a standard USB power bank in your pocket rather than a sealed battery. That's deliberate. The bank you already carry for your phone runs it — around 8 hours from a 10 000 mAh — and when it's flat you swap another in and carry on rather than heading down. The power bank isn't included.

And it's 320 g, sleeveless and slim. It goes under the shell you already own without adding a thing through the arms. Bank goes flat and you've still got a quilted gilet.

The gear it replaces:

Heavier insulated jacket — bulk carried all day for warmth you only need when stationary
Second midlayer — another thing in the rucksack, and still passive
Hand warmers — single use, one spot, gone in a few hours

What's in it:

Four heating zones. Neck, back, abdomen and waist — the neck being the one most gilets skip.
Three settings up to 50°C. Turn it down rather than unzipping and losing the lot.
One chest button. Works through gloves without looking down.
Runs from a standard power bank. Swap banks and the day carries on. Not included.
320 g and sleeveless. Layers under a shell without touching your arms.
Water-resistant outer. Copes with the weather you'd expect to sit out in.

Specifications:

Product type: Heated sleeveless gilet
Heating zones: Four — neck, back, abdomen, waist
Heat settings: Three — 30°C, 40°C, 50°C
Power source: USB power bank, not included
Run time: Up to 8 hours with a 10 000 mAh power bank
Outer: Nylon, water-resistant
Insulation: Cotton
Heating elements: Carbon fibre
Weight: Approx. 320 g
Fit: Slim, sleeveless

No waiting to warm back up. No bulk under your shell. No day ending because a battery went flat.