Amazfit T-Rex Dual Solar really does have two solar panels

Zepp Health is getting much more specific about how its upcoming Amazfit T-Rex Dual Solar will work. The latest build of its software reveals that the watch is being built with photovoltaic panels on both the front and the case back. And surprisingly, the panel underneath is the larger of the two.

So now we know what “Dual Solar” actually means. This is not just a solar ring around the display with some clever branding attached to it, Zepp Health is working on two separate surfaces that can generate power.

There really are two solar panels

We first started seeing signs of this setup earlier in the summer, when Zepp Health was working with separate measurements for power coming from the watch face and the case back. At the time, that was interesting but not enough to say with confidence that the T-Rex Dual Solar would actually have photovoltaic panels on both sides.

That ambiguity has now disappeared. The latest APK describes the watch face and case back as both having built-in photovoltaic panels. And each is getting its own charging measurement.

Even more interesting is the difference between them. The photovoltaic panel on the case back is larger than the panel around or beneath the watch face. So Zepp Health says its charging index will normally be higher when both receive the same amount of light.

That is a pretty unusual way to build a solar watch. Most solar wearables concentrate on getting light through or around the display. That’s because the display is obviously the part of the watch exposed to the sun while you are wearing it.

The rear panel creates a slightly different proposition. While the watch is strapped to your wrist, it is not going to see much sunlight at all. The obvious use would be taking the watch off and leaving it somewhere with both surfaces exposed, although exactly how Zepp Health expects owners to position it remains to be seen.

Zepp Health has created a Solar Charging Index

There is also a new way of measuring how much useful light each panel is receiving.

Zepp Health is calling it the Solar Charging Index. The watch can show separate values for the watch-face panel and the case-back panel, rather than simply telling you that solar charging is taking place.

The reference point is 50 kLux. When the watch face receives that level of light, Zepp Health defines its Solar Charging Index as 100%. That does not mean the battery is charging at 100% or that the watch has reached full solar output, it is essentially a reference scale for the amount of usable solar energy reaching the panel.

The larger case-back panel changes the numbers. Under identical lighting, Zepp Health says the back should normally report a higher Charging Index because it has a greater photovoltaic area.

This should make the solar system much more transparent than simply sticking a little sun icon on the screen. Owners should be able to see whether moving the watch, changing its angle or exposing the rear panel is actually improving the amount of energy coming in.

Solar Boost Mode makes more sense now

This also adds some context to the Solar Boost Mode we previously uncovered. When activated, the watch cuts back to essentials such as the time, battery level and solar information, while temporarily disabling a number of normal functions.

The idea appears to be simple. Reduce how much power the watch is consuming while those two photovoltaic panels are bringing energy in, giving solar charging a better chance of making a significant difference to the battery level.

Zepp Health is also building temperature protection into the system. Solar charging can pause when the watch becomes too hot, which is particularly relevant if someone leaves it sitting directly in strong sunlight to take advantage of that larger rear panel.

There are dedicated solar statistics as well, including separate information for the two charging surfaces. So this is looking less like the passive solar top-up used on some watches and more like a system Zepp Health wants owners to actively interact with.

The big unanswered question is how much extra battery life all of this actually produces. Two photovoltaic panels sound good on paper, but panel size, efficiency, power consumption and the amount of time the rear of the watch spends exposed to light will ultimately decide whether Dual Solar makes a noticeable difference.

At least the hardware idea is much clearer now. The Amazfit T-Rex Dual Solar has already surfaced under model A2570, and Zepp Health is now putting together a fairly elaborate solar system around it, complete with two panels, individual charging measurements, Solar Boost Mode and temperature management.