whats the most energy efficient tablet which can charge via usb?

Hey Paul, efficiency is not everything. Our Samsung tablet is fairly efficient, but when it does charge it requires a high power (1.5 A) source. Most of our adapters and car USB converters can't charge it. What seems to work best on all converters and power sources is our LG. Don't recall the model but it is a mini tablet I guess. You can plug it into any dollar store cigarette lighter converter USB jack and it charges fine, it just takes a couple hours.

I'm curious, what is your solar setup? Do you have some sort of battery or just a panel with a USB outlet? Sometimes the panel/USB device with no battery causes a problem where each time the sun goes behind a cloud, for a moment the tablet senses no charge and the screen wakes up. This keeps happening and in the end all the power you harness is lost in the screen lighting up all day. The one I saw that works really well is a sport battery, about 4 Amp Hour (AH) if you can find it, with a automotive "battery solar maintainer" from an auto parts store charging it. You can mount the solar panel to the top of your back pack when your hiking and the battery inside the pack. Then put one of those 3 cigarette lighter outlet blocks with a USB jack connected to the battery. Now you can charge your phone, tablet, and run a small LED light for a couple hours at night just on the panel, and the tablet never lights up with varying sun. Have a great trip, Rudydoo

It takes a while to charge, but ASUS' Transformer Book T100 had battery life of 12+ hours