After a lot of reading and thinking, I've reached the following info and conclusions:

An iPhone 4 or 5 takes about 8 Wh to fully charge from dead.
A pair of AA NiMH (Eneloop) takes about 5 Wh.
A pair of AA lithium primaries hold about 9 Wh.

A Powerfilm USB/AA charger is 1.5W and weighs 6.3oz (with two Eneloop AAs, rqd for USB).

Raw battery specs:
NiMH AA 27g 2.3Wh 0.08 Wh/g (eneloop)
LiPo 86g 18.1Wh 0.21 Wh/g (many choices)
Li prime 15g 4.5Wh 0.31 Wh/g (energizer)

I don't like charging directly from solar to phone 'cause I'm moving. I like to charge an interim battery throughout the day and download electrons at night. It usually takes a couple days to deplete my iPhone, and a few days to deplete my 2 AAs. There are good and bad solar days, so I need extra capacity.

So first, the battery should be at least 10Wh. Second, the solar should be at least 2.5W for 8 hours of charging at half capacity. This should give me twice the power I need for my devices.

This stuff all weighs too much as purchased (other threads dig this in detail), so I will be repackaging (like many others have done.) What inspires me to have at this anew is that the naked powerfilm cells are only 10g per watt. The nylon canvas backing that comes with the USB/AA unit weighs 1.9oz!

I add up a total about 3.4oz including 2.4W panel, 10Wh LiPoly, PCB, AA holder, and case.
Add a USB cable, 2 NiMH AAs, and 2 Lithium AAs for 6.4 oz total.

I carry a Princeton Tec Byte:
13g strap
26g lamp body
15g 2xAAA Li
---
54g Total (1.9 oz) Mfg: 64g?
Add:
15g 2xAAA Li Spares (or more...)
---
69g with spares.

This got me to thinking about a rechargeable headlamp that has a mini-USB input.
Two Li-primary have 3.6Wh, enough to run the lamp in soft-white for 96h.
An equivalent Li-Poly is about 20g.

This headlamp would weigh < 60g with similar performance to the Byte.

Any interest?

Like Bosavi ? More info .

Edited by annapurna on 02/20/2013 11:20:44 MST.

Exactly! I missed the Tikka as well. I'm going to hack one out of a Byte and a cellphone battery.

I just got the Bosavi last week, charged it and left it on on the high setting and it lasted somewhere short of 8hrs. Recharged it and left it on the next to high setting and it lasted for a little less than 12hrs. I didn't time it on the lowest setting. I don't remember how long it took to charge but it seemed to be within 3hrs. Used it this last weekend for a 3 nighter and liked the way it worked.

I have been very happy with mine so far also,but of course it is still pretty new to me.

I understand the bosavi can be programmed for different light intensities and will remember the last setting you used.

Would you describe this feature as:
awesome, useful, meh, annoying?

Is the super intense setting good to have?
Are the mid and red lamps the right strength?

My older Petzl has the feature that by turning the lens I could change the beam width. I find that more useful than the bright/dim settings on LED headlamps.
Thoughts?

There are NO UL options for solar power at this time.
Unless you can spend 8-12 hrs at a spot in good sunshine to charge your device, it just isn't worth it.
For a headtorch, it is wayyyyyyyyyyyyyyyyyy cheaper and lighter to take a spare pair of batteries.
I use solar extensively here is Australia. I charge my BD Storm batts ( 4 x AAA), my iphone, and Samsung Galaxy Tab 2 7.0. use 2 x 28 Watt Powerfilm ( Iowa Thin Film ) rollable panels. The biggest issue with solar is that all these really good thin films are not very efficent, plus do not create the required Amps. Volts are fine, but what we need is Amps.
You guys in the US have lots of local options, but IMHO go on Alibaba and get a 4lb 60Watt panel that will charge anything from AAA's thru to Ipad and other devices that rquire more than .5Amps from China.
monocrystalline is the most efficient of the äffordable"consumer options - it just weighs more.
I love solar - just wish we could get more than 20% efficiency

I agree completly with Matthew. Sure, it is possible to carry your solar system and recharge batteries as you walk along. However, weight-wise, it is better to simply carry extra batteries.

Solar recharging is very efficient if you need to go to one remote place (without commercial power) and stay there for a few weeks. The typical example is Mount Everest Base Camp. Everybody rolls out their solar rig next to their tent and it stays there. You can orient the rig once to maximize sun.

It is possible to arrange a solar rig to get higher volts or else higher current, but if you need both then you are talking about a lot of surface area, and that typically means a good deal of weight and support structure weight.

--B.G.--