Barry - There's a good website blog you should check out -
http://www.crazyguyonabike.com/doc/?doc_id=9258. He's being doing tests on various panels for a while, and there are good reviews there from other users as well like
http://www.crazyguyonabike.com/doc/?doc_id=8860.
I have currently got a few 7W and 13W Goal Zero panels, a 10W Instapark Mercury panel and various USB chargers for AA/AAA batteries. I've also been testing my kit mainly last year before I went to Nepal for a month. I had used a stripped out USB cable conected to a multimeter, then connected to whatever I was charging to test the current output, but have also bought a few of the Charger Doctor usb gizmos that work well.
In the old days (around 2004-5) I had tried I think it was 3W iSun ICP Solar and Sunforce folding solar chargers, but the current output was really low (was 12V output to a cig lighter connector), and then bought a Brunton 3.5W panel, and a flexible Coleman 5w panel but the outputs again were so low charging AA/AAA batteries and USB lithium ion battery packs was painfully slow.
But with my current kit, I pair either the 7W or 10W panels with a 5000mAh lithium ion USB battery pack, and a USB AA/AAA charger - I have both Goal Zero Guide10 Plus AA charger, which when paired with the Goal Zero solar panels can use a dedicated pin socket to provide a higher unregulated current to the charger, so in theory the batteries charge faster. However, the hassle with the Guide 10 plus AA charger is that you HAVE to charge 4 batteries at once, you can't do just 2 at a time. You can still use the USB output to micro USB input on the charger, it will just be slower. I have also bought Fujicell SCH500 AA USB charger (
www.youtube.com/watch?v=fTVaeG6QJxE) instead of the Goal Zero Guide10 Plus charger, as the Fujicell would let me charge 2 AAs at a time instead of 4. I used it mainly for recharging AA batteries in my UV water filter, GPS handheld as well as headlamp/torches.
I took both a 7W and 10W panel on the month trip to Nepal where they both functioned well on two separate 2 week treks. The 7W panel without charger weighs about 350gms, and the 10W panel adds another panel and is about 450gms but is quicker. Given that I didn't use the Goal Zero AA charger, I will just take the 10W panel.
For reference, my Goal Zero 7w panel charges a 5000mAh battery from empty to full in about 10 hours (over 2 days). Via USB the Goal Zero outputs 5V at about 0.5A (this is for my version bought in 2012 – I think the latest version has upgraded the circuitry and now outputs 5V at up to 1A). My 10w (Instapark Mercury 10) panel cuts that time down to about 5-6 hours as it outputs a higher current, and a 13w panel (Goal Zero 13) brings it down to about 4-5hrs, which can output up to 1A. The 7w panel fully charged a Sony camera battery (1000mAh) directly via USB to the camera in about 2 hours in direct sunlight.
As an aside, I found it interesting with the Charger Doctor to notice the quality and length of USB charging cables and the impact it has on the charging current. So I have used the Charger Doctor to test and separate my various charging cables and keep the ones that let higher currents through to the connected device (these are usually thicker and shorter cables)