I have these things (2 of) The newer version and the older version.
The EBAY one featured is the older model and 12v only. Which by memory is old stock if any one has them.
Best thing since sliced bread.
The later one charges 12v and 24v, the earlier charges 12v.
I had 4 batteries (80ah-100ah each) The 24v worked brillantly.
I had a large 60lt waeco fridge (draws 7a at 12v, 3.5 at 24v), a laptop, 300w inverter and a large HID 24v 70w lamp (draws about 6amps to start then 1-3 amps running)
I ran this setup at Bathurst for the week. Never failed, never ran out of battery charge.
I also did a test as well, they work in full sun at at nearly an amp, slight cloud cover, reduces the amps a little, but full voltage.
They will still work in heavy cloud, but amps is low and voltage reduces a lot.
Heavy cloud, probably runs at 30-50% of it rating.
Do your sums on your current draw. Then work out a plan.
EDIT:
PIC (The one on the left is the 24v the other on the right is the 12v only one)
http://farm3.static.flickr.com/2765/...f96a15af_b.jpg