I've got the big Zareba and it works just fine. Lots of sun here, but even on cloudy days it puts out plenty of zap. We seldom get several cloudy days in a row, though.

The main reason I went solar is that this is also the lightening strike capital of the US, meaning we get a lot of surge through the power lines. Where I used to board had plug-in chargers and they lost three or four each summer. In 2 years here I've lost one, when the fence itself took a hit.

The two most important aspects to getting a fence to give a good zap are proper grounding and good connections. I have 3 ground rods connected in line using heavy-weight insulated wire. All junctions between pieces of wire are made with crimps--I was amazed how much more current I had when I did that versus looping the wire back on itself.