In my limited experience, you have to be a little careful using a
switching, or even a series pass, regulator with a solar panel. Most are
designed to regulate voltage coming from a relatively stiff source, and
some become unstable when hooked to a high impedance source like a solar
panel. This can often be overcome by putting a big capacitor across the
panel, and it can of course be overcome by designing the regulator to
function properly with the high impedance source in the first place. And
quite a few regulators work just fine without modification. But it's
something to keep in mind when using a regulator designed for more
conventional applications.

Roy Lewallen, W7EL

René wrote:

I have seen elegant ckts where a simple switcher was used, regulating
the *input* voltage coming from the solar cell, keeping it in max
efficiency mode at all loads. This obviously only works with flexible
loads such as slow chargers or such.