Solar panels are funny power sources: for each panel, if you draw no power, the voltage will rise to 15..40 V (depending on the type of panel), and when you short them out, a current of 5..12 A will flow (again, depending on type). My panels will do 30V @ 8A.
Note that in both cases just described, the power output will be zero: power = volts x amps, so when either one is zero, there’s no energy transfer! – to get power out of a solar panel, you have to adjust those parameters somewhere in between. And what’s even worse, that optimal point depends on the amount of sunlight hitting the panels…
As you draw more current, there’s a “knee” at which the predominantly voltage-controlled output drops, until the panel is asked to supply more than it has, after which the output voltage drops very rapidly.
Power is the product of V and A, which is equivalent to the surface of the area left of and under the current output point on the curve.
But how do you adjust the power demand to match that optimal point in the first place?
The trick is to vary the demand a bit (i.e. the current drawn) and then to closely watch what the voltage is doing. What we’re after is the slope of the line – or in mathematical terms, its derivative. If it’s too flat, we should increase the load current, if it’s too steep, we should back off a bit. By oscillating, we can estimate the slope – and that’s exactly what my inverter seems to be doing here (but only on down-slopes, as far as I can tell):
As the PV output changes due to the sun intensity and incidence angle changing, the SMA SB5000TL inverter adjusts the load it places on the panels to get the most juice out of ‘em.
Update – I just came across a related post at Dangerous Prototypes – synchronicity!