Where does a solar panel give its most power?

A solar panel gives its most power at its maximum power point, the knee of its current-voltage curve, roughly 80% of its open-circuit voltage. Short it and it pushes its full current at 0 V; leave it open and it shows its full voltage at 0 A. Both give no power. A good charger holds the panel near the knee, and a blocking diode stops the battery draining back through the panel at night.

A current source with a ceiling

Each photon a solar cell absorbs frees a charge, so in steady light a cell pushes a steady current: a panel is a current source. But each cell is also a big diode, wired across that source. Load the panel lightly and it pushes its full current, here 100 mA. Raise the load's resistance and the voltage rises, until the cells' own diodes start taking the current for themselves. At the open-circuit voltage, 4.9 V for this six-cell panel, they take all of it.

Find the knee

Power is zero at both ends and peaks at the knee, about 0.8 × 4.9 ≈ 4 V here. A resistor load sits there when it equals the knee's volts over its amps:

A 10 Ω load gets the full 100 mA but only 1 V: 0.1 W. At 100 Ω the panel hits its ceiling with only 47 mA flowing. And in half the light the knee moves, so the best load becomes about 82 Ω.

MPPT: chase the knee

The knee moves with light and temperature: a hot panel loses roughly 0.3 to 0.4% of its voltage for every degree. So good chargers don't rely on a fixed load. A maximum power point tracker is a buck or boost converter that nudges its input voltage up and down, many times a second, and stays wherever the power is highest. A buck can only lower the voltage and a boost only raise it, so which one you need depends on whether the panel's knee sits above or below the battery.

The blocking diode

After dark the light's current stops, but the diodes are still there, and a charged battery would push current back through them all night. A diode between panel and battery blocks that. Choose a Schottky: it drops about 0.3 V instead of a silicon diode's 0.7 to 0.8 V, so less of the day is wasted, and a smaller drop keeps the panel nearer its knee.

With a current source already limiting the amps, a panel is half a lithium charger. Add a Zener across the cell to cap the voltage and it's a crude constant-current, constant-voltage charger; leave the Zener out and the panel pushes the cell past its 4.2 V limit, toward the panel's own ceiling.

Where you'll meet it

A calculator's strip of cells, a garden light and a roof full of panels all work on this curve. A home inverter's MPPT holds the panels at their knee as clouds pass.