Wiring panels: series vs parallel vs series-parallel

Series adds voltage. Parallel adds current. Series-parallel does both. Which configuration works depends on your charge controller and battery bank.

Two identical 100W panels wired differently produce the same total power (200W) but at different voltage and current combinations. The charge controller cares about the difference. Getting the wiring wrong means lost power, damaged equipment, or a system that works at half capacity on cloudy days.

Series wiring

Connect the positive terminal of one panel to the negative terminal of the next. The output voltage is the sum of both panels. The current stays the same as a single panel.

Two 100W panels (Vmp 18V, Imp 5.56A) in series: output is 36V at 5.56A = 200W.

When to use series:

With an MPPT charge controller. MPPT controllers convert high-voltage, low-current panel input into the lower voltage your battery needs, and they do it efficiently. Higher input voltage from series wiring gives the MPPT algorithm more headroom to work with, especially in hot weather when panel voltage drops.
When wire runs are long. Higher voltage means lower current for the same power, which means less voltage drop in the wires. A 36V circuit at 5.5A loses less power over a 10-meter cable run than an 18V circuit at 11A.

When not to use series:

With a PWM charge controller. PWM controllers don't convert voltage; they just connect the panel directly to the battery and clip the excess voltage. Two 18V panels in series produce 36V, but a PWM controller charging a 12V battery wastes the difference as heat. You'd be throwing away nearly half the power.
When cold-weather Voc exceeds the controller's maximum input. Each panel added in series increases the total Voc. Two panels with Voc 22V in series produce 44V open circuit, which rises further in cold temperatures. Check the solar array calculator to make sure the cold-adjusted Voc stays within your controller's rated maximum.

Shade vulnerability: Series wiring is sensitive to partial shading. If one panel is shaded, its reduced current limits the entire string. Bypass diodes (built into most modern panels) help, but shaded performance in series is still worse than in parallel.

Parallel wiring

Connect all positive terminals together, all negative terminals together. The output voltage stays the same as a single panel. The current is the sum of all panels.

Two 100W panels (Vmp 18V, Imp 5.56A) in parallel: output is 18V at 11.12A = 200W.

When to use parallel:

With a PWM charge controller. PWM needs the panel voltage to be close to (but a few volts above) the battery voltage. Two panels in parallel keep the voltage at 18V, which is correct for a 12V battery through PWM.
When partial shading is likely. In parallel, a shaded panel produces less current but doesn't drag down the other panels. Each panel contributes independently. If one panel drops to 50% output from shade, the other still produces full current.

When not to use parallel:

When wire runs are long. Parallel wiring produces high current at low voltage, which increases voltage drop losses in the cables. At 11A and 18V over a 10-meter run with undersized wire, you might lose 5-10% of your power just in the cable.
When you have many panels. As you add panels in parallel, total current increases and you need thicker cables, larger fuses, and a charge controller rated for the combined current.

Series-parallel wiring

Combine both. Wire panels into groups (strings) in series, then connect the strings in parallel.

Four 100W panels (Vmp 18V, Imp 5.56A): wire two panels in series to make a 36V, 5.56A string. Make two such strings. Connect the strings in parallel: output is 36V at 11.12A = 400W.

When to use series-parallel:

With four or more panels and an MPPT controller. Series-parallel gives you the voltage boost that MPPT controllers prefer while keeping current per string manageable.
When you want shade tolerance across strings. If one string is partially shaded, the other string continues at full output.

Matching panels. All panels within a series string should be identical (same model, same wattage, same Vmp and Imp). Mismatched panels in series reduce the string's output to the weakest panel's current. Strings wired in parallel should have matching voltage. Mismatched string voltages cause the higher-voltage string to push current into the lower-voltage string rather than into the controller, wasting power.

Quick decision table

PWM controller + a twelve-volt battery: panels in parallel (or one panel alone). Keep panel voltage near eighteen volts.

PWM controller + a twenty-four-volt battery: two panels in series (Vmp around thirty-six volts) or panels designed for twenty-four-volt systems.

MPPT controller + a twelve-volt or twenty-four-volt battery: series (preferred for efficiency) or series-parallel for three or more panels. Check that total Voc stays within the controller's maximum input voltage rating.

Any controller, partial shade expected: parallel gives better shade performance per panel, at the cost of higher current and thicker wiring.

The solar array calculator checks controller compatibility, wire sizing, and shade tolerance for any configuration you enter. Start there before committing to a wiring plan.

Practical install tips

MC4 connectors. Modern solar panels come with MC4 connectors pre-attached on flying leads. These are weatherproof, tool-free (snap together by hand, require a disconnect tool to separate), and rated for the voltages and currents in residential solar. Use MC4 branch connectors for parallel wiring rather than stripping wire and using wire nuts (which aren't weatherproof and can arc in DC circuits).

Wire management. Route cables neatly along the panel frame or mounting rail. Secure with UV-resistant cable ties or clips every 30-60 cm. Loose cables flap in the wind, which fatigues the wire at stress points and can eventually break the conductor or wear through the insulation.

Don't exceed the charge controller's input voltage. In series wiring, panel voltages add up. Three 40V panels in series produce 120V at the controller input. If your charge controller is rated for a maximum of 100V input, this configuration destroys it. Always check the controller's maximum input voltage specification before wiring panels in series. MPPT controllers have higher voltage limits (typically 100-150V) than PWM controllers (typically 25-50V).

Label your wiring. At the combiner box and charge controller, label which cable comes from which panel or string. When troubleshooting a production problem months later, knowing which wire is which without tracing cables across the roof saves significant time.

The solar array calculator models series, parallel, and series-parallel configurations to help you select the right topology for your charge controller.