The short answer: in series, panel voltages add while current stays the same; in parallel, currents add while voltage stays the same. Match currents within a series string, match voltages across parallel strings, keep your cold-weather voltage under the charge controller's ceiling, and fuse parallel strings.
The one-minute comparison
| Series | Parallel | |
|---|---|---|
| Voltage | Adds up | Stays the same |
| Current | Stays the same | Adds up |
| Wiring hardware | MC4s chain panel-to-panel; extension pairs as needed | Y-branch MC4 connectors + inline fuses |
| Long cable runs | Friendlier — higher voltage, lower loss | Needs thicker copper for the same power |
| Partial shading | One shaded cell drags the whole string | Other strings keep producing |
| Watch out for | Cold-weather voltage vs controller limit | Current stacking vs wire and fuse ratings |
Series: voltage stacks, current doesn't
Connect the positive lead of one panel to the negative lead of the next — which is exactly what the factory MC4 connectors are shaped to do. Two panels with a 20 V open-circuit voltage become a 40 V string; add a third and you're at 60 V, all at the same current. This is why higher-voltage strings are friendlier on long cable runs: less current means less voltage drop in the same copper (see the sizing guide).
Parallel: current stacks, voltage doesn't
Connect positives together and negatives together using Y-branch MC4 connectors. The string voltage stays at one panel's level while the currents stack — which is exactly the case where the wiring, not the panels, becomes the limit. Common practice: two parallel strings usually get by without string fuses; three or more need an inline fuse per string, sized above the string's short-circuit current. The hardware exists for this: Y-branches plus MC4 inline fuse holders.
The ceiling that kills controllers
Every MPPT charge controller has a maximum input voltage — a never-exceed limit, not a target. And here's the trap: panel voltage rises as temperature falls, so a string that measures fine on a summer afternoon can sail past the limit on a −10 °C morning. A common engineering convention is to keep your estimated cold-weather open-circuit voltage under roughly 90% of the controller's maximum rating, then confirm with the panel datasheet's temperature coefficient (for mono-crystalline silicon it's commonly around −0.3 %/°C — check your exact panel).
Worked illustration: a string with 100 V of nameplate Voc, a −0.3 %/°C coefficient, and a record low of −10 °C: Voccold ≈ 100 V × (1 + 0.003 × 35) ≈ 110.5 V. If your controller's max input is 100 V, that string destroys it on the first cold snap. If the limit is 150 V, you're comfortably inside it.
Mixing mismatched panels
- In series, match currents. The panel with the lowest current caps the whole string — a 10 A panel dragging alongside 13 A panels throws away the difference.
- In parallel, match voltages. The lowest voltage panel sets the bus; the others run throttled back to meet it.
- Mismatched mixes are legal, just wasteful. When in doubt, keep each series string uniform and treat the string as one unit.
Which one should you pick?
- Start from the controller, not the panels: its max input voltage and rated current define the window you must stay inside, cold morning included.
- Long cable runs → favor series (higher voltage, thinner copper).
- Mixed shading or mixed panel types → favor parallel-by-string: build uniform series strings, then parallel the strings with fusing.
- Then pick wire gauge with the sizing guide and verify every connector joint with the crimping guide.