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When a cabin battery bank needs more charge than a lone panel can give, wiring solar panels in series is the way to raise the string voltage without adding cables: link the panels positive to negative, and the string feeds the charge controller at a higher voltage than any panel alone. The steps below show how to check the panel labels, link the string, run it to the controller and check it before switching on. Anything past the charge controller toward home wiring is work for a licensed electrician, and this guide does not cover it.
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The quick answer: wiring solar panels in series
In series wiring, the positive lead of a panel joins the negative lead of the next panel, down the string, until the free ends run to the charge controller. The voltage across the string adds up from panel to panel, while the current stays at the level of a panel on its own. That higher voltage suits a charge controller with a high input limit and lets the string run over a longer cable with less loss, while the panels must match so no panel drags the rest of the string down. Check the panel labels and the controller label before you link anything, and keep every connection dry and tight.
Check the panel labels
Start at the label on the back of each panel, which lists the voltage and current figures the maker states. Use panels with matching figures in a string, since a panel with lower figures caps what the string can give, and the rest of the panels cannot make up the gap. Note the open-circuit voltage figure on each label as well, since the controller must take the full string voltage on a cold morning when the figure runs at its highest. If a label is worn or missing, do not guess the figures: use a panel with a readable label instead.
| Wiring | Voltage | Current |
|---|---|---|
| Series | Adds across panels | Same as a panel on its own |
| Parallel | Same as a panel on its own | Adds across panels |
The table above is the whole rule: series trades voltage for current, parallel trades current for voltage, and the controller must suit whichever side grows. A series string suits a long cable run and a controller with a high input limit, while a parallel set suits a low-voltage controller and a short run.
Connect positive to negative
Link the string with the MC4 leads on the panels: the positive lead of the first panel joins the negative lead of the next, and so on down the string, until the last panel leaves a free positive and the first panel leaves a free negative. Push each MC4 pair together until it clicks, then give the joint a firm pull by hand to confirm it holds. Keep the free ends apart and capped while you work, so they cannot touch each other or the frame. Work in shade or with the panels face down while linking, since a panel in sun is live at its leads. See the best MC4 connectors for skoolie for how MC4 leads differ from other connectors.
Run the string to the controller
Run the free ends of the string to the solar input of the charge controller, positive to positive and negative to negative, using an MC4 extension cable where the run is too long for the panel leads. Check the controller label for its highest input voltage first, and keep the string voltage below it with room to spare. Keep the cable run tidy and off the ground, with a drip loop where it meets the controller, so rain runs off instead of into the housing. Anything past the controller toward home wiring is work for a licensed electrician.
Check the string before you switch on
Walk the string before you switch the controller on: each MC4 pair clicked shut, no bare metal showing, no lead pinched under a frame edge, and the free ends on the right terminals. Look at the controller display as the sun hits the panels: it should show the string charging, with the voltage settling near the sum of the panel figures. If the display shows nothing, switch off and re-check each joint in turn, since a loose MC4 pair is the usual fault. Never work on a live string with wet hands, and never let the free ends touch while the panels are in sun.
Ways to keep the string healthy
- Keep every panel in the string in the same sun: shade on a panel drags the whole string down, so trim branches and mind shadows through the day.
- Use panels with matching figures in a string, and never mix a shaded panel into a sunny string.
- Check the MC4 joints after storms and at the start of each season, and re-seat any joint that feels loose.
- Keep cables off the ground and out of walkways, with slack at the panel so wind cannot tug the joints.
- For a bigger array, add a parallel string through its own run to the controller rather than stretching a string past the controller limit.
FAQ: power away from the grid
Can I wire different panels in series?
Only where the figures match: use panels with the same voltage and current figures on their labels, since a panel with lower figures caps the string. A panel from another batch with the same figures on its label is fine, but a panel with different figures is not a match for the string.
Does series wiring need a special controller?
It needs a controller that takes the higher string voltage: an MPPT controller suits series strings, while a PWM controller suits a lower-voltage string. Check the controller label for its highest input voltage before you link the string, and keep the string below it.
What happens if a panel in the string is shaded?
The shaded panel limits the whole string, so the string gives only what the shaded panel can pass. Keep the string in the same sun, trim branches that throw shade, and site the string where shadows do not cross it through the day.
Can I add more panels to a series string later?
Yes, where the controller allows it: check the controller label for its highest input voltage first, since each panel added to the string raises the string voltage. Add panels with matching figures, link positive to negative as before, and check the string again before switching on.
Related solar guides
See the DIY solar panel kit, the solar panel by size, the MC4 vs Anderson connectors for solar and the solar panel kit with inverter vs without.
