The short answer: Wiring solar panels in series increases the total voltage while keeping the amperage the same. Wiring in parallel increases the total amperage while keeping the voltage the same. Most modern MPPT systems use a combination of both (series-parallel) to keep voltage high enough for efficiency but low enough to protect the charge controller.
Choosing how to wire your solar array dictates what size wires you need, what charge controller you can use, and how your system handles partial shading.
Series Wiring: Increasing Voltage
When you wire panels in series, you connect the positive terminal of the first panel to the negative terminal of the second panel, and so on.
- Voltage: Adds up. (e.g., Two 20V panels in series = 40V)
- Amperage: Stays the same as a single panel. (e.g., Two 5A panels in series = 5A)
- Best for: Systems with MPPT charge controllers, long wire runs (higher voltage means less voltage drop), and unshaded locations.
Parallel Wiring: Increasing Amperage
When you wire panels in parallel, you connect all the positive terminals together (usually using branch connectors or a combiner box) and all the negative terminals together.
- Voltage: Stays the same as a single panel. (e.g., Two 20V panels in parallel = 20V)
- Amperage: Adds up. (e.g., Two 5A panels in parallel = 10A)
- Best for: Systems with PWM charge controllers, 12V RV systems, and locations with heavy partial shading.
Illustrative Worked Example
Let’s look at how wiring changes the output of four identical 100W solar panels.
- Panel Specs: 20 Volts (Voc), 5 Amps (Isc)
Scenario A: All 4 in Series
- Total Voltage: 20V + 20V + 20V + 20V = 80 Volts
- Total Amperage: 5 Amps
- Total Power: 80V × 5A = 400 Watts
Scenario B: All 4 in Parallel
- Total Voltage: 20 Volts
- Total Amperage: 5A + 5A + 5A + 5A = 20 Amps
- Total Power: 20V × 20A = 400 Watts
Scenario C: Series-Parallel (2 strings of 2)
- Total Voltage: 20V + 20V = 40 Volts
- Total Amperage: 5A + 5A = 10 Amps
- Total Power: 40V × 10A = 400 Watts
(Note: This is an illustrative example. The total wattage remains 400W in all scenarios, but the voltage and amperage delivered to the charge controller change drastically.)
System Design Nuances Often Ignored
Many basic guides explain the math but miss the practical implications of wiring choices:
- The Shading Penalty in Series: If a single leaf or chimney shadow covers just 10% of one panel in a series string, the output of the entire string drops to match the shaded panel. Parallel wiring isolates the shaded panel, allowing the others to operate at full power.
- Wire Thickness and Cost: High amperage requires thick, expensive copper wire to prevent dangerous heating and voltage drop. By wiring in series to increase voltage and keep amperage low, you can safely use thinner, cheaper wire for long runs from the roof to the charge controller.
- Cold Weather Voltage Spikes: Solar panel voltage increases in freezing temperatures. If you wire too many panels in series, the cold-weather voltage spike can exceed your charge controller's maximum input limit, destroying the unit.
Practical Checklist for Wiring
Before climbing on the roof, review your plan:
- Check Controller Limits: Does your series string voltage (Voc) stay at least 15% below the charge controller's maximum input voltage?
- Assess Shading: Will a tree shade part of the array at 3 PM? If so, consider parallel wiring or a series-parallel hybrid.
- Size Your Wires: Use a WattSizing Calculator or voltage drop chart to ensure your wire gauge can handle the total amperage of your parallel connections safely.
- Use Fuses: When wiring three or more panels in parallel, you must use inline fuses on each positive wire before the combiner box to prevent electrical fires in case of a short circuit.
Frequently Asked Questions
Can I mix different size solar panels in series?
It is highly discouraged. If you wire a 5A panel and a 10A panel in series, the entire string will be bottlenecked by the lowest amperage panel, limiting the whole system to 5A.
Can I mix different size solar panels in parallel?
Yes, but only if their voltages are nearly identical (within 0.5V to 1V of each other). If you wire a 20V panel and a 40V panel in parallel, the higher voltage panel will push current into the lower voltage panel, potentially causing damage.
Which is better for an RV: Series or Parallel?
For most RVs, parallel is safer because RV roofs frequently experience partial shading from AC units, vents, and trees. However, if you have a large array and an MPPT controller, a series-parallel setup offers the best balance of shade tolerance and wire efficiency.
Do I need a combiner box for series wiring?
No. Series wiring simply connects the positive lead of one panel to the negative lead of the next, creating a single long chain. You only need a combiner box or branch connectors when wiring in parallel.
