Wiring RV solar panels in series, parallel or series-parallel

The two numbers on every panel

Every solar panel label lists several ratings. The ones that matter most for wiring are:

• Vmp and Imp: voltage and current at maximum power, under standard test conditions.
• Voc: open-circuit voltage, the highest voltage the panel produces when it's not connected to a load.
• Isc: short-circuit current, the highest current it produces.
• Maximum series fuse rating: the largest fuse allowed to protect the panel.
• Temperature coefficient of Voc: how much the voltage rises as the panel gets colder (as a % per °C).

The examples below use a made-up panel rated 20 V / 5 A at maximum power. Use your own panel's numbers.

Series wiring: voltages add

Connect the positive (+) of one panel to the negative (−) of the next. Voltage adds and current stays the same.

Three panels in series: 20 + 20 + 20 = 60 V at 5 A.

• Pros: Lower current means less voltage drop, so you can use thinner wire on long runs from roof to controller. Higher voltage also lets an MPPT controller start charging earlier in the morning and in weak light.
• Cons: Shade or snow on one panel reduces output of the whole string (bypass diodes in the panel help). Needs an MPPT controller. Cold-weather voltage must stay under the controller's limit.

Parallel wiring: currents add

Connect all positives together and all negatives together. Current adds and voltage stays the same.

Three panels in parallel: 20 V at 5 + 5 + 5 = 15 A.

• Pros: Shade on one panel only affects that panel. Works with PWM controllers when panels are 12V-nominal.
• Cons: Higher current needs heavier wire. With three or more panels or strings in parallel, each one generally needs its own fuse, because a fault in one could receive current from all the others. Check the panel's maximum series fuse rating and the controller manual.

Series-parallel: the best of both

Wire panels into identical series strings, then connect the strings in parallel.

Two strings of two panels: each string 40 V at 5 A; combined 40 V at 10 A.

Every string must have the same number and type of panels. Mixing panels or string lengths wastes power and can confuse the controller.

MPPT vs PWM, and what it means for your wiring

• PWM controllers connect the panels more or less directly to the battery, so panel voltage is pulled down to battery voltage. Any panel voltage above what the battery needs is wasted. Use 12V-nominal panels in parallel with a 12V battery.
• MPPT controllers convert higher panel voltage into more charging current. They accept series strings and generally harvest more energy, especially when panels are cold. They cost more.

The Alberta cold-voltage check

Panel voltage rises as temperature drops. On a clear, cold Alberta morning, a series string can exceed the controller's maximum input voltage and damage it. Always do this check:

1. Find the panel's Voc and temperature coefficient of Voc on the datasheet. For example, 22.5 V and −0.30 %/°C.
2. Pick the coldest temperature the panels might see. Readings of −30 °C or colder happen in Alberta, and if you store or camp in winter, plan for them.
3. Temperature difference from the 25 °C test condition: 25 − (−30) = 55 °C.
4. Voltage increase: 55 × 0.30% = 16.5%. Cold Voc ≈ 22.5 × 1.165 ≈ 26.2 V per panel.
5. Multiply by panels per string. 3 in series ≈ 78.6 V (fine for a 100 V controller). 4 in series ≈ 104.8 V, which exceeds a 100 V controller.

These are example numbers. Use your panel's datasheet and your controller's maximum PV input voltage. Some controller makers publish string-sizing calculators that do this for you.

Wire sizing and voltage drop

Wire must handle the current safely (ampacity), and long runs should keep voltage drop low.

• Voltage drop (V) ≈ 2 × one-way length × current × wire resistance per unit length. The "2" accounts for the positive and negative conductors.
• A common target for charging circuits is about 3% or less of system voltage.
• Size PV wire from the array's short-circuit current with a safety factor. Multiplying Isc by 1.25 is a widely used rule. Then check it against the fuse rating and voltage drop.
• Size the controller-to-battery cable for the controller's maximum output current, and keep that run short.

Wire manufacturers and controller makers publish ampacity and voltage-drop tables. Use them, and remember that wire inside hot roofs or bundled with other cables carries less current safely.

Quick comparison

When to call a pro

If you're unsure about fusing, cold-weather voltage, or combining different panels, get advice from a qualified solar or RV electrical technician before you buy parts.

Related guides

• Planning an RV solar system for Alberta: energy audit, panels, batteries and controller
• How to install a roof-mounted RV solar kit, step by step
• RV solar and battery fusing, breakers, grounding and safety