When you wire solar panels or batteries to your campervan electrical system, you have a choice: series, parallel, or a hybrid. Getting this wrong means poor performance, damaged components, or a fire risk.
This guide covers the difference between series and parallel wiring for both solar panels and batteries, and how to choose the right configuration for a UK van build.
Summary
| Application | Recommended | Why |
|---|---|---|
| Solar panels with MPPT | Series (or series-parallel) | Higher voltage = better efficiency |
| Solar panels with PWM | Parallel | Series doesn't help PWM |
| Batteries for 12V system | Parallel (with busbar) | Keep existing appliances |
| Batteries for 24V system | Series | Higher voltage for large inverters |
| Long cable run (solar) | Series | Lower current = thinner cable |
| Shaded roof | Parallel | Shade tolerance matters more than efficiency |
The Basic Difference
| Property | Series | Parallel |
|---|---|---|
| Voltage | Adds up (12V + 12V = 24V) | Stays the same (12V + 12V = 12V) |
| Current | Stays the same (5A + 5A = 5A) | Adds up (5A + 5A = 10A) |
| Total power | Same (both give same total wattage) | Same |
| Shading tolerance | Poor — one shaded panel kills the string | Good — shaded panel doesn't affect others |
Series = voltage adds, current stays the same. Parallel = voltage stays the same, current adds.
Solar Panels: Series Wiring
Wire two 100W panels in series:
- Each panel: 18V (Voc), 5.6A (Isc)
- Combined: 36V, 5.6A
- Total: 200W
When to use series:
- MPPT needs higher voltage. An MPPT solar charge controller is more efficient at higher input voltages. With 36V input instead of 18V, an MPPT controller works at 95-98% efficiency instead of 85-90%.
- Long cable runs. Higher voltage means lower current means thinner cables. A 36V system at 5.6A can use 2.5mm² cable for a 10m run. A 12V system at 11.2A (parallel) would need 4mm² or 6mm² for the same run.
- Future expansion. You can add more panels in series to reach 48V or 72V without adding more cable capacity.
Problems with series in the UK:
- Shading. Series wiring is vulnerable to partial shading. If one panel on a Transit Custom roof is shaded by a roof vent, a bike rack, or a tree branch, its current drops — and the current through the entire string drops. Your 200W system might output 50W.
- The UK has a lot of shade. Overcast UK skies mean low light — series doesn't help with that because the voltage is already high enough, and the current is already low. The shading sensitivity is a real problem.
Solar Panels: Parallel Wiring
Wire two 100W panels in parallel:
- Each panel: 18V, 5.6A
- Combined: 18V, 11.2A
- Total: 200W
When to use parallel:
- Shaded conditions. If one panel is shaded, the other still produces full current. In a UK winter with patchy cloud, parallel consistently outperforms series on a van roof.
- PWM controllers. A PWM controller (cheaper than MPPT, less efficient) needs the panel voltage close to the battery voltage. 18V is ideal for a 12V battery. 36V in series would be wasted on a PWM controller.
- Simple installation. Parallel wiring is simpler — no need for fuses between panels (though you should still fuse each string).
Problems with parallel:
- Higher current. 11.2A at 12V needs thicker cables than 5.6A at 36V. A 10m run from roof panels to the controller needs 6mm² cable for 11.2A to stay under 3% voltage drop.
- Lower controller efficiency. Even with an MPPT, lower voltage input means the controller works harder to boost the voltage up to battery charging level. Efficiency drops from 95% to 85-88%.
- Blocking diodes required. Without blocking diodes, current from one panel can flow backwards through the other panel at night, draining your battery (solar panels produce negative voltage at night if wired in parallel without protection).
The Right Choice for UK Van Solar
| Scenario | Configuration | Why |
|---|---|---|
| Small panels (100-200W total), MPPT controller | Series | Higher voltage = better MPPT efficiency |
| Large panels (300-400W total), MPPT controller | Series-parallel (2 strings of 2 in series, then parallel) | Limits current to 11.2A per string, keeps voltage at 36V |
| PWM controller | Parallel | Series doesn't help PWM — parallel gives better shade tolerance |
| Heavy shading (roof vents, bike racks) | Parallel | Shade on one panel doesn't kill the other |
| Long cable run (5m+) | Series | Higher voltage = lower voltage drop |
My recommendation for UK vans: Wire two 100W or 200W panels in series (if using MPPT) because the efficiency gain outweighs the shading risk. If you have roof obstructions (vents, AC, bike rack), wire in parallel for shade tolerance.
Batteries: Series Wiring
Wire two 100Ah 12V batteries in series:
- Each battery: 12V, 100Ah
- Combined bank: 24V, 100Ah
- Total: 2,400Wh
When to use series:
- 24V or 48V systems. If your inverter runs on 24V (some larger inverters are 24V-only), you need series. 24V systems are more efficient for large power draws (over 2,000W).
- Long cable runs. 24V at 50A carries the same power as 12V at 100A but with half the cable thickness.
- Higher voltage appliances. Unusual in campervans but some marine and off-grid equipment runs on 24V.
Problems with series in vans:
- Everything in a van runs on 12V. Your lights, fridge, water pump, USB sockets — they're all 12V. You'd need a 24V-to-12V DC-DC converter (£80-150) to power them.
- Imbalanced charging. If the two batteries in series have slightly different internal resistance (they always do), one will charge slightly more than the other. Over time, this imbalance reduces overall capacity.
- Safety. 24V is more dangerous than 12V. The risk of arcing is higher. 48V is genuinely unsafe for DIY work — sparks can weld tools to terminals.
Batteries: Parallel Wiring
Wire two 100Ah 12V batteries in parallel:
- Each battery: 12V, 100Ah
- Combined bank: 12V, 200Ah
- Total: 2,400Wh
When to use parallel:
- Standard 12V systems. Almost every campervan runs on 12V. Parallel keeps the voltage at 12V.
- Increasing capacity. Add batteries in parallel to extend run time.
- Mixing existing batteries. If you add a new battery to an existing bank, parallel is the only practical option.
Problems with parallel:
- Imbalanced charging. Batteries in parallel share the current unevenly based on their internal resistance. The battery closest to the positive terminal gets charged first and discharged last. Over time, this causes one battery to age faster.
- Fusing complexity. Each parallel string needs its own fuse. If one battery shorts internally, the other batteries dump their full power through it — instantly melting cables starting fires.
Best Practice for Parallel Battery Banks
- Use identical batteries. Same brand, same age, same capacity. Mixing old and new batteries causes imbalance.
- Wire as a busbar, not daisy-chain. Connect each battery's positive and negative to a common busbar, not from one battery to the next. This ensures even current sharing.
- Fuse each battery. Each positive terminal needs a fuse rated for the total bank current divided by the number of batteries.
- Use a battery balancer (Victron Battery Balancer, £50-80) to keep two 12V batteries in parallel evenly charged.
Series-Parallel Hybrid
If you have 4 solar panels or 4 batteries, you can use a series-parallel combo.
Solar example (4x 100W panels):
- Wire 2 panels in series (36V, 5.6A) — one pair
- Wire the other 2 in series (36V, 5.6A) — second pair
- Wire both pairs in parallel (36V, 11.2A) — combined
This gives you 36V (good MPPT efficiency) with 11.2A (acceptable cable thickness). Shading one panel only affects one pair — the other pair still produces full power.
Battery example (4x 100Ah batteries):
- Wire 2 in parallel (12V, 200Ah) — first bank
- Wire 2 in parallel (12V, 200Ah) — second bank
- Wire both banks in parallel (12V, 400Ah)
This gives you 400Ah at 12V with better balance than a single 4-battery parallel bank.
Common Mistakes
| Mistake | Result |
|---|---|
| Series wiring solar panels with a PWM controller | Wasted potential — the extra voltage is dumped as heat |
| Parallel wiring batteries without individual fuses | Fire risk if one battery shorts internally |
| Mixing old and new batteries in series or parallel | Reduced capacity, one battery fails early |
| Series wiring 12V solar panels for a 24V system | Works, but you need a 24V-to-12V converter for your 12V loads |
| Not using blocking diodes for parallel solar panels | Battery drains through the panels at night |
