Connecting Solar Panels: Series vs Parallel

Connecting solar panels correctly is one of the most important steps in any solar installation. Get the wiring right and your system will generate electricity safely and efficiently for decades. Get it wrong and you risk underperformance, equipment damage, or worse, a serious electrical hazard.

Whether you’re planning a simple 4kWp rooftop system or a larger off-grid setup, understanding how series and parallel connections work will help you ask the right questions when talking to installers and make sense of the system your installer proposes.

Key Takeaways

• Solar panels can be wired in series (to increase voltage), in parallel (to increase current), or in a combination of both
• Series wiring is standard for string inverter systems; parallel wiring suits lower-voltage battery setups
• Cable sizing must account for the maximum current your string will carry to prevent overheating and voltage drop
• Shading affects series strings severely but has less impact on parallel or microinverter setups
• All UK solar PV wiring must comply with BS 7671 (IET Wiring Regulations) and Part P of the Building Regulations
• MCS-certified installers handle all wiring to regulation standard, including grid connection requirements

Series vs Parallel: The Fundamental Choice

When you connect solar panels together, you have two basic options: series or parallel. Each has different effects on your system’s voltage and current output, and the right choice depends on your inverter type, system voltage, and whether shading is a concern.

Connecting Panels in Series

In a series connection, the positive terminal of one panel connects to the negative terminal of the next, continuing down the string. The voltages of each panel add together, while the current stays the same as a single panel.

So if you have six 400W panels each rated at 40V and 10A, wiring them in series gives you a string voltage of 240V at 10A. This higher voltage suits string inverters, which typically require an input voltage between 200V and 800V to operate efficiently.

Series wiring is the standard approach for grid-tied residential installations in the UK. It minimises wiring complexity and works well with most string inverter products from brands like SolarEdge, Fronius, and SMA.

The main drawback is shading sensitivity. If one panel in a series string is shaded or underperforms, the entire string’s output drops to match it, much like the weakest link in a chain. This is where power optimisers or microinverters become valuable (see shading section below).

Connecting Panels in Parallel

In a parallel connection, all positive terminals connect together and all negative terminals connect together. The currents of each panel add together, while the voltage stays the same as a single panel.

Using the same six panels, a parallel connection gives you 40V at 60A. This lower voltage suits 12V, 24V, or 48V battery-based systems, such as off-grid setups, caravans, or garden buildings. Many MPPT charge controllers are designed for parallel panel arrays feeding 12V or 24V battery banks.

Parallel wiring is more shade-tolerant because shading one panel only reduces its contribution, rather than dragging down the whole array. However, parallel connections require thicker, higher-rated cables to handle the combined current, and the wiring is more complex as the string grows.

Series-Parallel Combinations

Larger systems often use a combination of both. Panels are grouped into series strings to build voltage, and those strings are then connected in parallel to increase current capacity. This approach balances voltage optimisation for the inverter with redundancy, so that one underperforming string does not shut down the entire array.

A typical 20-panel, 8kWp system might use two strings of 10 panels each, wired in series, with both strings connected in parallel at the inverter’s DC input. Your installer will design the string configuration based on your panel specifications and inverter’s accepted voltage and current ranges.

Cable Sizing and Voltage Drop

Choosing the correct cable size is critical. Undersized cables cause resistive losses, generate heat, and can be a fire risk. Oversized cables are safe but wasteful. The goal is to select a cable cross-section that keeps voltage drop within acceptable limits while safely carrying the maximum expected current.

Standard Cable Sizes for Solar PV

In the UK, solar PV installations use single-core, double-insulated cables rated for outdoor and UV exposure. Common cross-sections are:

• 4mm² cable, suitable for most residential string currents up to around 10A per conductor
• 6mm², used where strings carry higher currents or cable runs are long
• 10mm², for main DC tails from combiner boxes to the inverter on larger systems

Your installer calculates the required cross-section using the maximum string current (typically 1.25 × Isc to account for fault conditions), the cable run length, and the permissible voltage drop (generally no more than 1% on the DC side for a well-designed system).

Voltage Drop Calculation

Voltage drop is the loss of voltage along a cable due to its resistance. For DC circuits, the formula is:

Voltage Drop (V) = Current (A) × Resistance per metre (Ω/m) × Cable length (m) × 2

The factor of 2 accounts for both the positive and negative conductors. As an example, a 10A string running 20 metres each way through 4mm² cable (resistance approximately 4.6 mΩ/m) loses around 1.8V, or roughly 0.75% of a 240V string voltage. That’s within acceptable limits. But if the same string used 2.5mm² cable (resistance 7.4 mΩ/m), the drop would be around 3V, affecting generation output noticeably over the system’s lifetime.

Shading and How Connection Method Affects It

Shading is one of the most significant factors affecting solar panel performance in the UK, where chimneys, dormers, trees, and neighbouring buildings frequently cast shadows across roof surfaces.

Series Strings and Bypass Diodes

Every solar panel contains bypass diodes, typically one per group of 20 cells within the panel. These diodes activate when one cell group is heavily shaded, routing current around the shaded section rather than letting it become a resistive bottleneck. This limits, but does not eliminate, the power loss from shading.

When a panel in a series string is partially shaded, the operating current of the entire string is constrained. Even with bypass diodes, a shadow covering a third of one panel in a 10-panel string can reduce the string’s output by significantly more than one-tenth. The MPPT algorithm in your inverter will try to find the best operating point, but partial shading creates multiple peaks in the power curve that can confuse single MPPT tracking.

Power Optimisers

Power optimisers (made by SolarEdge and others) attach to each panel individually and perform MPPT tracking at panel level before passing DC power to the string inverter. This means each panel operates at its own best output point regardless of what neighbouring panels are doing. Shaded panels no longer drag down the string.

Power optimisers add cost, typically £40-80 per panel, but on roofs with shading obstacles they can recover enough lost generation to justify the investment, often paying back within three to five years through higher output.

Microinverters

Microinverters go further by converting DC to AC at each panel rather than at a central unit. Each panel operates independently, so shading, soiling, or degradation on one panel has zero effect on its neighbours. Brands like Enphase dominate this segment.

Microinverter systems cost more upfront but offer the best shade tolerance, panel-level monitoring, and design flexibility for complex roofs. They also eliminate the single point of failure that a central string inverter represents.

UK Regulatory Requirements

All solar PV wiring in the UK must comply with BS 7671 (the IET Wiring Regulations, 18th Edition) and Part P of the Building Regulations, which covers electrical safety in dwellings.

Part P Compliance

Part P requires that electrical installation work in homes, including solar PV systems, is either carried out by a registered competent person or notified to and inspected by your local building control authority. Using an MCS-certified installer satisfies Part P, as MCS certification requires installers to demonstrate electrical competency and compliance.

DIY grid-tied solar panel wiring is technically possible if you are a qualified electrician, but it is not a path most homeowners can legally take without professional oversight. Off-grid systems on outbuildings not connected to the domestic supply have fewer restrictions, but best practice still means complying with BS 7671.

G98 and G99 Notification

Connecting a solar system to the grid also requires DNO (Distribution Network Operator) notification. Systems up to 3.68kW per phase can self-certify under G98, which your installer handles automatically. Larger systems exceeding 3.68kW per phase require a G99 application, which involves a formal review by the DNO before installation can be commissioned. G99 applications can take four to twelve weeks, so planning ahead matters for larger systems.

Electrical Safety Checks

Before commissioning, your installer should test:

• Open-circuit voltage (Voc) of each string
• Short-circuit current (Isc) to verify panel connections
• Insulation resistance of all DC cabling
• Earth continuity for the array frame and mounting system
• Correct polarity throughout the DC circuit

You should receive an Electrical Installation Certificate (EIC) on completion, which you’ll need for MCS certification, warranty claims, and future property sales.

Case Study: 6kWp South-Facing Installation in Surrey

Background

A homeowner in Guildford, Surrey, installed a 6kWp system on a south-facing pitched roof with a chimney stack on the east edge of the array. Initial quotes used a single series string of 14 panels. The homeowner was concerned about chimney shading affecting morning output.

Project Overview

The installer redesigned the array as two strings: a main string of 10 panels on the unobstructed western section of the roof, and a second string of 4 panels on the eastern section closest to the chimney. Both strings fed the same dual-MPPT inverter, allowing each string to track its own optimal operating point independently.

Implementation

The 4mm² DC cabling from each string ran through separate conduits to a DC isolator, then to the inverter in the garage below. Cable runs were kept under 15 metres per string to minimise voltage drop below 0.5%. All cabling was clipped and protected to BS 7671 standard. The system was G98 notified and commissioned with full EIC documentation.

Results

First-year generation came in at 5,480kWh, compared to the installer’s projection of 5,200kWh for a single-string design with an east-end shade penalty. The two-string approach recovered approximately 280kWh annually. At 27p/kWh for self-consumed electricity plus 15p/kWh SEG export income, the more complex string design added around £75 per year to system income, paying back the additional design cost within two years.

Expert Insights From Our Solar Panel Installers About Connecting Solar Panels

One of our senior solar panel installers with over 14 years of experience shared their perspective on wiring decisions:

“The biggest mistake we see in substandard installations is undersized DC cabling on long runs. Installers sometimes use 4mm² cable on a 25-metre run where 6mm² is really needed. Over 25 years, that extra resistive loss easily costs more than the saving on cable. We always calculate voltage drop before specifying cable, it’s a fundamental design step.”

“On shading, the right answer depends entirely on the roof. If there’s a clear, unobstructed south-facing pitch, a simple series string with a quality string inverter is perfectly fine and more cost-effective than microinverters. But if there are dormers, rooflights, or an irregular shape, we’ll always recommend optimisers or micros. The technology cost has come down enough that it’s often justified even for moderate shading.”

Frequently Asked Questions

Summing Up

Getting your solar panel connections right from the outset sets up your system for a lifetime of reliable generation. Series wiring builds the voltage your inverter needs; parallel wiring suits battery systems and improves shade tolerance; and a combination of both handles the middle ground. Beyond the wiring topology, cable sizing and voltage drop calculations protect your investment and comply with UK electrical regulations. For grid-tied systems, an MCS-certified installer handles all of this, including Part P notification and DNO registration, so you don’t have to navigate it alone.

Updated April 2026