Should solar panels be in series or parallel?

Solar panels should be connected in series for most grid-tied, inverter-based systems, but parallel wiring is better for systems with shading issues or using microinverters, according to the Energy Saving Trust (Energy Saving Trust, 2026).

The choice between series and parallel wiring depends on your inverter type, shading conditions, and system voltage. In a series connection, panels are linked positive-to-negative, increasing voltage while current stays constant. In parallel, all positives connect together and all negatives connect together, keeping voltage constant but increasing current. Most UK homes use string inverters, where series wiring is standard because it achieves the higher voltage needed for inverter operation. However, if any panel is shaded, it can drag down the entire series string’s output. Microinverters or power optimisers bypass this by treating each panel independently.

Series wiring is standard for string inverters

For a typical UK solar PV system using a single string inverter, panels are wired in series. This raises the voltage to the inverter’s operating range, typically 300–600V DC for modern units (GOV.UK Solar PV Installation Guide, 2026). Higher voltage means lower current, reducing cable losses and allowing thinner, cheaper wiring. A standard 4kWp system with 10 panels of 400W each in series will produce around 400V DC. This setup is efficient and cost-effective for unshaded roofs. The Energy Saving Trust confirms that series wiring is the most common configuration for UK homes with good south-facing exposure (Energy Saving Trust, 2026).

Parallel wiring helps with shading issues

If a roof has partial shading from chimneys, trees, or dormers, parallel wiring, or a series-parallel hybrid, can prevent one shaded panel from crippling the entire array. In a pure parallel setup, each panel operates independently at its own voltage, so shading on one panel only affects that panel’s output. However, parallel wiring requires higher current capacity cables and may need a charge controller or inverter that accepts low voltage, such as a microinverter system. The Microgeneration Certification Scheme (MCS) recommends power optimisers or microinverters for shaded roofs, which effectively make each panel run in its own parallel-like circuit (MCS Certified, 2026).

Microinverters and optimisers remove the choice

With microinverters or DC power optimisers, the series-or-parallel decision becomes irrelevant because each panel handles voltage conversion independently. Microinverters convert DC to AC at each panel, so panels are typically connected in parallel to the AC mains at 230V AC. Power optimisers keep panels in series but individually regulate voltage, preventing shading losses from affecting the string. Ofgem notes that optimised systems can recover 15–25% more energy on partially shaded roofs compared to standard series strings (Ofgem, 2026). For new installations, many installers now default to microinverters or optimisers for flexibility, though this adds roughly £300–£600 to a typical 4kWp system.

A worked example

A typical UK household with a 1930s semi-detached house in Manchester installing a 4 kW solar PV system of ten 400W panels will save roughly £515 per year on electricity bills. With a series-wired string inverter setup costing around £6,500 before any support, the homeowner can apply the 0% VAT rate until March 2027 and may qualify for the ECO4 scheme if on a low income or certain benefits. The Energy Saving Trust estimates a payback period of about 12 to 15 years for this configuration, with 25-year lifetime savings of approximately £8,000 to £9,000 once you factor in the Smart Export Guarantee payments of roughly 5.5p per kWh exported. A parallel-wired system with microinverters on the same roof would cost nearer £8,000 but performs better if a chimney or dormer casts afternoon shade on three panels.

What homeowners often get wrong

The most common mistake is assuming parallel wiring always gives higher total output for any shaded roof. Here are three frequent errors that can cost you money or invalidate your warranty.

1. Thinking parallel cures all shading problems Many homeowners believe parallel wiring lets every panel work at full output when one is shaded. In reality, parallel wiring keeps voltage constant but current drops on the shaded panel, so you still lose that panel’s contribution and the inverter may not start if overall voltage stays too low for the MPPT range.
2. Ignoring the inverter’s minimum voltage requirement A string inverter typically needs 300V DC to operate, but a parallel-wired system of ten 400W panels delivers only about 40V. This means the inverter won’t turn on at all, wasting the entire £6,500 installation unless you add a voltage-boosting device like a power optimizer.
3. Believing series wiring voids the warranty on shaded panels Some installers claim series wiring damages shaded panels by forcing current through them. This is wrong, shading reduces output but does not harm the panels themselves, and the MCS certification standards (MCS 010) explicitly allow series strings with partial shading as long as bypass diodes are present.

Quick reference

• A 4 kW series-wired system on a south-facing roof in the UK generates roughly 3,400 kWh per year, saving £515 annually at current electricity rates.
• Parallel wiring is only suitable for microinverters or DC optimisers, which add £1,500 to £2,500 to the upfront cost compared to a standard series string inverter.
• You qualify for 0% VAT on solar panel installations until March 2027 if the installer is MCS-certified and the system meets the Microgeneration Certification Scheme standards.
• Payback on a series-wired system is typically 12 to 15 years, while parallel-wired microinverter systems stretch to 16 to 20 years due to higher equipment costs.
• Never mix panels of different wattages in a series string, the lowest-rated panel limits the entire string’s current, cutting total output by up to 30% and voiding the manufacturer’s performance warranty.