AC Cable vs DC Cable: Which Should You Run Further?

How does a plug-in solar kit create AC and DC power?

Every solar panel produces direct current (DC). The photovoltaic cells generate electricity when sunlight hits them, and that raw power flows out through MC4 connectors as DC.

Your microinverter, which bolts directly to the back of the panel or sits between two panels, converts that DC into alternating current (AC) at 230V. The AC then travels along a cable to your 13A plug or hardwired connection at the consumer unit.

In a standard plug-in solar kit, the DC cable run is very short — the distance from the panel's MC4 output to the microinverter input, typically under 1 metre. The AC cable run is everything else: from the microinverter to your socket or consumer unit.

What happens when you run a longer AC cable?

When your panels are on a shed roof 15 metres from the house, or mounted on a south-facing garden wall 20 metres from your nearest socket, that AC cable needs to cover the distance.

The good news: AC at 230V handles distance well. The current in an 800W kit at 230V is only about 3.5A. That's a small amount of current, and small current means small losses.

For a typical 800W kit using 2.5mm² cable:

• 10 metre run: voltage drop of about 0.5V (0.2% loss)
• 20 metre run: voltage drop of about 1.0V (0.4% loss)
• 30 metre run: voltage drop of about 1.5V (0.7% loss)

Even at 30 metres, you're losing less than 1% of your power. That's roughly £2–3 per year on an 800W kit. You won't notice it.

The cable itself is standard 2.5mm² flex or 2.5mm² SWA (steel wire armoured) for buried runs. Both are cheap and available at any electrical wholesaler or Screwfix. A 25-metre roll of outdoor-rated 2.5mm² flex costs around £20–30.

You do need to protect the cable from damage. For runs across a garden, bury SWA cable at least 450mm deep, or run standard flex through conduit clipped to a fence or wall. Don't leave cables lying across paths.

What happens when you run a longer DC cable?

Extending the DC cable means moving the microinverter away from the panels and running the raw panel output a longer distance before conversion.

With a plug-in solar kit, this is unusual. Most kits are designed with the microinverter mounted at the panel. But some people consider it when they want the microinverter indoors for easier access or when using a string inverter instead of a microinverter.

DC cable losses are calculated differently from AC. Solar panels typically output 30–40V DC (for a single panel), and at 400W that means a current of about 10–13A. That's 3–4 times the current on the AC side for the same power.

Higher current means higher losses. For 4mm² DC cable at 400W per panel:

• 10 metre run: voltage drop of about 1.4V (3.5% loss)
• 20 metre run: voltage drop of about 2.8V (7% loss)
• 30 metre run: voltage drop of about 4.2V (10.5% loss)

At 20 metres you've already blown past the 3% maximum recommended voltage drop. At 30 metres you're losing over 10% of your power. That's £20–30 per year wasted as heat in the cable, and it can also cause the microinverter to underperform or shut down if the input voltage drops below its operating range.

You can reduce DC losses by using thicker cable (6mm² or 10mm²), but the cable gets expensive, stiff, and harder to route. A 25-metre roll of 6mm² solar cable costs £50–80.

Is it safer to extend the AC cable or the DC cable?

This is where the difference is stark. DC electricity is harder to make safe than AC, and the statistics back that up.

About 96% of solar-related fires involve DC wiring. When a DC cable is damaged, shorted, or develops a loose connection, it can create an arc that burns continuously. DC arcs don't cross zero the way AC does 100 times per second. A DC arc can sustain itself, melt through metal, and ignite nearby materials.

AC is inherently safer for cable runs. If an AC cable is damaged, the circuit breaker in your consumer unit trips and cuts the power instantly. The kit shuts down. With a microinverter, there's no stored DC energy in the cable to sustain a fault.

DC cables also stay live whenever the panels are in daylight. You can't switch them off from inside the house. If a rodent chews through a DC cable running across your garden, that cable is still energised. An AC cable can be isolated at the consumer unit or simply by unplugging it.

When would you actually need to extend the DC cable?

In a standard plug-in solar kit, almost never. The microinverter mounts directly behind or between the panels. The DC cables are pre-wired with MC4 connectors and typically measure 50cm to 1m.

The only scenarios where you'd consider a longer DC run:

Using MC4 extension cables to separate panels

If you're mounting two panels on different parts of a shed roof or splitting them between a shed and a fence, you might need 3–5 metres of DC extension cable to connect both panels to a central microinverter. At this distance, DC losses are manageable (under 2% for 4mm² cable).

Moving the microinverter indoors

Some people want the inverter inside for weather protection or monitoring access. This requires extending both the positive and negative DC cables from the panels to the indoor inverter. Don't do this with a plug-in kit. The microinverter is IP67-rated (waterproof) and designed for outdoor mounting. Moving it indoors adds unnecessary DC cable, increases fire risk, and voids the design intent of the kit.

How should you route a long AC cable from your panels to your house?

The most common scenario in the UK is panels on a detached shed or garage, with a cable run of 10–25 metres back to the house. Here are the three main methods.

For runs over 20 metres, consider stepping up to 4mm² cable. The extra cost (about £10–15 for 25 metres) buys you an even lower voltage drop and a bit of future-proofing if you ever upgrade to a higher-output kit.

Does cable length affect my microinverter's performance?

A long AC cable does not affect the microinverter's ability to convert power. The microinverter doesn't care how far the AC cable runs. It just feeds current into the grid at 230V.

A long DC cable can cause problems. If the voltage at the microinverter's DC input drops below its minimum operating voltage (typically 16–22V for panel-level microinverters), the inverter won't start or will reduce its output. This is called the MPPT (Maximum Power Point Tracking) window.

With a Hoymiles microinverter, the MPPT range is typically 16–60V. A single 400W panel produces about 34V at maximum power. On a dull winter day, that might drop to 25V. If you've added 10 metres of undersized DC cable and lost 3–4V, you're operating right at the edge of the MPPT range. The inverter might start cycling on and off.

What cable do you actually need to buy?

Here's a clear shopping list for both scenarios. Most people only need the AC column.

Quick reference: AC cable vs DC cable