Is Plug-in Solar Safe With Old Wiring? What the IET Actually Says

This is the question nobody in the plug-in solar industry wants to answer clearly: is your existing wiring actually safe for plug-in solar?

The honest answer: it depends entirely on the age and condition of your electrical installation. A modern home with an up-to-date consumer unit is fine. A 1970s property with original wiring and an old fuse board? That needs checking first.

What Actually Happens When You Plug In Solar

When you connect a plug-in solar system to a wall socket, electricity flows backwards through your home's wiring. Normally, power comes from the grid, through your consumer unit, along the ring main, and out to your sockets. Solar reverses this — power enters through a socket and flows back towards the consumer unit.

This is called backfeed. In a modern electrical installation, backfeed is perfectly safe. The wiring is designed to carry current in both directions, and modern protective devices handle it without issue.

The problem arises in older installations where protective devices weren't designed with backfeed in mind.

The Three Things That Matter

1. Your Consumer Unit (Fuse Board)

If your consumer unit still has rewirable fuses (the old ceramic holders with fuse wire) or cartridge fuses rather than modern MCBs (miniature circuit breakers), it's a sign your installation predates current safety standards.

Old fuse boards don't provide RCD (residual current device) protection — the device that trips instantly if electricity finds a path to earth through your body. Modern consumer units with RCDs are required under current wiring regulations.

The risk: Without RCD protection, a fault in your solar system could create a shock hazard that your existing protection won't catch.

What to do: If you have an old fuse board, get an electrician to assess it before connecting solar. A consumer unit upgrade costs £300–600 and brings your whole installation up to current standards — which is worth doing regardless of solar.

2. Your RCD Type

Even if you have a modern consumer unit with RCDs, the type matters. There are three main types:

Type AC RCDs detect AC fault currents only. This is the older, cheaper type. Some micro inverters can introduce a small DC component during faults, which Type AC RCDs may not detect.

Type A RCDs detect both AC and pulsating DC fault currents. This is the standard required in modern installations and handles plug-in solar safely.

Type B RCDs detect AC, pulsating DC, and smooth DC fault currents. This is the highest protection level, typically used in EV charger installations.

Most UK homes built or rewired after 2008 have Type A RCDs. If yours is Type AC (check the label on the device in your consumer unit — it's printed on the front), you should upgrade before connecting solar.

The cost: Replacing a Type AC RCD with a Type A costs an electrician roughly £80–150 including the part. It's a 30-minute job.

3. Your Wiring Condition

Old wiring — particularly rubber-insulated cable from pre-1960s installations, or early PVC cable from the 1960s–70s — may have degraded insulation. Degraded insulation plus backfeed from a solar system increases the risk of current leakage and, in the worst case, fire.

Modern PVC-insulated cable from the 1980s onwards is generally fine for plug-in solar, provided it's in good condition.

How to check: An EICR (Electrical Installation Condition Report) is the proper way to assess your wiring. An electrician inspects your installation, tests the circuits, and rates them as satisfactory (C1/C2/C3 codes). An EICR costs £150–300 for a typical house.

The IET (Institution of Engineering and Technology) recommends an EICR before connecting any generation to a domestic installation. Most plug-in solar marketing doesn't mention this — but it's sensible advice, particularly for older homes.

How Old Is "Old"?

As a rough guide:

Built or fully rewired after 2008: Almost certainly fine. You'll have a modern consumer unit with Type A RCDs, MCBs, and current-standard wiring. Plug in and go.

Built or rewired 1990–2008: Probably fine, but check your RCD type. If it's Type AC, get it swapped. Otherwise, you're likely good.

Built or rewired 1970–1990: Worth getting an EICR. The wiring may be adequate, but the consumer unit likely needs assessment. Budget £150–300 for the report plus potentially £300–600 for a consumer unit upgrade if needed.

Built before 1970, never rewired: Get an EICR before doing anything. Original pre-1970 wiring is unlikely to be safe for backfeed without assessment and likely upgrades.

The EICR: What to Expect

An EICR takes 2–4 hours for a typical 3-bed house. The electrician will:

• Visually inspect your consumer unit and wiring
• Test earth fault loop impedance on each circuit
• Test RCD operation and trip times
• Check insulation resistance
• Identify any defects or non-compliances

You'll receive a report with codes:

• C1: Danger present — immediate action required
• C2: Potentially dangerous — urgent remedial action required
• C3: Improvement recommended — not immediately dangerous but should be addressed
• Satisfactory: All good

If your report comes back satisfactory or C3 only, your wiring is fine for plug-in solar. If C1 or C2, fix those issues first — they're dangerous regardless of solar.

Testing Your System Once Connected

Once your wiring checks out and your solar is connected, a multimeter like the Fluke 117 lets you verify the system is working correctly. Check the voltage at the socket while generating — it should be between 230V and 253V. If it's higher, the backfeed may be raising voltage above safe limits, which can indicate a wiring issue.

A smart plug like the TP-Link Tapo P110 between the inverter and wall socket gives you continuous monitoring of power output, so you can spot any anomalies early.

The Bottom Line

Plug-in solar is safe — in a home with adequate wiring. The 800W limit and modern micro inverter design mean the electrical risks are genuinely low. But "low risk" isn't "no risk," and older homes need checking first.

If your house was built or rewired after 2008, you're almost certainly fine. If it's older, spend £150–300 on an EICR before connecting. That's a small price for peace of mind — and it protects your home insurance position too (see our insurance guide).

The IET's advice is sound: know your wiring before you generate into it.

For the complete installation process, see our step-by-step guide. For checking your system is working correctly after connection, see our troubleshooting guide.