Plug-in batteries seem like a great fit for your new plug-in solar panels, but don't rush to buy one just yet — the rules are more complicated than you'd think

Plug-in solar panels became legal in the UK today (September 27).

These panels plug directly into an ordinary mains power socket, and could cut your electricity bill considerably; in fact, experts have told us that a plug-in solar system could effectively pay for itself in three to five years .

However, although plug-in solar panels are now legal, plug-in batteries (which store electricity then discharge it back into the circuit later) are not.

"Without a battery, solar energy generated during the day is either used immediately or exported to the grid, and consumers have to buy electricity at a higher price in the evening to meet their need," explains Anker Solix , which produces portable power stations and solar generators, and today launched the Solarbank 4 Pro solar battery .

The Solarbank 4 Pro is described as "plug-in ready", but can currently only be installed in your home via a hardwired connection, and must be fitted by a Part P registered electrician — for now.

Anker Solix is among many companies banking on plug-in batteries becoming legal in the UK soon, but we don't yet know when that might happen.

What's the problem with plug-in batteries? The problem (as described by Regen , a not-for-profit organisation promoting renewable energy and energy efficiency), is that wiring in UK homes wasn't designed for energy to be discharged back into it.

Your home has circuit breakers, which protect against overloading and fires.

They turn off the power to a circuit if the load gets too high, or there's a major system fault.

Each circuit breaker has a lower rating than the physical limit of the circuit, which provides a buffer for safety.

"The problem with a discharging battery is known as load masking," explains Regen .

"Circuit breakers only detect the current flowing past them from the consumer unit, and they cannot ‘see’ that a battery is plugged in.

So when a battery discharges into a wall socket, the appliances on that circuit can draw power from the battery instead of the consumer unit — and the breaker continues to allow current up to its rating on top of what the battery is supplying.

"The wiring closest to the appliances can therefore carry more current than the circuit breaker is set to protect against.