PV Electricity Distribution and Storage

Grid connected systems

Most PV systems are grid-connected and are linked to a main or a local distribution board. The system operates in parallel with the normal mains supply so that when the PV array is not generating enough energy, mains electricity is used. If more electricity is generated by the PV system than is needed, the excess can be either exported to the grid or stored in batteries for later use.

Where electricity is exported to the grid, it must be synchronised with the electricity supply and it is necessary to have connection approval from the Distribution Network Operator (DNO). To connect the system to the grid, you will need to apply the Energy Networks Association (ENA).  

The ENA recommendations set the connection and commissioning requirements for connecting an electricity generating system to the UK distribution network. ENA engineering recommendations G98 or G99 apply, depending on the size of the total generation on the site, so it is essential that this is calculated. The ENA have guidance documents and forms available online in their resource library.

Battery performance and storage

Battery storage is often used alongside PV arrays to store energy at times of low demand or when there is a surplus generation on a particularly sunny day which can then be discharged when energy is needed later.

Battery technology is evolving rapidly and there are several key considerations in order to install and use them appropriately in historic buildings. More information can be found on our Installing Electrical Energy Storage Systems and Batteries in Historic Buildings web page.

Distribution equipment

Electrical equipment should be located to permit easy access for maintenance and repair. All parts of the installation system should be fully labelled to make maintenance engineers aware of the presence of the system and the location of isolation devices. Where equipment is fixed to the building wall, the number of fixing points should be minimised by attaching the equipment first to a fire-resistant board or mild steel frame system, and then fixing this to the wall or floor.

Electrical cabling

Cabling to and from the PV array to the inverters will need to pass through the roof covering several times.

For thick tiles, it is common practice to notch the back of the tile, sleeve the cable to protect against rubbing, and pass it under the tile. For thinner tiles and slates a lead flashing or proprietary cable gland product can be used.

Before designing the installation, it would be advisable to consult an experienced lead contractor on the positioning of fixings and cabling, to prevent unintended damage to the lead and ensure its longevity.

Inverters

The inverter is the electrical device that converts the direct current (DC) electricity generated by the PV panel into alternating current (AC) that can either be used or exported to the grid.

There are 3 types of inverters that convert DC current to AC current:

• String. convert electricity from multiple panels
  In domestic installations these are usually single phase (230 volts) and can have 1 to 3 strings of panels connected. In commercial installations they are generally three phase (400 volts) and can have a larger number of strings connected. Inverters also have the option of charging batteries, electric vehicle charging, switching, and controlling the system for ‘island mode’ operation
• Central. These are large three phase components and are used in large systems and therefore not relevant for domestic or small-scale commercial systems
• Micro. convert electricity from a single panel and are mounted at the back of the panel. It is good to place the inverter(s) as near to the array as possible, to minimise the DC cable runs, whilst also ensuring that there is good access as they will require regular maintenance. The inverter will need to be rated suitable for the environment it is placed in, so will need to be suitably Ingress Protected (IP) rated to withstand environmental factors. Inverters require good ventilation around them to ensure they don’t overheat and shut down. For roof mounted arrays they are commonly fitted in the loft space which can overheat in summer months, so in this instance an alternative location should be found