Adding Battery Storage to an Existing Feed-in Tariff PV System
A domestic property has a 4 kWp solar PV array installed in 2015 under the Feed-in Tariff (FIT). The system uses a 3.6 kW single-phase inverter with a G98 connection (16 A per phase). The homeowner wants to add a 5 kWh battery and a new hybrid inverter to store excess generation and use it at night. The existing consumer unit has no spare ways, and the PV system is metered for FIT payments. The homeowner wants to keep the FIT payments and also export battery power to the grid. The DNO connection is single-phase 100 A TN-C-S. The installer proposes an AC-coupled battery system with a 3.6 kW inverter, connected via a new sub-board. The homeowner asks whether they can keep the FIT and what approvals are needed.
1. Compare AC-coupled and DC-coupled battery storage options for this property, focusing on FIT eligibility and technical implications.
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An AC-coupled battery has its own inverter and connects to the AC side of the installation, usually through a new sub-board. The existing PV inverter and array remain unchanged, so the FIT meter and the accredited generation equipment are less likely to be affected. A DC-coupled system connects the battery to the DC side and normally requires replacing the existing PV inverter with a hybrid inverter. That is a material alteration to the accredited PV system and could affect FIT eligibility or require prior approval from the FIT licensee. Technically, AC coupling is often simpler to retrofit because it avoids DC array modification and preserves the existing inverter’s warranty. However, AC coupling introduces additional conversion losses. DC coupling can be more efficient but is more invasive. The homeowner must notify the FIT licensee before any work and should not assume that keeping the existing array untouched automatically preserves payments; the metering and accreditation conditions must be checked.
2. What DNO notifications and approvals are required for the battery system, and how does the existing G98 connection affect this?
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The existing PV inverter is connected under G98, which applies to single-phase generation up to 16 A per phase. Adding a 3.6 kW battery inverter could increase the total export capacity beyond the existing connection agreement. Even if the new inverter is also rated at 16 A or less, the combined export from PV and battery may exceed the G98 limit, so the DNO must be consulted. If the total export is within 16 A per phase, a G98 application may be possible, but if it exceeds that threshold a G99 application is likely. The DNO may require an export limitation device or a revised connection agreement. The installer must not energise the battery until the DNO approval is in place. The FIT licensee should also be notified separately because the generation and export metering arrangements may change. Exporting battery power to the grid may affect the FIT export payments and must be declared.
3. Identify the key safety and installation requirements for the battery and associated inverter, including protection and labelling.
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The battery system must be installed according to the manufacturer’s instructions and relevant guidance for electrical energy storage. The DC side requires suitable isolation, fusing, and cable protection, and the inverter must have loss-of-mains protection so it cannot island the property during a power cut. The AC side must be protected by an appropriate circuit breaker and RCD; the RCD type must be compatible with the inverter’s DC fault characteristics, often Type A with 6 mA DC detection or Type B. The battery location must be assessed for fire risk, ventilation, temperature, and access, and the installation must be labelled with warning signs indicating multiple sources of supply. The existing consumer unit has no spare ways, so the new sub-board must be properly rated and the earthing and bonding checked. The system should be commissioned and tested, including verification of the export limitation and anti-islanding, and the homeowner should receive clear operating and emergency instructions.