Battery Storage Systems
Battery energy storage systems — sizing, installation, earthing, and safety requirements for electricians
System Configurations
Battery storage systems connect to the electrical installation in two main ways, each with distinct advantages.
AC-coupled vs DC-coupled systems
Industry guidance| Feature | AC-Coupled | DC-Coupled (Hybrid) |
|---|---|---|
| Connection point | AC side via battery inverter | DC bus via hybrid inverter |
| Conversion steps (PV to battery) | DC-AC-DC (3 conversions) | DC-DC (1 conversion) |
| Efficiency | 85-90% | 90-97% |
| Retrofit suitability | Excellent — works with any PV | Requires compatible hybrid inverter |
| Cost (new install) | Higher — two inverters needed | Lower — single hybrid inverter |
| Typical brands | Tesla Powerwall, SolarEdge | GivEnergy, Fox ESS, Huawei |
Battery Sizing
Correct battery sizing maximises self-consumption and return on investment.
Usable capacity = Total capacity x Depth of Discharge (DoD)- Total capacity
- = Manufacturer's rated capacity in kWh
- DoD
- = Depth of discharge — typically 90-100% for lithium-ion
Manufacturer specifications
A typical 3-4 bedroom UK home consumes 8-10 kWh per day. Evening and overnight usage is roughly 50-60% of this. A 5 kWh battery covers essential evening loads, while a 10 kWh battery provides full overnight coverage. Pair battery sizing with solar PV generation data and time-of-use tariff windows (such as Octopus Go charging between 00:30-04:30) for optimal financial return.
Time-of-Use Tariffs
Electrical Installation Requirements
Key installation requirements
BS 7671, MCS MIS 3012| Requirement | Detail |
|---|---|
| AC protection | Dedicated MCB (typically 32A Type B) and 30 mA RCD |
| DC isolation | DC isolator between battery and inverter |
| AC isolation | Lockable AC isolator adjacent to inverter |
| Cable sizing | Sized to inverter maximum output current plus voltage drop |
| Earthing | Inverter and battery chassis bonded to main earth terminal |
| Surge protection | Type 2 SPD recommended on AC side |
Fire Safety
Grid Connection and Export
Battery systems that can export to the grid must comply with the same G98/G99 requirements as solar PV. Most hybrid inverters include export limitation firmware that can be set to zero export if the DNO requires it. Systems under 3.68 kW (single phase) use the simplified G98 notification process.
EPS / Backup Power