Battery Storage (BESS) Sizing Engine
Capacity, EPS Runtime & Financial Payback Modeller
Estimated EPS Runtime
Continuous Backup Power Duration
Battery Storage Systems & BS 7671:2018+A4:2026
Installing a Battery Energy Storage System (BESS) allows homeowners to capture excess Solar PV generation or exploit cheap overnight electricity tariffs. However, to accurately forecast returns and ensure compliance with BS 7671, specific engineering and financial metrics must be understood.
1. Depth of Discharge (DoD)
A battery rated at 5.2 kWh does not yield 5.2 kWh of usable energy. To protect internal cell chemistry and prevent permanent voltage collapse, the Battery Management System (BMS) enforces a Depth of Discharge limit. Standard Lithium-Ion cells operate at roughly 90% DoD (leaving 10% reserve). Modern Lithium Iron Phosphate (LiFePO4) chemistries can safely be cycled at 100% DoD.
2. Island Mode & Emergency Power Supply (EPS)
A common misconception is that a solar battery will automatically keep the lights on during a grid blackout. By default, grid-tied inverters are mandated by the G98/G99 network regulations to immediately shut down (anti-islanding protection) during a power cut, to prevent back-feeding lethal voltages to DNO engineers repairing lines in the street.
To utilise EPS (Emergency Power Supply) mode, the system requires highly specific electrical infrastructure:
- An automatic or manual changeover switch to mechanically sever the connection to the grid.
- Because the DNO supplier's Earth connection (PME/TN-C-S) is lost during a blackout, the property's backup circuits must instantly convert to a TT Earthing Arrangement. This requires a dedicated earth rod installed specifically for the inverter.
3. Financial Payback Assumptions
The ROI model in this calculator assumes optimal conditions: that the battery successfully completes one full cycle per day. For example, it charges completely for free via excess Solar PV during the day, and discharges its full usable capacity during the evening, directly offsetting peak-rate grid import. In winter, payback can be maintained by charging the battery overnight on a cheap off-peak EV tariff (e.g., 7p/kWh) and discharging it during the day.