Inverters and Balance of System: Grid-Tied, Off-Grid, Hybrid, PWM/MPPT, Batteries and Monitoring
This section covers inverters and balance of system: grid-tied, off-grid and hybrid inverters, PWM and MPPT charge controllers, batteries and storage, meters and monitoring. The exam tests component matching, because a good array with the wrong BOS underperforms or fails.
Inverter types
Grid-tied inverters synchronise to the grid, use anti-islanding protection, and have no battery. Off-grid inverters form their own voltage and must handle motor starting surge. Hybrid inverters combine grid, battery and PV. Match the inverter's MPPT voltage window and maximum DC input current to the string design, and respect the DC/AC ratio.
Charge controllers
PWM controllers clamp array voltage near battery voltage; they are cheap but waste power when array Vmp is much higher. MPPT controllers convert excess voltage to current and typically harvest 10–30% more. Size MPPT current as array W ÷ battery V, then add about 25% margin.
Batteries
Flooded lead-acid needs ventilation and water topping; VRLA, AGM and gel are maintenance-free; LiFePO4 gives more usable capacity but needs a BMS. Capacity falls with high temperature and high discharge rate. Never mix battery types, ages or states of charge in one bank.
Worked example
A 2 kW array charges a 48 V battery. Average controller current = 2000 ÷ 48 = 41.7 A. Add 25%: 52 A, so select a 60 A MPPT. If the inverter is 3 kW with a 6 kW surge for 5 seconds, the battery BMS and DC cables must support the surge current, not just the continuous rating.
Traps
Using PWM with a high-voltage grid-type module; undersizing the controller; mixing old and new batteries; ignoring inverter standby consumption; assuming anti-islanding is optional; connecting lithium without a BMS.