There is increasing demand for battery-based solar PV systems everywhere. As utility agreements change, batteries offer improved economics and independence, however it is critical to choose the right type for the application.
First, let's review the most common types of PV types:
1) Grid-direct (utility interactive) - Owner sells energy to utility grid, no batteries involved
2) Stand-alone (off-grid) - Owner is not connected to utility, solar and batteries must provide for all electrical loads
3) Multi-mode - Solar and battery system combination is able to sell energy to the utility grid AND store it for self-consumption in the case of grid-outage or unfavorable utility payment schemes
The simplest battery-based system is a direct DC stand-alone system, the x in a circle represents electrical loads.
The next step up is a DC stand-alone system with a load controller integrated in the charge controller (CC)
If the system involves AC loads, such as most household appliances, lights and chargers, you will need an inverter. The simplest is an AC stand-alone system, wherein the charge controller either charges a battery, or directs the DC PV power directly to an off-grid inverter. That inverter powers AC loads in turn, discharging the battery power if the PV is not producing enough at the time.
If the owner is concerned about reliability, a small generator comes into play, creating a Hybrid AC stand-alone system. The generator feeds loads through the inverter.
Moving from off-grid to multi-mode we need to look at more robust multi-mode inverters. The simplest is a DC-coupled multi-mode system. The only addition besides the inverter is a "back-up loads" panel for critical loads in case of an outage. These systems can export and sell energy to the utility grid.
If you are retrofitting batteries to an existing solar system, you will install an AC-coupled multi-mode system. The existing solar inverter(s) output should be re-directed to the back-up loads panel, and the new multi-mode inverter will feed the main service panel. This allows the grid-tie solar inverter to operate even in the case of a grid outage! These are typically the most expensive but dynamic systems. They usually involve a battery management system (BMS)
Now, let's review modes of operation for these system types.
Grid-tied Multimode systems:
- Battery back-up - Net-metered (NEM) or otherwise compensated for production, batteries usually floating at 100% charge in case of power outage. Size battery bank to critical loads.
- Time of Use (TOU) - Batteries used to shift loads to off-peak prices or self-consumption for lack of NEM. Surplus solar production is stored and then discharged regularly for economic benefit.
- Peak Load Shaving - Batteries programmed to cap power (kW, not kWh) draw from grid below peak load price tiers. Typically used in commercial systems.