Most grid-tied PV systems are batteryless. They rely on moving energy to and from the grid. Under “net-billing” agreements, the utility provides billing credit for any surplus electricity produced by the system. System owners can then draw on that credit when more energy is being used than the system is producing. Depending on system size and the energy-use habits of the system owners, some systems can earn enough credit during the summer months (greater production) to make up for winter’s lower production, becoming a net-zero annual energy user.
The disadvantage of a batteryless system is that the system relies on the grid to operate, so if the utility grid stops working, the home is also without power—despite having an on-site source of power (PV system). For most people, this is no big deal because utility outages are usually short in duration. But there are situations in which a home must be able to continue using energy even when the grid is not working, such as relying on life-saving medical equipment, keeping computers running, or keeping food cold.
Some homes or businesses might be able get by with an uninterruptible power supply (UPS). But unfortunately, those normally operate only long enough to properly turn off computers or electronics without “crashing,” which can cause data loss.
Other homes or businesses might be able to get by with a new class of batteryless inverters that, when the sun is shining and the grid is down, can provide a limited amount of AC energy (see this article). These systems can be useful for running a fridge during the day, which will probably keep food cold enough to make it through the night until the next day. They can charge phones and other battery devices which otherwise could run out of battery energy. But they won’t be helpful at night or during cloudy days. Only batteries can make up for that.
So why doesn’t everyone include a few batteries for backup? First, including batteries adds complexity to a PV system, which adds expense. Second, battery-based systems are 10% to 15% less efficient compared to batteryless systems, since some energy is used to keep the batteries at a full state of charge—and that means less energy goes out to the grid. The larger the battery, the more self-discharge there will be, and the less net-billing credit you will get out of your system. Finally, batteries will likely need replacing sooner than the rest of the equipment in the system, generally two to four times over the lifetime of the inverter and other system electronics. If you decide to design and install a battery-based grid-tied system, use the smallest battery capacity that will serve your backup needs—to keep costs down and limit efficiency losses.