Maintenance requirements for all batteries include keeping the terminals and tops of batteries free from corrosion, dirt, and debris. This helps keep electricity flowing equally through the entire battery bank. The battery should be charged to 100% on a weekly basis (and daily is better)— keeping batteries in a discharged state can decrease their life. The electrolyte level must never expose the lead plates to air. Flooded batteries also need to be equalized—a controlled overcharging that is commonly done every few months. Equalization helps to rebalance cell voltage and improves the health of the battery by mixing up the electrolyte, which can stratify over time. Properly venting explosive hydrogen gases (produced by charging lead-acid batteries) from a battery box to the outdoors is extremely important. Passive venting can be accomplished by intake air vents installed at the bottom of a battery box combined with higher outlet vents at the top. This allows the lighter hydrogen gases to rise up and out. Active venting by a DC fan can also be used.
Sizing a battery bank starts with load analysis. The battery needs to store the amount of energy needed for the daily loads. If the loads need to work on some days where there is no sun available (but before the generator kicks in), then the battery bank needs to be larger—known as days of autonomy. Beyond that, batteries will yield more cycles when less is drawn out of them on a daily basis. For example, a battery that’s discharged 20% may provide 3,300 cycles; if discharged 80%, it may only provide 675 cycles.
Backup power sources are usually included in off-grid PV systems for when it’s not sunny. Engine generators, which may run on gasoline, propane, or diesel-compatible fuel, are the most common backup source because they provide power on demand. Generators are used more during the shorter days of the winter and for periods of cloudy weather. They can be set up to auto-start if a compatible inverter/charger is used, though many off-gridders do not recommend it—generators should be checked for fluid and fuel before starting. Generators are also important for equalization charging, as it is difficult to get enough power and energy from a PV array to perform this function.
Generator options include fuel type, size (kW), and fuel storage. Noise, exhaust fumes, and access for maintenance influence generator location. Regular maintenance—checking the oil, changing filters, and tuning—is needed so the generator can be available when needed. A generator typically needs to be sized to both handle charging batteries and run the loads simultaneously. Inefficiencies due to high elevations and temperature, and the limitations of the charging capacity available in the inverter/charger, will also influence sizing.
Meters and data monitoring for an off-grid system are even more critical than for a grid-tied system, since you’ll need to make sure the batteries are reaching 100% full on a regular basis; track the trends of loads versus charging over time; and monitor the battery’s state of charge. Meters and monitoring also help you gain insight on future system needs—for example, if more modules are needed to keep up with household usage and to determine when the batteries need replacing.
Data monitoring can show trends about battery charging and load profiles and to help spot potential problems. Some inverters and controllers can log the daily energy consumption and production data, and the minimum and maximum battery state of charge.
Living off-grid requires much more interaction with the energy system than does living with a grid-tied system. In a grid-connected system, if there is no sun or if more energy gets used on one day than another, the grid is a (mostly) reliable backup source. In an off-grid system, the user has to strategize to ensure adequate energy to meet the loads—every day. Users with stand-alone systems have to monitor and adjust their energy consumption, watch weather patterns and time usage accordingly, and make sure there’s fuel for the backup generator. Additionally, troubleshooting is more difficult in this complex system—and the stakes are higher when the utility is not there as a backup.
Carol Weis is a NABCEP-certified PV installer and ISPQ Master PV trainer. She writes curricula and teaches international PV classes to local technicians and end users, and is an instructor for Solar Energy International. She has worked as a licensed electrician and solar installer in Colorado.