Battery Bank Design & Sizing

Batteries are used in renewable energy (RE) systems for many reasons. For example, if your home is off-grid and powered by a PV array, you’ll need to store the solar energy in batteries for use at night or during cloudy weather. If your home is on the grid, a battery bank can provide electricity during a utility outage. In areas with utilities unfriendly to net billing, some systems are designed for self-consumption—with batteries to store RE energy as it’s created and all of the RE-generated electricity used on-site.

Which Batteries?

In these situations, you’ll need to figure out which batteries are best for your situation and how many of them you will need. There are many battery types to choose from and battery bank designs depend upon factors such as application, budget, and maintenance preferences. For background information on the common battery types used in RE systems, see “Battery Chemistry“ in HP179. The battery types discussed here are being mass-produced—there is a formalized testing process in place with material safety handling data sheets, etc.—and are currently available for the U.S. residential storage market.

While each battery chemistry could hypothetically be used for any system, certain battery types are more commonly deployed in specific types of systems. And while this may change as prices for different battery chemistries fluctuate, the following is currently how we are seeing batteries being deployed in homes.

Flooded Lead-Acid

Flooded lead-acid (FLA) batteries are most common in off-grid systems. These battery banks tend to be larger than for on-grid. Since there is no electric utility to depend on, they have to support all of the household loads. The lower purchase price of FLA batteries compared to other options is attractive when you need a lot of storage. FLAs require regular watering and maintenance. However, folks living off-grid generally are more self-reliant due to living farther way from in-town services. They are usually very hands-on, often dealing with pumping their own water, maintaining road access, etc. Thus, maintaining a battery bank becomes another part of the overall property upkeep (see “Methods” in this issue for information on maintenance and watering systems).

Valve Regulated Lead-Acid

Valve regulated lead-acid (VRLA, also known as “sealed” lead-acid) batteries are often used for battery backup in grid-tied systems. These systems spend most of their time in “float” service (fully charged), waiting for a utility outage—they are cycled less than in off-grid and self-consumption systems, which cycle batteries daily. Compared to other chemistries, sealed batteries have a low to moderate cycle life. They also have a lower self-discharge rate (how fast they lose energy while sitting unused), which is helpful since they spend so much time in standby mode.

Sealed batteries are more expensive than their flooded counterparts. However, since battery backup systems usually need to provide energy only for the home’s “critical” loads (such as refrigeration, medical equipment, and communications) during an outage, the battery bank can be smaller, which can make up for their higher cost. Since they do not need to be watered, they can make sense for those on-grid folks who may not be willing to perform the upkeep required of FLA batteries. Additionally, some off-grid systems will also employ sealed batteries to avoid the required maintenance of FLAs (and thus reduce the risk of ruining those batteries if they are not watered).

Lithium-Ion

While lithium-ion (Li-ion) batteries have a high cycle life and no maintenance, their purchase price can be high. As such, these batteries are commonly found in systems with small battery banks that require frequent cycling. This can be the case for self-consumption grid-tied systems that are designed to utilize as much PV energy during the day as possible and need a battery only large enough to store excess PV-generated energy for use during non-solar hours. These systems are sometimes designed to take advantage of time-of-use (TOU) utility rate structures, which charge higher rates during peak energy usage periods, like late afternoon and early evening—during those times, the home uses PV energy stored in the battery bank, instead of high-rate utility electricity (see “Maximizing Solar Self-Consumption” in HP178).

Self-consumption systems can offer battery backup when the utility is down. However, these may require larger battery banks than those designed for self-consumption alone, as they need to support all the loads on the critical load subpanel during an outage. On the flipside, Li-ion banks can be cycled more deeply (and still maintain high cycle life), compensating somewhat for their higher cost. The common depth of discharge is 80% or 90%. In comparison, LA batteries usually are designed to have a lower DOD—30% to 50%—to help maintain their cycle life. As such, you can accomplish the same goal with a smaller Li-ion bank, since the loads can use more of its capacity.

Nickel Iron

Nickel-iron (NiFe) battery banks are used mostly in off-grid systems. These battery banks are very robust (tolerant of over- and undercharging, and freezing), and offer extremely long cycle life. They also provide some protection from the occasional deep discharges and daily cycling an off-grid battery bank experiences. Like Li-ion, NiFe banks are often designed for an 80% DOD, which allows a smaller battery bank. Their disadvantage is their frequent watering requirement, which is about two to three times that of FLAs. Again, this maintenance is generally a better match for off-grid systems, rather than for on-grid ones. These batteries are good candidates for single-point battery watering systems, which allow you to add water to multiple cells simultaneously. Their higher self-discharge rate makes them an unlikely choice for backup systems, which spend most of their time in standby mode. Due to their high purchase price, NiFE battery banks aren’t as common as FLA banks in off-grid systems.

Sizing Methods

No matter what type of system you have, there are some basic pieces of information you will need for sizing. This includes the daily energy the battery bank will need to supply and the DOD recommended for that battery type.

Off-grid

Off-grid systems need enough battery capacity to power loads in the home at night and during cloudy stretches of weather. Calculate your daily energy consumption by tallying up each electrical appliance’s energy use over 24 hours. Although this sounds simple, it often involves researching each electrical load and determining its consumption. This may be presented as “average yearly kWh” (as on an Energy Star label). More often, this information is given on electrical labels or needs to be measured with a watt meter. Handheld energy meters, such as P3’s Kill A Watt, plug between the wall socket and appliances to show amps, volts, watts, and watt-hours. If you leave them plugged in over 24 hours, they can show watt-hours per day. Alternatively, you can estimate how often the appliance is consuming power and multiply the load watts by number of hours per day.

DC battery bank voltage needs to be determined for off-grid system design. Smaller systems might use 24 volts, and very small systems (for example, a cabin system) might use 12 V. Modern residential systems are most commonly 48 V. Operating at this voltage means smaller, cheaper DC wiring can be used; and it matches the input voltage of the larger off-grid inverters.

A primary factor in off-grid battery bank design is “days of autonomy”—the number of days that the bank should meet loads without being recharged due to clouds hampering PV output. A common approach is to design for two to three days of autonomy and include a backup generator to charge the battery bank and power loads after cloudy days.

Inverter inefficiency needs to be accounted for as the conversion from low-voltage DC to household-voltage AC takes energy to process. Off-grid inverter manufacturers list “peak efficiency” in their specifications, but this isn’t necessarily representative of an inverter’s average efficiency. Designers have to make an educated guess about the actual output level at which the inverter will spend the majority of its time, and use the inverter’s efficiency curve to determine the system’s average efficiency.

Battery temperature also needs to be considered. FLA batteries will put out hydrogen (H2) gas and require ventilation, so need to be housed in a battery box usually located in an unconditioned space. Knowing the average low temperature for the battery box location lets you adjust the battery capacity used in your bank sizing. The temperature correction factor can be found in the battery specifications.

Grid-tied with backup

With these systems, it’s important to determine the “critical” loads that you want to run during a utility outage and how long you need to run them. These may include refrigeration, lighting, and communications (phone charging, Internet router, computer, radio, and TV). Perhaps your area has severe storms that can interrupt utility power for several days, or maybe outages are a rarity and short in duration. You need to know the case for your location, so you can make an educated choice for number of days of energy storage required.

Once you know the critical loads in kWh/day and the amount of time you want to keep those loads powered, sizing the backup battery bank for grid-tied systems is very similar to off-grid battery bank sizing. You still need to account for inverter efficiency and battery bank voltage, as well as proposed DOD and the batteries’ temperature.

In a backup system, a design DOD of 80% can be justified since these batteries spend most of their time in standby mode, are not cycled daily, and are usually quickly recharged by the grid. If you’re looking for fewer maintenance requirements, sealed batteries can be a good choice since they do not require watering. Although they generally aren’t installed in living spaces (they have pressure valves to allow small amounts of gassing if overcharged), they are commonly installed in a garage or equipment room, which can mean less capacity loss due to temperature, since this can be a semiconditioned space.

Self-Consumption Grid-Tied Systems

These systems represent a newer breed of battery-based PV systems and the sizing process hasn’t been standardized. One method involves obtaining a detailed load-versus-time profile with energy-monitoring equipment. Current transformers (CTs) are installed around the conductors in the home’s service panel,  and data is collected and sent to a monitoring system. This information can be used to size a PV array to offset all of or the desired portion of the home’s energy consumption. The battery bank is sized to store the portion of the PV array’s daily energy that would normally be exported to the grid in a net-metered system, so that it can be consumed by the home instead.

This data also can be used for load-shifting strategies, such as using timers for appliances, that can maximize the energy being consumed as it is produced by the PV system and minimize the energy draw in the evening, which reduces the size of the battery bank needed.

Once you know the kWh per day the PV system will produce, and the percentage of that energy that is consumed outside of the daytime window, the sizing is pretty straightforward. Since the batteries will be cycling daily, lithium-ion batteries, which do not require watering and are often discharged down to 80%, are usually chosen. Li-ion capacity isn’t as affected by low temperatures and, since these batteries don’t produce gas, they can be kept in an enclosure inside living spaces if the authority having jurisdiction approves. Actually they should be kept in a somewhat conditioned space, since it is generally recommended that they not undergo charging while they are below 32°F.

To save installation time and avoid equipment compatibility issues stemming from using a newer-generation battery technology, a full energy-storage system (ESS) may be a good choice. This self-contained enclosure includes the batteries, possibly an inverter integrated with the BMS, and the required BOS components. ESS units are specified by their kWh capacity, so there is no need convert to amp-hours for these calculations.

Self-consumption grid-tied systems with backup. These designs will need to accommodate the nonsolar load and the critical load profiles. One sizing method is based on cycling the battery bank less each day so there is reserve capacity left for unexpected outages. This requires increasing the size of the battery bank—that extra capacity may or may not get used, depending on the length and frequency of outages. Using the previous sizing example for self-consumption, but using a 75% DOD means that a 12 kWh ESS ($18,750) could be used. This would provide a minimum of 3 kWh of backup storage after normal daily discharge.

An option offered by some ESS units that doesn’t require more battery capacity is to reprogram them to shift from a self-consumption system to a purely backup system (useful if the system owner knows a storm is coming). This makes the entire battery bank available to critical loads during an outage.