Off-Grid Battery Bank Sizing
How much energy storage do you need?
Figuring out how many batteries you need can be daunting.
If you don’t have enough battery capacity, you run out of power and need to fire up the backup generator.
On the other hand, if you buy too many batteries, you add unnecessary expense to your system, with extra components, complexity and maintenance.
Sizing solar batteries is one of the first steps in designing your off-grid system.
The amount of battery storage you need is based on your energy usage. Energy usage is measured in kilowatt hours over a period of time.
1,000 watts x 10 hours per day = 10 kWh per day
Check out our off-grid load evaluation calculator.
After estimating daily usage we need to consider which type of battery will work best, as they have unique performance characteristics and are sized differently.
Picking the Right Battery
There are a few different battery types commonly used for off-grid storage systems:
Flooded Lead Acid
- Lowest upfront cost $ $ $ $
- Typical lifespan: 5-7 years
- Requires maintenance - add distilled water and equalize charge on a monthly basis
- Enclosure needs to be vented outside to expel built-up hydrogen gas
Sealed Lead Acid
- More expensive $ $ $ $
- Typical lifespan: 3-5 years
- No maintenance
- Enclosure should still be vented, batteries could offgas in certain conditions
- Most expensive $ $ $ $
- Typical lifespan: 10+ years
- No maintenance, no venting
- Highest efficiency, faster charging, more usable capacity (deeper discharge depth)
The two main battery chemistries for off-grid are Lead Acid (flooded or sealed) and Lithium. These two chemistries have unique characteristics. Lithium batteries are more efficient, which means less power is wasted in the charge/discharge process. They also have a greater discharge depth allowing you to fully utilize all of your battery capacity.
Lead acid batteries are sensitive and need to be fully recharged every day, where Lithium batteries can stay at a partial charge without any adverse effect. Lead acid batteries also have a more limited amount of usable capacity and are typically discharged only 50%.
Because of the better efficiency and deeper discharge depth, Lithium battery banks tend to be only 50-60% of the size of a comparable lead acid bank! The Lithium batteries we use are purpose-built for off-grid solar, and utilize a special Lithium chemistry called Lithium Ferro Phosphate (LiFePO4, commonly called “LFP”).
This type of Lithium battery is engineered to provide a long service life (over 10 years) while also being safe, with a stable chemistry and sophisticated electronic protection features.
Sizing Your Battery Bank
The exact math for sizing your battery system is based on your daily power usage and the battery type. Based on usage of 10kWh per day, here are some examples:
Lead Acid Sizing
10kWh x 2 (for 50% depth of discharge) x 1.2 (inefficiency factor) = 24 kWh
10kWh x 1.2 (for 80% depth of discharge) x 1.05 (inefficiency factor) = 12.6 kWh
Battery capacity is specified either in kilowatt hours, or amp hours.
For example, 24 kWh = 500 amp hours at 48 volts → 500 Ah x 48V = 24 kWh
It’s usually a good idea to round up, to help cover inverter inefficiencies, voltage drop and other losses. Think of this as the minimum battery bank size based on your typical usage. You may want to consider 600-800 amp hours of capacity, based on this example, depending on your budget and other factors.
Battery banks are typically wired for either 12 volts, 24 volts or 48 volts depending on the size of the system. Here are example battery banks for both lead acid and Lithium, based on an off-grid home using 10 kWh per day:
For Lead Acid, 24kWh is equal to:
- 2,000 amp hours at 12 volts
- 1,000 amp hours at 24 volts
- 500 amp hours at 48 volts
For Lithium, 12.6 kWh is equal to:
- 1,050 amp hours at 12 volts
- 525 amp hours at 24 volts
- 262.5 amp hours at 48 volts
We offer off-grid packages complete with solar panels, racking, cable, and a power center. Each system has several battery options and everything is sized proportionally, so the solar panels, inverter and battery all work optimally together. Here are examples of complete systems with battery banks:
Check out our complete list of battery banks. All of our battery banks include high quality, UL listed interconnect cables. Our Flooded lead acid battery banks include a refractometer for measuring battery state of charge.
Other factors influence battery sizing:
- Ambient Temperature - Heat or cold has a big impact on battery performance and capacity.
- Seasonal Factors - People use more power at different times of the year. The sun produces more power in the summer than in the winter.
- Budget - Battery bank size is often a compromise between what you want to spend on batteries and how often you'll need to run your backup generator.
This is not intended to be a comprehensive guide on off-grid design.
Use this information, based on your energy usage, to get an idea of the minimum battery bank size, and then call us at 1-800-472-1142 for help picking the best solution for your needs.
Questions? Chat with us!
Todd built his own 3 kW solar system to power his Northern California home, where he lives off the grid with his wife and children. Todd and the rest of the Unbound Solar team are here to answer any questions you have about designing a system that will fit your needs. Give us a call—we’re here to help.