The Battle Born Educational Series | System Planning & Sizing
When sizing a deep cycle lithium battery bank, the first question is simple: how much energy do you actually need?
Everything starts with your loads. Understanding what you plan to power, and how much energy those devices use, is the foundation of building a battery system that performs the way you expect.
Start with Your Loads
Begin by identifying everything you want your battery bank to power.
This could include:
- A refrigerator
- Lights
- A water pump
- Electronics
- Inverter-powered appliances
Each of these devices consumes power, typically measured in watts. The goal is to determine how much energy they use over the course of a day.
Calculate Daily Energy Use
To estimate daily energy consumption, use this formula:
Watts × Hours Used Per Day = Watt-hours
For example, if a refrigerator uses 100 watts and runs for 5 hours per day, that equals 500 watt-hours.
Repeat this calculation for each device in your system. Once complete, add all of those values together to get your total daily energy use.
This number, measured in watt-hours, represents how much energy your battery bank needs to supply in a typical day.
Convert Energy to Battery Capacity
Once you know your total watt-hours, the next step is to convert that into battery capacity.
Use the following formula:
Watt-hours ÷ System Voltage = Amp-hours
For example, if your total daily energy use is 2,400 watt-hours in a 12-volt system:
2,400 ÷ 12 = 200 amp-hours
This tells you how much battery capacity you need to support one day of use.
Estimate Battery Count
With your required amp-hours calculated, you can determine how many batteries you need.
Battle Born batteries are rated in amp-hours, so use this formula:
Required Amp-hours ÷ Battery Amp-hour Rating = Number of Batteries
For example:
200 amp-hours ÷ 100 amp-hour battery = 2 batteries
This gives you a baseline battery bank size based on your daily energy use.
Factor in Runtime
Battery sizing is not just about daily usage. It also depends on how long you want to operate between charges.
If you plan to recharge daily, you can size your system for one day of use.
If you want to run for multiple days without recharging, you need to increase your capacity accordingly.
For example:
- One day of runtime = base calculation
- Two days off-grid = double the required capacity
This step is especially important for off-grid systems where charging availability may be limited.
Consider Charging Sources
It is important to remember that battery sizing does not exist in isolation.
Your ability to recharge the system plays a major role in how much capacity you actually need. Charging sources may include:
- Solar panels
- Alternator charging
- Shore power
- Generators
If your system has strong, consistent charging, you may be able to operate with less battery capacity. If charging is limited or inconsistent, additional storage may be necessary to maintain reliability.
The Bottom Line
Sizing a deep cycle battery bank comes down to three core factors:
- Your loads
- Your daily energy use
- How long you want to run between charges
Once those are defined, the math becomes straightforward. You can calculate watt-hours, convert to amp-hours, and determine how many batteries are needed to meet your goals.
From there, your charging setup and system design will help fine-tune how much total capacity makes the most sense.
Next, we look at how to size your inverter to match your loads, including both continuous power and startup surge requirements. Read: How to Size an Inverter
Or, explore the full series at the Battle Born Academy and build your knowledge from the ground up.
