The Battle Born Educational Series | System Planning & Sizing
When sizing solar for a deep cycle lithium battery system, the goal is to match your solar production to both your battery capacity and your daily energy use.
A properly sized solar array ensures your system can recharge efficiently and keep up with real-world demand. Too little solar can leave your batteries undercharged, while too much may be limited by available space or system constraints.
Start with Your Battery Bank
A simple place to begin is your battery capacity.
A common guideline for lithium systems is:
Approximately 200 watts of solar per 100 amp-hours of LiFePO4 battery capacity.
For example, if your system includes two 100 amp-hour batteries:
- 200Ah total capacity
- Recommended solar range: 400 to 600 watts
This provides a balanced recharge rate under typical conditions, allowing your solar array to replenish the battery effectively throughout the day.
Size Based on Daily Energy Use
Another approach is to size solar based on how much energy your system consumes each day.
Use the following formula:
Daily Energy (Watt-hours) ÷ Peak Sun Hours = Solar Watts Needed
For example:
If your system uses 2,400 watt-hours per day and you average 4 peak sun hours:
2,400 ÷ 4 = 600 watts of solar
This method aligns solar production directly with your daily usage, helping ensure your system can maintain charge over time.
Understand Peak Sun Hours
Peak sun hours represent the equivalent number of hours per day when solar panels operate at full output.
This value varies depending on location, season, and weather patterns. In many areas, an average of 4 to 5 peak sun hours is a reasonable baseline, but it’s important to consider your specific conditions when sizing a system.
Account for Real-World Conditions
Solar output is not constant, and real-world factors will impact performance.
These include:
- Cloud cover and weather
- Panel angle and orientation
- Shading from nearby objects
- Temperature and system losses
Because of these variables, it is common to add an additional 20 to 30 percent solar capacity when space allows.
This extra capacity helps ensure your system can still perform under less-than-ideal conditions and recover more effectively after periods of high usage.
Consider Space and Installation Constraints
In many mobile and off-grid applications, available space becomes the limiting factor.
On RVs, vans, or boats, roof space may restrict how many panels can be installed. In these cases, panel layout and efficiency become important considerations.
Maximizing usable space and minimizing shading can make a significant difference in overall system performance, even when total panel wattage is limited.
The Balance of Solar Sizing
Solar sizing is not based on a single number. It is a balance between:
- How much energy you use
- How quickly you want to recharge your battery bank
- How much physical space is available
A system designed with these factors in mind will be better equipped to handle real-world conditions and maintain consistent performance over time.
The Bottom Line
Sizing solar panels for a lithium battery system starts with understanding your battery capacity and daily energy use.
From there, you can estimate required solar wattage, adjust for real-world conditions, and design a system that fits within your available space.
When properly sized, solar becomes a reliable and consistent source of energy, helping your system stay charged and ready across a wide range of conditions.
Next, we look at how to choose the right solar charge controller, including the differences between PWM and MPPT and how to properly size your controller for your system. Read: How to Choose the Right Solar Charge Controller
Or, explore the full series at the Battle Born Academy and build your knowledge from the ground up.
