Solar panels are an excellent energy source for boats, but understanding their real-world output is essential to designing a reliable system. While panels may be rated for a specific wattage under ideal conditions, various factors such as sunlight hours, efficiency losses, and wiring configurations can significantly impact the actual wattage delivered to your battery bank. This guide unpacks the details behind how wattage is calculated, the role of MPPT controllers, and how to optimize your solar setup to maximize efficiency.
Solar Panel Wattage and Peak Sun Hours
The wattage of a solar panel is based on standard test conditions (STC), typically 1,000W/m² of sunlight at 25°C. For instance, a 100W panel produces 100 watts under perfect sunlight with no shading or inefficiencies. However, real-world conditions rarely match STC. Solar output depends on your location, season, weather, and panel placement, which are collectively measured in peak sun hours (PSH).
Peak sun hours represent the total equivalent hours of full sunlight your panels receive in a day. For example:
Miami averages 5.5 PSH in summer, so a 100W panel produces 100×5.5=550 Wh/day.
Boston, with 4 PSH in winter, sees a drop in output to 100×4=400 Wh/day.
Check the Peak Sun Hours for your city here.
Factors such as shading from masts or sails can further reduce the PSH, sometimes by as much as 50%. To combat this, careful placement of panels and the use of multiple panels on both port and starboard side is crucial.
The Role of MPPT Efficiency
Solar panels produce variable voltage and current throughout the day, influenced by sunlight intensity and shading. A Maximum Power Point Tracking (MPPT) charge controller optimizes this by adjusting the panel’s voltage and current to match the battery’s charging needs while minimizing losses. Unlike simpler PWM controllers, MPPT controllers increase system efficiency, especially when panel voltage is much higher than battery voltage.
How MPPT Controllers Work:
An MPPT controller adjusts the higher voltage from solar panels into usable current for the battery. For example:
A 100W panel generating 20V and 5A in ideal conditions would output 20V×5A=100W
If connected to a 12V battery bank, an MPPT converts the 20V to 12V and increases the current proportionally, delivering approximately 12V×8.3A=100W, minus minor conversion losses.
MPPT controllers are not 100% efficient. Typical conversion efficiencies range from 92% to 97%, so the actual output would be closer to 100W×0.95=95W.
Voltage Requirements and Minimum Thresholds
For an MPPT controller to operate effectively, the input voltage from the solar array must exceed the battery’s voltage by a specific margin, known as the voltage threshold. Most MPPT controllers require at least 5V above the battery’s nominal voltage to function. For example:
A 12V battery requires at least 17V from the solar array to initiate charging.
For a 24V battery bank, the minimum solar array voltage is approximately 29V.
If the panel voltage drops below this threshold due to shading, poor weather, or improper wiring, the MPPT cannot produce any wattage, and the system will idle. This is why wiring panels in series are preferred, however it is critical to understand panel wiring implications to achieve adequate voltage across MPPT.
Check this blog to understand the impact of Series, Parallel, and Mixed Solar Wiring on Efficiency
Understanding Wattage Ratings of MPPT Controllers
The maximum solar panel wattage an MPPT controller can handle is closely tied to the voltage of your battery bank. A 60A MPPT, for instance, can manage up to 800W when paired with a 12V battery bank, but this capacity doubles to 1,600W with a 24V bank and increases further to with a 48V bank. This scaling happens because as the battery voltage rises, the controller is able to process the same current delivering more power.
This relationship is a crucial consideration when designing your solar system, as higher battery voltages allow you to connect more panels or higher-wattage arrays to the same MPPT. However, exceeding the MPPT’s maximum input wattage is not recommended as it can lead to system instability, or even failure.
Refer to your manufacturer’s specifications for the maximum PV input rating to avoid the risks of under-sizing or overloading the system.
Losses in a Solar Setup
In addition to MPPT inefficiencies, other factors can reduce solar output:
Shading: Even partial shading on one panel can significantly reduce the output of the entire array in series connections. In parallel configurations, shading affects only the shaded panel but may reduce the voltage below the MPPT’s threshold.
Heat: Solar panels lose efficiency as they heat up. Every degree above 25°C typically reduces panel efficiency by 0.5%. For example, on a hot deck at 40°C, a 100W panel may only produce around 92.5W.
Wiring Losses: Voltage drops across long wires can reduce output, especially in high-current parallel setups. It is suggested to mount MPPTs close to the panels, alternatively opt for series set-up to minimize copper losses.
Do I need more than one MPPT?
If your boat has multiple solar panels, instead of connecting all panels to a single MPPT, multiple controllers allow each group of panels to work independently, making the system more efficient.
Take, for example, a boat with panels on both the port and starboard sides. By connecting each side to its own MPPT, shading on one side won’t impact the other. On the other hand, if panels are combined into a single MPPT using a series-parallel setup, the system’s output can drop to match the weakest performing string, reducing overall efficiency. With multiple MPPTs, each panel group operates at its best giving maximum Ah to your battery bank.
How to Calculate Actual Solar Output for Your Boat
To estimate the real-world wattage from your solar panels, use the following formula:
Actual Output (W)= Panel Wattage × Peak Sun Hours × MPPT Efficiency × System Efficiency
Example Calculation:
Panel Wattage: 400 W
Peak Sun Hours: 5h
MPPT Efficiency: 95%=0.95
System Efficiency (wiring, heat, etc.): 90%=0.90
Actual Output in a day = Panel Wattage x Peak Sun Hours x MPPT Efficiency x System Efficiency = 400×5×0.95×0.90 = 1710 Wh/day

With this calculation, you can now determine your solar system’s watt-hours (Wh) and evaluate additional loads can be powered with solar energy.
Maximizing wattage out of your solar investment requires understanding of sun hours, MPPT efficiency, panel locations and wiring configurations. Whether you’re designing a small setup or a complex multi-panel system, the right planning and components will ensure you make the most of your solar investment.
Ready to begin your solar journey? Explore Xantrex’s marine-grade solar panels or reach out to our team for personalized solar system design.