CHAPTER 4: Determine How Much Solar You Need
Designing a Balanced Marine Solar System
CMP's approach starts with a simple question:
What do you want your solar system to accomplish?
From there, we use a five-step methodology and the CMP Solar System Design Worksheet to estimate power consumption, determine solar requirements, evaluate battery capacity, and build a balanced system.
4.1. What Do You Want to Achieve With Your Solar System?
Every boat has different cruising habits and power requirements.
Before selecting solar panels or batteries, it's important to decide what you want your solar system to accomplish.
Define Your Solar Goals First
For some boat owners, solar simply keeps the batteries topped off while the boat is at the dock or on a mooring. Others want to reduce engine or generator run time while cruising. Many long-distance cruisers aim to generate enough energy to operate independently for days at anchor.
Typical objectives include:
- Keep your batteries charged while the boat is unattended.
- Reduce engine or generator use by having solar supplement your existing charging sources.
- Generate all the power you need at anchor, allowing you to enjoy extended periods without running the engine or generator.
- Generate the power you need both at anchor and while underway, minimizing dependence on other power sources.
For this Learning Center example, we'll use the same objective as Tom's case study:
Design a solar system capable of supplying all daily electrical needs while anchored, without relying on the engine or generator.
Understanding Your Boat's Energy Flow
Every marine electrical system follows the same basic principle:
- Energy enters the system from one or more charging sources.
- Energy is stored in the battery bank.
- Energy leaves the batteries to power onboard equipment.
Charging sources may include:
- Solar panels
- Alternator
- Shore power charger
- Generator
- Wind generator (if installed)
Stored energy is then distributed to:
- DC equipment such as lighting, electronics, pumps, and navigation systems
- AC appliances through an inverter, including refrigeration, galley appliances, and other household equipment
Understanding this flow of energy is the foundation of designing an efficient and reliable solar system.
The Five-Step Solar Design Method
Custom Marine Products uses a simple five-step process to size every marine solar system. By following these steps in order, you can design a balanced system that matches your actual energy needs instead of relying on guesswork.
☑️ Step 1 — Estimate Daily Energy Consumption
Begin by estimating how much electricity your boat uses during a typical day.
This includes every DC and AC appliance, such as:
- Refrigeration
- Navigation electronics
- Cabin lighting
- Water pumps
- Autopilot
- Inverters
- Entertainment equipment
Knowing your daily energy consumption is the foundation of every solar system design.
☑️ Step 2 — Estimate Power Generation from Alternative Sources
Next, determine how much energy you're willing to provide from sources other than solar.
Ask yourself questions such as:
- Will you run the engine every day?
- Do you plan to use a generator?
- Will shore power be available regularly?
- Or do you want solar to provide nearly all of your daily energy?
The less supplemental charging you intend to use, the more solar capacity you'll need.
☑️ Step 3 — Estimate Solar Generation Capacity Needed
Once your daily energy demand is known, you can calculate approximately how much solar power is required to replace that energy each day.
This determines the size of your solar array.
☑️ Step 4 — Select Solar Panel(s) and Controller – Mounting Considerations
With the required solar capacity established, the next step is selecting the appropriate equipment, including:
- Solar panels
- Charge controller
- Panel mounting locations
- Wiring configuration
Available installation space often influences panel selection and final system size.
☑️ Step 5 — Estimate Optimum Size of Your Battery Bank
Finally, confirm that your battery bank is large enough to store the energy your solar panels produce.
For most cruising boats, a battery bank capable of providing 24–48 hours of reserve capacity offers a practical balance between usable storage, weight, and cost.
4.2. Using the Marine Solar System Design Worksheet
- Download - Boat Power Consumption Worksheet - Excel Google Docs (Gray fields are editable. Go to File->Make A Copy for your own use.)
Why Use a Solar Design Worksheet?
Designing a marine solar system involves more than selecting solar panels. The entire system; including batteries, charging sources, and onboard electrical loads—must work together as a balanced energy system.
To simplify this process, Custom Marine Products developed a comprehensive Marine Solar System Design Worksheet that guides you through every stage of system planning.
The worksheet follows the same five-step methodology introduced earlier in this chapter and allows you to adjust different variables to see how they affect your final system design.
Whether you're upgrading an existing boat or designing a new installation, the worksheet provides a practical way to estimate your energy requirements before purchasing equipment.
Step 1 — Estimate Daily Energy Consumption
The first step is to determine how much electricity your boat uses during a typical day.
Begin by listing every onboard electrical device, including both DC equipment and AC appliances.
Typical DC loads include:
- Refrigeration
- Radar
- Computer - Laptop
- Autopilot
- Cabin Lights (LED)
- Nav/Anchor Lights
- Stereo
- VHF Radio
- Instruments
- Pressure Water
- Phone Charger
- Other
For each appliance, record:
- Current draw (Amps)
- Estimated operating time per day (Hours)
The worksheet automatically calculates your total daily DC energy consumption in Amp-hours (Ah).
If your boat has different operating modes, such as anchored versus underway, you can calculate each scenario separately.
Step 2 — Estimate Power Generation from Alternative Sources
Next, estimate the power consumed by AC appliances operating through an inverter.
The worksheet automatically converts AC wattage into the equivalent battery demand while accounting for inverter losses.
You can then estimate how much energy will come from charging sources other than solar, such as:
- Engine alternator
- Generator
- Shore power
- Wind generator
For example, you may decide to run your engine for one or two hours each day, reducing the amount of energy your solar array must supply.
This step establishes your net daily energy requirement.
Step 3 — Estimate Solar Generation Capacity Needed
Once your daily energy consumption is known, the worksheet estimates how much solar capacity is required to replace that energy.
The calculation considers several important variables, including:
- Average daily peak sun hours
- Battery charging efficiency
- Solar panel wattage
- Number of solar panels
- Charge controller size
For many cruising areas, approximately five peak sun hours per day provides a practical planning value for fixed horizontal solar panels. The worksheet can easily be adjusted for different cruising regions or mounting configurations.
It also estimates excess solar production available during favorable weather conditions.
Step 4 — Select Solar Panel(s) and Controller – Mounting Considerations
Plan for cloudy days.
Solar production changes with weather conditions.
The worksheet allows you to estimate reduced solar output during extended periods of cloud cover and calculate the additional battery reserve needed to maintain reliable operation.
Rather than designing only for perfect sunshine, the worksheet helps create a system capable of supporting several days of reduced solar production.
This additional reserve is especially valuable for cruising boats that spend extended periods at anchor.
Step 5 — Estimate Optimum Size of Your Battery Bank
The final calculation determines the battery capacity required to support your daily energy usage.
The worksheet automatically adjusts battery requirements based on battery chemistry.
For example:
- Lead-acid batteries typically provide only about 50% usable capacity.
- AGM batteries provide slightly greater usable capacity.
- LiFePO₄ batteries allow approximately 90–95% usable capacity, meaning fewer batteries are often required to provide the same usable energy.
This comparison clearly illustrates why battery chemistry plays a major role in overall system design.
4.3. Review Your Solar System Design Findings
The worksheet provides three key findings:
- Power consumption — how much energy the boat uses each day
- Battery capacity — how much energy storage is needed to support that consumption
- Solar capacity — how much panel wattage is needed to replace the energy used
At Anchor and On Passage
For the Erickson 38 case study, the worksheet shows different requirements depending on whether the boat is at anchor or underway and whether solar power is available.
The example ultimately indicates a solar requirement of approximately 255 watts at anchor under the stated assumptions, with additional capacity needed to provide greater protection during cloudy periods.
The purpose of these numbers is not to provide a universal solar-system size. They demonstrate how the worksheet turns your boat's actual energy consumption and operating conditions into a practical system design.
Our Findings
The worksheet gives you a calculated solar requirement, but there can be more than one way to approach the design.
In the case study, the results show approximately:
- 82–177 Ah of daily power consumption
- 200–400 Ah of optimum battery capacity using lead-acid batteries
- Approximately 300–550 watts of solar panel capacity
These figures demonstrate how the five-step process connects energy consumption, battery storage, and solar generation.
The goal is not simply to select the largest battery or the most solar panels possible. Instead, the system should be sized around the boat's actual energy needs, operating conditions, available charging sources, and desired level of independence from the engine or generator.
4.4. Another Way to Approach Solar Sizing
For example, in our case study, we can start with a 240 Ah lead-acid battery bank and look at how much energy is consumed over a 24-hour period. By accounting for the battery bank's usable capacity, average daily sun hours, and system inefficiencies, we can estimate the solar capacity needed to replenish the energy used.
In this example, the calculation indicates approximately 295 watts of solar under the stated conditions.
However, designing for average conditions alone may not provide enough energy during periods of reduced sunlight. To provide additional capacity for two consecutive cloudy days, the example increases the solar array to approximately 450 watts.
☑️ The Key Idea
Solar sizing is not only about replacing the energy you use on an average day. A well-designed system should also consider charging efficiency, available sunlight, and the amount of reserve you want during periods of reduced solar production.
This battery-bank-first approach is an alternative way to understand the relationship between battery capacity, energy consumption, and solar generation. For a complete system design, CMP's five-step worksheet brings these factors together to determine the appropriate balance between your solar array, battery bank, and other charging sources.
4.5. A Complete and Balanced Solar Power System
Once you have determined your energy consumption, battery capacity, solar capacity, and controller size, the next step is understanding how to use the solar power your system produces throughout the day.
A balanced marine solar system is not simply about generating enough power. It is also about understanding when power is being consumed, when solar power is available, and how much energy is being stored in the batteries.
Understanding the Daily Power Cycle
Your boat's power consumption changes throughout the day. Some loads, such as refrigeration, may operate continuously, while other equipment creates higher demand during certain periods.
Without solar generation, these loads gradually draw energy from the battery bank, causing the battery's state of charge to decline.
When solar is added, the daily energy cycle changes:
- Overnight: The boat continues to consume energy while solar production is unavailable.
- Morning: Solar production begins and starts replacing the energy being consumed.
- Midday: Solar production reaches its highest level and may exceed the boat's immediate power consumption.
- Afternoon: The batteries may reach full charge, creating an opportunity to use available solar energy directly.
- Evening: Solar production decreases and the battery bank again supplies the boat's loads.
Put Excess Solar Power to Work
A properly sized system may produce more solar power than the boat needs at certain times of the day. Rather than letting that available energy go unused once the batteries are fully charged, this can be an ideal time to run higher-power equipment.
Depending on your boat and system, this could include:
- Air conditioning
- Ice makers
- Water makers
- Water heaters
- Other high-power electrical loads
This is the basic concept of power management: use energy-intensive equipment when solar production is high and available, while allowing the battery bank to supply the boat when solar production is low.
The Goal of a Balanced System
Power consumption + Solar generation + Battery storage + Other charging sources
The result is a system that captures available solar energy, stores what you need, and makes effective use of excess energy when solar production is at its highest.
With the system sized and balanced, we can now move on to the next stage: selecting the appropriate solar panels, controller, battery bank, and installation configuration for the boat.
Chapter 4 Summary
In this chapter, we used CMP's five-step Solar Design Worksheet to work through the process:
- Estimate Daily Energy Consumption — Identify your DC and AC loads and estimate how much energy your boat uses each day.
- Estimate Power Generation from Alternative Sources — Consider how much energy will come from your alternator, generator, wind generator, or other charging sources.
- Estimate Solar Generation Capacity Needed — Use your energy needs, available sun hours, charging efficiency, and desired cloudy-day reserve to determine the solar capacity you need.
- Select Solar Panel(s) and Controller – Mounting Considerations — Determine the appropriate solar panels, charge controller, and mounting configuration based on your calculated requirements and available space.
- Estimate Optimum Size of Your Battery Bank — Make sure your battery bank provides enough usable energy to support your loads and maintain an appropriate reserve.
The goal is a balanced solar system, one that produces enough energy to meet your needs, stores enough power for periods of low solar production, and makes effective use of available solar energy.
Once the system is properly sized, power management becomes an important part of getting the most from your solar investment. Use high-power loads when solar production is high, while allowing the battery bank to provide energy when solar production is low.